阅读说明：本技术 流体感测装置 (Fluid sensing device ) 是由 D·阿祖尼 M·迪亚兹 T·布朗佩 于 2020-04-14 设计创作，主要内容包括：提供了一种流体感测装置(120),该流体感测装置包括：流体流动通道(121)；至少一个流体导管(132,134),该至少一个流体导管与该流体流动通道(121)处于流体连通；以及流体传感器(130),该流体传感器具有外壳(131)和至少一个传感器端口(135,136),该至少一个传感器端口与该至少一个流体导管(132,134)处于流体连通并且提供进入该外壳(137)的通路。该流体感测装置(120)还包括压力补偿室(142),该流体传感器(130)的外壳(137)被围在该压力补偿室中。该装置进一步包括至少一个压力补偿导管(144),该至少一个压力补偿导管与该压力补偿室(142)和该流体流动通道(121)处于流体连通。还提供了一种包括这种流体感测装置的质量流量控制器(100)。(A fluid sensing apparatus (120) is provided, the fluid sensing apparatus comprising: a fluid flow channel (121); at least one fluid conduit (132, 134) in fluid communication with the fluid flow channel (121); and a fluid sensor (130) having a housing (131) and at least one sensor port (135, 136) in fluid communication with the at least one fluid conduit (132, 134) and providing access to the housing (137). The fluid sensing device (120) also includes a pressure compensation chamber (142) in which the housing (137) of the fluid sensor (130) is enclosed. The device further includes at least one pressure compensation conduit (144) in fluid communication with the pressure compensation chamber (142) and the fluid flow channel (121). A mass flow controller (100) comprising such a fluid sensing device is also provided.)

1. A fluid sensing device comprising:

a fluid flow channel having an inlet and an outlet;

at least one fluid conduit in fluid communication with the fluid flow channel;

a fluid sensor having a housing and at least one sensor port in fluid communication with the at least one fluid conduit and providing access to the housing;

a pressure compensation chamber in which a housing of the fluid sensor is enclosed; and

at least one pressure compensation conduit in fluid communication with the pressure compensation chamber and the fluid flow passage,

Wherein the at least one pressure compensation conduit extends between the pressure compensation chamber and a location along the fluid flow channel such that fluid flowing along the fluid flow channel enters the pressure compensation chamber during use via the at least one pressure compensation conduit at the location of the at least one pressure compensation conduit, and

wherein the at least one fluid conduit extends between the at least one sensor port and a location along the fluid flow channel such that fluid flowing along the fluid flow channel enters a housing of the fluid sensor via the at least one fluid conduit at the location of the at least one fluid conduit during use.

2. A fluid sensing apparatus as defined in claim 1, wherein the fluid sensor is mounted on a printed circuit board forming a portion of the pressure compensation chamber.

3. The fluid sensing device of claim 2, wherein the pressure compensation chamber further comprises a reservoir against which the printed circuit board is sealed to enclose the reservoir and thereby define the pressure compensation chamber.

4. The fluid sensing device of claim 3, further comprising an elastomeric seal between the printed circuit board and the reservoir, wherein the printed circuit board is removably sealed against the reservoir by the elastomeric seal.

5. The fluid sensing device of claim 3 or claim 4, further comprising a housing comprising a solid body having the at least one fluid conduit and the at least one pressure compensation conduit formed therein, wherein the reservoir is defined by a cavity in the solid body.

6. A fluid sensing apparatus as claimed in any of claims 2 to 5, wherein the printed circuit board is a main printed circuit board on which the control electronics and the fluid sensor are mounted.

7. A fluid sensing apparatus as claimed in any of claims 2 to 5, wherein the printed circuit board is an auxiliary printed circuit board and the fluid sensing apparatus further comprises a main printed circuit board on which control electronics are mounted, the main printed circuit board being electrically connected to the auxiliary printed circuit board by one or more electrical connectors.

8. A fluid sensing apparatus as claimed in any preceding claim, further comprising a sensor seal between the at least one sensor port and the at least one fluid conduit, wherein the at least one fluid conduit is isolated from the pressure compensation chamber by the sensor seal.

9. A fluid sensing apparatus as claimed in any preceding claim, wherein the at least one fluid conduit comprises a first fluid conduit extending from a first location along the fluid flow channel and a second fluid conduit extending from a second location along the fluid flow channel, and wherein the at least one sensor port comprises a first sensor port in fluid communication with the first fluid conduit and a second sensor port in fluid communication with the second fluid conduit.

10. The fluid sensing device of claim 9, wherein the fluid flow channel comprises a flow restrictor disposed between the first and second positions, and wherein the fluid sensor is configured to measure a first pressure in the first fluid conduit and to measure a second pressure in the second fluid conduit.

11. The fluid sensing device of claim 10, wherein the first fluid conduit, the sensor housing, and the second fluid conduit together form a bypass channel along which a portion of the fluid flow channel is directed during use, and wherein the fluid sensor is configured to measure bypass flow through the bypass channel.

12. A fluid sensing apparatus as claimed in any preceding claim, wherein the fluid flow channel is bounded by and defined by an outer wall of the fluid flow channel, and wherein the at least one pressure compensation conduit and the at least one fluid conduit extend through the outer wall of the fluid flow channel.

13. A fluid sensing apparatus as claimed in any preceding claim, wherein the at least one pressure compensation conduit extends between the pressure compensation chamber and a location along the fluid flow channel such that a portion of fluid flowing along the fluid flow channel enters the pressure compensation chamber via the at least one pressure compensation conduit at the location of the at least one pressure compensation conduit during use.

14. A fluid sensing apparatus as claimed in any preceding claim, wherein the at least one fluid conduit extends between the at least one sensor port and a location along the fluid flow channel such that a portion of fluid flowing along the fluid flow channel enters a housing of the fluid sensor via the at least one fluid conduit at the location of the at least one fluid conduit during use.

15. A fluid sensing apparatus as claimed in any preceding claim, wherein the pressure compensation chamber is remote from the fluid flow passage.

16. A fluid sensing apparatus as claimed in any preceding claim, wherein a portion of the fluid flowing along the fluid flow channel exits the fluid flow channel at the location of the at least one pressure compensation conduit during use to enter the pressure compensation chamber via the at least one pressure compensation conduit.

17. A fluid sensing apparatus as claimed in any preceding claim, wherein a portion of the fluid flowing along the fluid flow channel exits the fluid flow channel at the location of the at least one fluid conduit during use to enter a housing of the fluid sensor via the at least one fluid conduit.

18. A fluid sensing apparatus as claimed in any preceding claim, wherein the at least one pressure compensation conduit extends between the pressure compensation chamber and at least one location along the length of the fluid flow channel between the inlet and the outlet.

19. A fluid sensing apparatus as claimed in any preceding claim, wherein the at least one fluid conduit extends between the at least one sensor port and at least one location along the length of the fluid flow channel between the inlet and the outlet.

20. A mass flow controller comprising:

a fluid control valve;

control electronics; and

the fluid sensing device of any one of claims 1 to 19,

wherein the control electronics are configured to control the fluid control valve based on a sensor signal provided by the fluid sensing device.

21. The mass flow controller of claim 20, wherein the fluid sensing device further comprises:

a main printed circuit board on which the control electronics are mounted; and

an auxiliary printed circuit board on which the fluid sensor is mounted and which forms a portion of the pressure compensation chamber,

wherein the main printed circuit board is spaced apart from the auxiliary printed circuit board in a direction perpendicular to a plane of the auxiliary printed circuit board and is electrically connected to the auxiliary printed circuit board by one or more electrical connectors.

22. A mass flow controller according to claim 20 or claim 21, wherein the mass flow controller is a miniature mass flow controller.

Technical Field

The present invention relates to a fluid sensing device, and in particular to a fluid sensing device for use with a fluid control valve, such as in a mass flow controller or a mass flow meter.

Background

Fluid control valves are used in a wide variety of applications to control the flow of a fluid. The controlled fluid may include a gas, a liquid, or a combination thereof. In some cases, the fluid may also include suspended particles. While fluid control valves vary widely in the particular configuration used to open and close the fluid communication path through the valve, one particular type of valve actuation is performed using a solenoid. In solenoid actuated valves, current is passed through an electromagnetic coil, where the coil is typically formed around a magnetic core. A coil typically comprises a wire which is wound around a bobbin a plurality of times, thereby creating a plurality of so-called turns. The energized solenoid produces a magnetic field. The strength of the magnetic field is proportional to the number of turns and the current supplied to the wire. As is well known in the art, to enhance the magnetic field provided by the solenoid, the number of turns may be increased and/or the current provided to the wire may be increased. The magnetic field typically operates on a movable armature connected to a plunger configured to engage a valve seat surrounding an inlet and/or outlet through which fluid may pass to vary the flow restriction created by the valve seat and a sealing portion of the plunger. Other types of actuation may be used, such as piezoelectric actuation.

Mass flow controllers ("MFCs") are widely used to measure and control the flow of fluids. A typical MFC includes a fluid sensing device, a fluid control valve, and a controller for controlling the fluid control valve. The fluid sensing device generally includes a flow channel extending between an inlet and an outlet, and a fluid sensor in communication with the flow channel. During operation of the MFC, the controller determines the flow through the flow passage based on the sensor signal of the fluid sensor and operates the control valve accordingly to maintain the desired flow. There are two main types of MFCs: heat based and pressure based.

Pressure-based MFCs typically use a flow restriction, such as a nozzle or orifice, along the flow path to create a pressure drop from which the flow rate can be determined. In such MFCs, the flow may be determined by physically measuring the bypass flow resulting from the pressure differential, or may be determined by mathematically calculating the flow based on the following principles: the flow rate of fluid through the restriction is a function of the pressure drop across the restriction. The pressure drop may be calculated and the flow determined by sensing the fluid pressure p1 upstream of the flow restriction and the fluid pressure p2 downstream of the flow restriction. In this or other applications, the fluid sensor may be a simple enclosure having a housing and two or more ports through which fluid may enter and exit the housing, whereby the flow rate along the flow passage may be determined by measuring the flow rate through the sensor housing. Alternatively, the fluid sensing device may have a single fluid conduit through which fluid enters the housing of the fluid sensor through a single sensor port, whereby the flow rate along the flow channel may be determined by measuring the pressure of the fluid entering the housing of the fluid sensor.

In each of the above types of fluid sensing devices, fluid passes from a flow channel along at least one fluid conduit and into a fluid sensor housing. This means that the sensor housing must be able to resist the pressure of the fluid once it has entered the sensor housing. However, the fluid sensor housing has a limited ability to withstand internal pressures. For example, the housing of a typical fluid sensor may have a maximum pressure rating of about 3 bar. This means that any pressure spike exceeding 3 bar can cause the sensor to malfunction and ultimately cause fluid leakage. To address this problem, it is known to reinforce the sensor housing to ensure that it can withstand the internal pressures experienced during use. However, such fluid sensors tend to be large, heavy, and more expensive, and are therefore not suitable for all applications, such as micro MFCs. Increased robustness of the fluid sensor may also lead to increased manufacturing complexity and cost.

The present invention seeks to provide an improved flow sensing device which overcomes or mitigates one or more of these problems associated with the prior art.

Disclosure of Invention

According to a first aspect of the present invention, there is provided a fluid sensing device comprising: a fluid flow channel having an inlet and an outlet; at least one fluid conduit in fluid communication with the fluid flow channel; a fluid sensor having a housing and at least one sensor port in fluid communication with the at least one fluid conduit and providing access to the housing; a pressure compensation chamber in which a housing of the fluid sensor is enclosed; and at least one pressure compensation conduit in fluid communication with the pressure compensation chamber and the fluid flow channel, wherein the at least one pressure compensation conduit extends between the pressure compensation chamber and a location along the fluid flow channel such that fluid flowing along the fluid flow channel enters the pressure compensation chamber via the at least one pressure compensation conduit at the location of the at least one pressure compensation conduit during use, and wherein the at least one fluid conduit extends between the at least one sensor port and a location along the fluid flow channel such that fluid flowing along the fluid flow channel enters a housing of the fluid sensor via the at least one fluid conduit at the location of the at least one fluid conduit during use.

Optionally, the fluid sensor is mounted on a printed circuit board which forms part of the pressure compensation chamber.

Optionally, the pressure compensation chamber further comprises a reservoir against which the printed circuit board is sealed to enclose the reservoir and thereby define the pressure compensation chamber. The fluid sensing device further comprises an elastomeric seal between the printed circuit board and the reservoir, wherein the printed circuit board is removably sealed against the reservoir by the elastomeric seal.

Optionally, the fluid sensing device further comprises a housing comprising a solid body having the at least one fluid conduit and the at least one pressure compensating conduit formed therein, wherein the reservoir is defined by a cavity in the solid body.

Optionally, the printed circuit board is a main printed circuit board on which the control electronics and the fluid sensor are mounted.

Optionally, the printed circuit board is an auxiliary printed circuit board, and the fluid sensing device further comprises a main printed circuit board having control electronics mounted thereon, the main printed circuit board being electrically connected to the auxiliary printed circuit board by one or more electrical connectors.

The fluid sensing device may further comprise a sensor seal between the at least one sensor port and the at least one fluid conduit, wherein the at least one fluid conduit is isolated from the pressure compensation chamber by the sensor seal.

The at least one fluid conduit includes a first fluid conduit extending from a first location along the fluid flow channel and a second fluid conduit extending from a second location along the fluid flow channel, and the at least one sensor port includes a first sensor port in fluid communication with the first fluid conduit and a second sensor port in fluid communication with the second fluid conduit.

The fluid flow channel includes a flow restrictor disposed between the first and second locations, and wherein the fluid sensor is configured to measure a first pressure in the first fluid conduit and to measure a second pressure in the second fluid conduit.

The first fluid conduit, the sensor housing and the second fluid conduit together form a bypass channel along which a portion of the fluid flow channel is directed during use, and the fluid sensor is configured to measure a bypass flow through the bypass channel.

The fluid flow channel is enclosed and defined by an outer wall of the fluid flow channel, and wherein the at least one pressure compensating conduit and the at least one fluid conduit extend through the outer wall of the fluid flow channel.

The at least one pressure compensation conduit extends between the pressure compensation chamber and a location along the fluid flow channel such that a portion of the fluid flowing along the fluid flow channel enters the pressure compensation chamber at the location of the at least one pressure compensation conduit via the at least one pressure compensation conduit during use.

The at least one fluid conduit extends between the at least one sensor port and a location along the fluid flow channel such that a portion of fluid flowing along the fluid flow channel enters a housing of the fluid sensor via the at least one fluid conduit at the location of the at least one fluid conduit during use.

The pressure compensation chamber is remote from the fluid flow passage.

A portion of the fluid flowing along the fluid flow channel is directed out of the fluid flow channel at the location of the at least one pressure compensating conduit to enter the pressure compensating chamber via the at least one pressure compensating conduit during use.

A portion of the fluid flowing along the fluid flow channel exits the fluid flow channel at the location of the at least one fluid conduit during use to enter the housing of the fluid sensor via the at least one fluid conduit.

The at least one pressure compensating conduit extends between the pressure compensating chamber and at least one location along the length of the fluid flow channel between the inlet and the outlet.

The at least one fluid conduit extends between the at least one sensor port and at least one location along the length of the fluid flow channel between the inlet and the outlet.

According to a second aspect of the present invention there is provided a mass flow controller comprising: a fluid control valve; control electronics; and a fluid sensing apparatus according to the above, wherein the control electronics are configured to control the fluid control valve based on a sensor signal provided by the fluid sensing apparatus.

The fluid sensing device further comprises: a main printed circuit board on which the control electronics are mounted; and an auxiliary printed circuit board on which the fluid sensor is mounted and which forms part of the pressure compensation chamber, wherein the main printed circuit board is spaced from the auxiliary printed circuit board in a direction perpendicular to the plane of the auxiliary printed circuit board and is electrically connected to the auxiliary printed circuit board by one or more electrical connectors.

The mass flow controller is a micro mass flow controller.

According to a third aspect of the present invention, there is provided a fluid sensing device comprising: a fluid flow channel having an inlet and an outlet; at least one fluid conduit in fluid communication with the fluid flow channel; a fluid sensor having a housing and at least one sensor port in fluid communication with the at least one fluid conduit and providing access to the housing; a pressure compensation chamber in which a housing of the fluid sensor is enclosed; and at least one pressure compensating conduit in fluid communication with the pressure compensating chamber.

By this arrangement, fluid can be supplied to the pressure compensation chamber via the at least one pressure compensation conduit, so that the fluid pressure outside the housing can be balanced with the fluid pressure inside the housing. The pressure compensation chamber thus compensates for the internal pressure within the housing, and the housing therefore only needs to resist a relatively small difference between the fluid pressure in the pressure compensation chamber and the fluid pressure in the at least one fluid conduit. This reduces the risk of leakage or sensor failure compared to sensor housings that need to withstand a complete difference between internal fluid pressure and atmospheric pressure. The reduced burden on the housing also allows for simplified construction and manufacture of the fluid sensor and may facilitate the use of lighter, smaller, and less complex components. This can be particularly beneficial when the fluid sensing device is intended for use in a compact device, such as a micro MFC.

The one or more locations along the fluid flow path to which the at least one pressure compensating conduit extends may be considered pressure compensating locations. The one or more locations to which the at least one fluid conduit along the fluid flow channel extends may be considered fluid sensing locations.

The fluid sensing device of the present invention can be easily used in many different applications, for example, for controlling industrial processes, performing laboratory experiments or for safety reasons. The fluid sensing device may be used for flow information only, or may be used as a means of flow regulation, such as in a device for mass flow control or volumetric flow control. The fluid sensing apparatus of the present invention finds particular utility in accurate fluid sensing for flow control, such as in mass flow controllers. The fluid sensing device of the present invention may be configured for use with a gas or liquid.

Preferably, the at least one pressure compensation conduit is in fluid communication with the fluid flow passage and extends between the pressure compensation chamber and the fluid flow passage.

With this arrangement, fluid in the fluid flow channel during use enters the pressure compensation chamber via the at least one pressure compensation conduit and enters the housing of the fluid sensor via the at least one fluid conduit. Thus, the pressure compensation chamber is filled with fluid at the same or similar elevated pressure as fluid flowing along the flow channel at the location of the pressure compensation conduit, while the sensor housing contains fluid at the same or similar elevated pressure as fluid flowing along the flow channel at the location of the at least one sensor port. The pressure compensation chamber thus compensates for the internal pressure within the housing, and the housing therefore only needs to resist a relatively small difference between the fluid pressure at the location of the pressure compensation conduit and the fluid pressure at the location of the at least one fluid conduit. This reduces the risk of leakage or sensor failure compared to sensor housings that need to withstand the difference between the elevated internal fluid pressure and atmospheric pressure. The reduced burden on the housing also allows for simplified construction and manufacture of the fluid sensor and may facilitate the use of lighter, smaller, and less complex components. This can be particularly beneficial when the fluid sensing device is intended for use in a compact device, such as a micro MFC.

In other embodiments, fluid may be supplied to the pressure compensation chamber from a fluid source other than the fluid flow passage via at least one pressure compensation conduit. At least one pressure compensation conduit may extend between the pressure compensation chamber and one or more fluid conduits separate from the fluid flow channel. Where the fluid sensing device comprises a housing, the housing may comprise a plurality of fluid conduits, and at least one pressure compensation conduit may extend between the pressure compensation chamber and one or more of the plurality of fluid conduits. For example, the fluid sensing device may be provided as part of a manifold or flow controller having a housing. The housing may comprise a first fluid inlet connected to the inlet of the fluid flow channel, a first fluid outlet connected to the outlet of the fluid flow channel, and a second fluid inlet, wherein at least one pressure compensation conduit extends between the pressure compensation chamber and the second fluid inlet. In such examples, the fluid sensing device may include a first fluid line connected to the first fluid inlet or the first fluid outlet, and a second fluid line extending between the first fluid line and the second fluid inlet. With this arrangement, fluid in the first fluid line enters the pressure compensation chamber during use via the second fluid line, the second fluid inlet, and the at least one pressure compensation conduit. Thus, the pressure compensation chamber is filled with fluid at a similar pressure as the fluid flowing along the first fluid line to compensate for the internal pressure within the housing. Additionally, this arrangement may facilitate cleaning of the fluid sensing device by allowing flushing of the pressure compensation chamber via the second fluid inlet and the at least one pressure compensation conduit. Where a single pressure compensating conduit is provided, during purging, fluid may be flushed from the chamber via the at least one fluid conduit and the fluid flow passage. In case a plurality of pressure compensation conduits is provided, at least a part of the fluid may be flushed out of the chamber via the additional pressure compensation conduit during cleaning. For example, the housing may include a first pressure compensation conduit extending between the pressure compensation chamber and the second fluid inlet, and a pressure compensation conduit extending between the pressure compensation chamber and the second fluid outlet. In such examples, fluid may be flushed through the pressure compensation chamber via the first and second pressure compensation conduits.

Preferably, the fluid sensor is mounted on a printed circuit board which forms part of the pressure compensation chamber. This has been found to provide a particularly compact arrangement and avoids the need to provide a separate electrical connector extending from the fluid sensor and through the wall of the pressure compensation chamber. Such electrical connectors are difficult to seal effectively and therefore represent a potential leak point. This arrangement also has the advantage of close proximity between any control electronics on the printed circuit board and the fluid sensor to reduce noise or other interference with the signal generated by the fluid sensor. The printed circuit board may form a wall of the pressure compensation chamber. The printed circuit board should have sufficient strength to withstand the difference between the fluid pressure in its lower pressure compensation chamber and the atmospheric pressure on its upper side.

Preferably, the pressure compensation chamber further comprises a reservoir against which the printed circuit board is sealed to enclose the reservoir and thereby define the pressure compensation chamber. With this arrangement, the printed circuit board forms an upper wall, or lid, of the pressure compensation chamber.

Preferably, the fluid sensing device further comprises an elastomeric seal between the printed circuit board and the reservoir, wherein the printed circuit board is removably sealed against the reservoir by the elastomeric seal. By this arrangement, the resilient seal can compensate for variations in manufacturing tolerances between the reservoir and the printed circuit board. The elastomeric seal also allows the printed circuit board to be removed and replaced when needed without the need to apply a separate sealant to reseal the printed circuit board against the reservoir. The elastomeric seal may be a rubber seal or any other suitable elastomer, such as NBR, FPM, or EPDM. The resilient seal may be seated in a groove extending around the reservoir. The resilient seal is preferably continuous. That is, the elastomeric seal preferably surrounds the reservoir to form a continuous seal.

In other examples, the printed circuit board may be permanently sealed against the reservoir by a sealant that is applied after the printed circuit board is positioned against the reservoir.

Preferably, the fluid sensing device further comprises a housing.

The at least one fluid conduit, pressure compensation conduit, and/or pressure compensation chamber may be formed by separate components held within the housing. Preferably, the housing comprises a solid body in which the at least one fluid conduit and the at least one pressure compensating conduit are formed. The solid body may comprise a plurality of bores from which the at least one fluid conduit and the at least one pressure compensation conduit are formed. Preferably, the reservoir is defined by a cavity in the solid body. The cavity may be defined in an outer surface of the solid body. The fluid flow passage may be formed, in whole or at least in part, by one or more of the plurality of internal bores. The plurality of internal holes are preferably formed in the solid body by a subtractive manufacturing process, such as drilling or another machining operation.

Where the fluid sensing device comprises an elastomeric seal between the printed circuit board and the reservoir, the elastomeric seal may be seated in a groove in the outer surface of the solid body, the groove extending around the cavity such that the elastomeric seal forms a continuous seal around the cavity.

In some embodiments, the printed circuit board is a main printed circuit board on which the control electronics and the fluid sensor are mounted. With this arrangement, all or substantially all of the electrical components of the fluid sensing device may be provided on a single PCB.

In other embodiments, the printed circuit board is an auxiliary printed circuit board, and the fluid sensing device further comprises a main printed circuit board on which the control electronics are mounted. The main printed circuit board may be spaced apart from the auxiliary printed circuit board and electrically connected to the auxiliary printed circuit board by one or more electrical connectors. With this arrangement, the main printed circuit board is at atmospheric pressure on both its upper and lower sides, and unlike the auxiliary printed circuit board, does not need to be reinforced to withstand the pressure in the pressure compensation chamber. This may reduce the size, weight, complexity, and cost of the main circuit board. In addition, and somewhat counter-intuitively, where an auxiliary printed circuit board is provided in addition to the main printed circuit board, the overall size of the fluid sensing device can be reduced by reducing the space occupied by the fluid sensor on the main printed circuit board. With this arrangement, the main printed circuit board need only accommodate electrical connectors, such as pins, for the fluid sensor rather than the entire fluid sensor. Thus, other electrical components may occupy the space on the main PCB that would otherwise be required for the fluid sensor, allowing for a more compact overall arrangement. The main printed circuit board may be spaced apart from the auxiliary printed circuit board in a direction substantially perpendicular to a plane of the auxiliary printed circuit board. In such embodiments, the main printed circuit board and the auxiliary printed circuit board may be substantially parallel. The main printed circuit board may be spaced from the auxiliary printed circuit board in a direction substantially parallel to the plane of the auxiliary printed circuit board. In such embodiments, the main printed circuit board and the auxiliary printed circuit board may be arranged substantially perpendicular to each other.

The at least one sensor port may be flush with the at least one fluid conduit. The at least one sensor port may extend into the at least one fluid conduit. Some fluid leakage may be accommodated between the at least one sensor port and the at least one fluid conduit. Preferably, the at least one fluid conduit is isolated from the pressure compensation chamber. Preferably, the fluid sensing device further comprises a sensor seal between the at least one sensor port and the at least one fluid conduit, wherein the at least one fluid conduit is isolated from the pressure compensation chamber by the sensor seal. The sensor seal may comprise an O-ring extending around the at least one sensor port and/or the at least one fluid conduit. In a preferred embodiment, the at least one sensor port extends into the at least one fluid conduit, and the sensor seal may comprise an O-ring extending around the at least one sensor port to isolate the at least one fluid conduit from the pressure compensation chamber. The sensor seal is preferably an elastomeric sensor seal.

The at least one fluid conduit may comprise a single fluid conduit. The at least one sensor port may comprise a single sensor port. In such examples, the fluid sensor may be configured to measure fluid pressure in a single fluid conduit using a single sensor port. Preferably, the at least one fluid conduit comprises a first fluid conduit extending from a first location along the fluid flow channel and a second fluid conduit extending from a second location along the fluid flow channel. Preferably, the at least one sensor port comprises a first sensor port in fluid communication with the first fluid conduit and a second sensor port in fluid communication with the second fluid conduit.

The at least one pressure compensating conduit may comprise a plurality of pressure compensating conduits extending from different locations. For example, at different locations along the fluid flow path. The at least one pressure compensating conduit may comprise a single pressure compensating conduit. The at least one pressure compensating conduit may comprise a single pressure compensating conduit in fluid communication with the fluid flow passage. With this arrangement, the fluid flow passage is in fluid communication with the pressure compensation chamber via only a single pressure compensation conduit.

In certain preferred embodiments, the fluid flow passage includes a flow restriction disposed between the first position and the second position. The flow restriction may include an obstruction inserted into the fluid flow passage to create a pressure drop. The flow restriction may comprise an orifice plate or a nozzle. The flow restriction may comprise a reduction in the diameter of an outer wall of the fluid flow passage. The diameter reduction may comprise a step change in the diameter of the outer wall. The diameter reduction may include a gradual change in the diameter of the outer wall. The reduction may be provided around only a portion of the circumference of the fluid flow channel. The reduction may be uniform around the circumference of the fluid flow channel. Preferably, the taper comprising the outer wall extending around the entire circumference of the flow passage is reduced. The fluid sensing device may include a laminar flow element including a flow stabilizer bar extending along the fluid flow channel from at least a first position to a second position. In such examples, the flow restriction may include an increase in the diameter of the flow restrictor. The increase may comprise a step change in diameter. Increasing may include a gradual change in diameter. The increase may be provided around only a portion of the circumference of the stabilizer bar. The increase may be uniform around the circumference of the stabilizer bar. Preferably, a taper extending around the entire circumference of the stabilizer bar is added including the stabilizer bar. In such embodiments, the diameter of the flow passage may be constant in the region of the flow restriction, such that the flow restriction is defined only by the increase in diameter of the flow stabilizer bar. This may be beneficial in that it allows the pressure drop across the restriction to be varied as needed for a given flux simply by varying the laminar flow element. The flow restriction may include a reduction in diameter of an outer wall of the fluid flow passage and an increase in diameter of the flow stabilizer.

The fluid sensor may be configured to measure a first pressure in the first fluid conduit and to measure a second pressure in the second fluid conduit. The flow rate through the fluid flow passage may then be determined based on the pressure differential.

The first fluid port and the second fluid port may be isolated from each other within the housing. The first fluid port and the second fluid port may be in fluid communication within the housing. In some embodiments, the first fluid conduit, the sensor housing, and the second fluid conduit together form a bypass channel along which a portion of the fluid flow along the fluid flow channel is directed during use. The fluid sensor may be configured to measure a bypass flow through the bypass channel. The flow rate through the fluid flow channel may then be determined from the bypass flow rate through the flow channel.

The fluid sensor may be a pressure sensor. The fluid sensor may be configured to sense a first fluid pressure at a first location along the fluid flow channel and to sense a second pressure at a second location along the fluid flow channel. The fluid sensor may be configured to sense a first fluid pressure at a first location via a first fluid conduit and a second pressure at a second location via a second fluid conduit. The fluid sensor may include a first sensor port positioned in the first fluid conduit and configured to sense a first fluid pressure at a first location. The fluid sensor may include a second sensor port positioned in the second pressure conduit and configured to sense a second fluid pressure at the second location. A first sensor seal may be disposed about the first sensor port to form a seal between an outer surface of the first sensor port and an inner surface of the first pressure conduit. A second sensor seal may be disposed about the second sensor port to form a seal between an outer surface of the second sensor port and an inner surface of the second pressure conduit. In this way, fluid in the first sensor port and/or the second sensor port may be prevented from bypassing the first sensor portion and/or the second sensor portion. Fluid in the first and second pressure conduits enters the housing of the fluid sensor via the first and second sensor portions.

The fluid sensor may be configured to output a sensor signal. The sensor signals may include a first fluid pressure signal and a second fluid pressure signal. The fluid sensor may be configured to calculate a pressure difference between the first fluid pressure and the second fluid pressure. The fluid sensor may be configured to output a sensor signal comprising a differential pressure signal comprising a plurality of calculated pressure differential values. The fluid sensor may be configured to calculate a flow rate through the fluid flow passage based on the sensed first and second fluid pressure values. The fluid sensor may be configured to output a sensor signal including a flow signal including a plurality of calculated flow values.

The fluid sensor may be a mass flow sensor. The fluid sensing device may include a bypass channel configured to draw flow in the fluid flow channel around the flow restriction. The fluid sensor may be a mass flow sensor configured to measure a bypass flow through the bypass channel. The first fluid conduit and the second fluid conduit may be connected to form a portion of the bypass channel.

The fluid sensor may comprise a single sensing head. The signals from the sensing heads may be amplified with different gains. This may increase the effective measurement range of flow that may be accurately measured by the fluid sensing device and may facilitate accurate flow readings from the fluid sensor even at very small flow rates. A fluid sensor may include multiple sensing heads within a single sensor. The signals from each sensing head may be amplified with different gains. This may increase the effective measurement range of flow that may be accurately measured by the fluid sensing device and may facilitate accurate flow readings from the fluid sensor even at very small flow rates.

The fluid flow passage may be bounded and defined by an outer wall of the fluid flow passage. In such embodiments, one or both of the at least one pressure compensating conduit and the at least one fluid conduit may extend through an outer wall of the fluid flow channel.

The at least one pressure compensation conduit may extend between the pressure compensation chamber and a location along the fluid flow channel such that a portion of the fluid flowing along the fluid flow channel at the location of the at least one pressure compensation conduit enters the pressure compensation chamber during use via the at least one pressure compensation conduit.

The at least one fluid conduit may extend between the at least one sensor port and a location along the fluid flow channel such that a portion of fluid flowing along the fluid flow channel at the location of the at least one fluid conduit enters a housing of the fluid sensor via the at least one fluid conduit during use.

The pressure compensation chamber may be remote from the fluid flow passage. This means that the pressure compensation chamber is located away from the fluid flow channel. The fluid flow passage may extend adjacent the pressure compensation chamber. The fluid flow passage may extend along the length of the pressure compensation chamber. The at least one pressure compensating conduit may form part of a separate fluid flow path to the fluid flow channel. In certain embodiments, the pressure compensation chamber does not form part of the same fluid flow path as the fluid flow channel.

At least one pressure compensation conduit may be ported into the fluid flow channel at a pressure compensation location along the length of the fluid flow channel such that the fluid pressure in the pressure compensation chamber is the same as the fluid pressure at the pressure compensation location in the fluid flow channel.

At least one fluid conduit may be accessed into the fluid flow channel at a fluid sensing location along the length of the fluid flow channel such that the fluid pressure at the at least one sensor port is the same as the fluid pressure at the fluid sensing location in the fluid flow channel.

The at least one pressure compensation conduit may be configured such that a portion of the fluid flowing along the fluid flow channel at the location of the at least one pressure compensation conduit is directed out of the fluid flow channel during use to enter the pressure compensation chamber via the at least one pressure compensation conduit.

The at least one fluid conduit may be configured such that a portion of the fluid flowing along the fluid flow channel at the location of the at least one fluid conduit is directed out of the fluid flow channel during use to enter the housing of the fluid sensor via the at least one fluid conduit.

The at least one pressure compensation conduit may extend between the pressure compensation chamber and at least one pressure compensation location along the length of the fluid flow channel between the inlet and the outlet.

The at least one fluid conduit may extend between the at least one sensor port and at least one fluid sensing location along a length of the fluid flow channel between the inlet and the outlet.

The fluid sensing device may be used in any suitable assembly. For example, the fluid sensing device may form part of a fluid manifold.

According to a fourth aspect of the present invention, there is provided a mass flow controller comprising: a fluid control valve; control electronics; and a fluid sensing apparatus according to the first aspect, wherein the control electronics are configured to control the fluid control valve based on a sensor signal provided by the fluid sensing apparatus. The fluid control valve may be a proportional valve.

In certain embodiments, the fluid sensing device of the mass flow controller further comprises: a main printed circuit board on which these control electronics are mounted; and an auxiliary printed circuit board mounted with the fluid sensor and forming a portion of the pressure compensation chamber. The main printed circuit board may be spaced apart from the auxiliary printed circuit board in a direction perpendicular to a plane of the auxiliary printed circuit board, and may be electrically connected to the auxiliary printed circuit board by one or more electrical connectors.

The mass flow controller may be a micro mass flow controller. As used herein, the term "miniature mass flow controller" refers to a mass flow controller having a housing with a maximum dimension in any direction of less than 100mm, preferably less than 80 mm. The micro flow controller may have a maximum length of less than 80mm and a maximum height of less than 50 mm.

Within the scope of the present application, it is expressly intended that the various aspects, embodiments, examples and alternatives set forth in the preceding paragraphs, in the claims and/or in the following description and drawings (and particularly individual features thereof) may be employed independently or in any combination. That is, all embodiments and/or features of any embodiment may be combined in any manner and/or combination unless such features are incompatible. The applicant reserves the right to amend any originally filed claim or to amend any new claim accordingly, including the right to amend any originally filed claim to rely on and/or incorporate any feature of any other claim although not originally filed in this way.

Drawings

Further features and advantages of the invention will be further described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a partial perspective cross-sectional view of a mass flow controller including a fluid sensing device according to a first embodiment;

FIG. 2 is a top view of the mass flow controller of FIG. 1;

FIG. 3 is a cross-sectional view taken through line III-III of FIG. 2;

FIG. 4 is a cross-sectional view taken through line IV-IV of FIG. 2;

FIG. 5 is a top perspective view of the solid body of the mass flow controller of FIGS. 1-4;

FIG. 6 is a schematic cross-section of a fluid sensing device of the mass flow controller of FIGS. 1-4 showing fluid flow through the device; and is

FIG. 7 is a partial perspective cross-sectional view of a mass flow controller including a fluid sensing device according to a second embodiment.

Detailed Description

Fig. 1-6 illustrate a first mass flow controller 100 that includes a fluid sensing device 120 according to a first embodiment of the present invention. The mass flow controller 100 includes a housing 101 having a fluid inlet 102 and a fluid outlet 103. The housing 101 houses a fluid control valve 104 (e.g., a proportional valve) and control electronics 105 mounted on a main PCB 106. In this example, the housing 101 includes a solid body 107 and a cover 108 removably secured to the solid body 107 by screws 109. A fluid control valve 104 is positioned along a fluid flow path extending between the fluid inlet 102 and the fluid outlet 103 and is configured to regulate the flow through the mass flow controller 100 based on control signals from the control electronics 105 in order to achieve or maintain a desired flow. As shown in fig. 1, the fluid inlet 102 and the fluid outlet 103 may be threaded to allow for easy coupling to a threaded connector. Alternatively, the fluid inlet and fluid outlet may have a cartridge-type fitting, or may have a flange-mounted manifold through which fluid lines may be connected without requiring threaded connections.

Fluid sensing device 120 includes a fluid flow channel 121 that is enclosed and defined by an outer wall 122 of channel 121 and forms a portion of a fluid flow path through mass flow controller 100. The fluid flow channel 121 extends between a channel inlet 123 in fluid communication with the fluid inlet 102 of the mass flow controller 100 and a channel outlet 124 in fluid communication with the fluid outlet 103 of the mass flow controller 100. In this example, the cross-section of the fluid flow channel 121 is circular, but other cross-sectional shapes may be suitable. The fluid sensing device further includes a restriction 125 located along the length of the fluid flow channel 121 and configured to create a pressure differential with fluid flowing along the channel 121. In this example, the restriction 125 comprises a gradual reduction in the diameter of the outer wall 122 of the fluid flow channel 121, such that the cross-sectional area of the fluid flow channel 121 is reduced in this region and the flow rate of the fluid increases as it passes through the restriction 125. This creates a pressure drop across the restriction 125. In other examples, the flow restriction may include a step change in diameter of the outer wall, and/or a change in diameter of an outer surface of a laminar flow element positioned in the flow channel 121 as described below.

The fluid sensing device 100 further includes a fluid sensor 130 mounted on an auxiliary printed circuit board 140. The main PCB 106 is spaced apart from the auxiliary printed circuit board 140 in a direction perpendicular to the plane of the auxiliary printed circuit board 140, and is electrically connected to the auxiliary printed circuit board 140 through an electrical connector 141. The main printed circuit board 106 is removably mounted on the solid body 107 by means of the same screws 109 as the cover 108. Because the main printed circuit board 106 supports the control electronics 105 of the mass flow controller 100, it may be considered a "main" PCB. The auxiliary printed circuit board 140 on which the fluid sensor 130 is mounted may be considered an "auxiliary" PCB. In other examples, such as the embodiment discussed below in connection with fig. 7, the mass flow controller may comprise a single PCB on which both the control electronics 105 and the fluid sensor 130 are mounted directly.

As best seen in fig. 6, the fluid sensor 130 is in fluid communication with a first location 131 of the fluid flow channel 121 upstream of the restriction 125 via a first fluid conduit 132, and is in fluid communication with a second location 133 of the fluid flow channel 121 downstream of the restriction 125 via a second fluid conduit 134. However, in other examples, the fluid sensor may be in fluid communication with only a single location of the fluid flow channel. The fluid sensor 130 is configured to generate a sensor signal indicative of the flow rate of the fluid passing along the fluid flow channel 121 so that the control electronics 105 can control the fluid control valve 104 accordingly to achieve a desired flow rate through the mass flow controller 100.

In this example, the fluid sensor 130 is a pressure sensor and includes a first sensor port 135 positioned in the first fluid conduit 132 and a second sensor port 136 positioned in the second fluid conduit 134. The first sensor port 135 and the second sensor port 136 are each provided with openings through which fluid may enter and/or exit the housing 137 of the fluid sensor 130. The first sensor port 135 enables the fluid sensor 130 to sense or measure a first fluid pressure P1 at the first location 131, and the second sensor port 136 enables the fluid sensor 130 to sense or measure a second fluid pressure P2 at the second location 133. First and second sensor ports 135 and 136 extend into first and second fluid conduits 132 and 134, respectively. An O-ring 138 is provided around each of the first and second sensor ports 135, 136 to form a resilient seal between the first sensor port 135 and the first fluid conduit 132, and between the second sensor port 136 and the second fluid conduit 134, respectively, to prevent fluid leakage.

Due to the restriction 125, the flow rate at the second location 133 tends to be higher than the flow rate at the first location. Thus, the second fluid pressure P2 tends to be lower than the first fluid pressure P1. The pressure differential ap across the restriction 125 may be calculated from the sensed P1 and P2 values, and the flow rate through the fluid flow channel 121 determined based on the following principles: the flow rate of the fluid through the restriction is proportional to the pressure differential across the restriction. The pressure difference ap may be determined by the fluid sensor 130 or by the control electronics 105. In the case where the pressure difference ap is determined by the control electronics, the sensor signals may include a first pressure signal over time of the first fluid pressure P1 and a second pressure signal over time of the second fluid pressure P2. In the case where the pressure difference ap is determined by a fluid sensor, the sensor signal may comprise a pressure difference signal of the pressure difference ap over time. The fluid sensor may be configured to determine flow, in which case the sensor signal may comprise a flow signal.

In other examples, the fluid sensor 130 may be a mass flow sensor. For example, the first fluid conduit 132 and the second fluid conduit 134 may be connected to form a bypass channel (not shown) where a portion of the fluid flow is directed out by the flow restriction 125, wherein the fluid sensor is configured to measure the bypass flow at the bypass channel. The fluid flow along the fluid flow path may then be calculated from the bypass flow.

As best seen in fig. 3-5, the solid body 107 is formed from a solid block of material having a plurality of holes defined therein to form the conduits, and the other components of the mass flow controller 100 are received by the solid block of material. The fluid sensing device 120 further includes a pressure compensation chamber 142 in fluid communication with a third location 143 along the fluid flow passageway 121 via a pressure compensation conduit 144. The pressure compensation chamber 142 is defined by a reservoir in the form of a cavity 145 in the outer surface of the solid body 107 and by the auxiliary printed circuit board 140 enclosing the cavity 145. The auxiliary printed circuit board 140 is held in place against the cavity 145 by a pair of screws 146 that extend into threaded holes 1071 in the solid body 107. The auxiliary printed circuit board 140 is sealed against the cavity 145 by an elastomeric seal 147 which sits in a continuous groove 148 formed in the outer surface of the solid body 107 and extending around the cavity 145. The elastic seal 147 prevents fluid from leaking between the auxiliary printed circuit board 140 and the solid body 107. In this way, the auxiliary printed circuit board 140 forms an upper wall of the pressure compensation chamber 142. Accordingly, the lower side of the auxiliary printed circuit board 140, on which the fluid sensor 130 is mounted, is exposed to the increased pressure in the pressure compensation chamber 142, and the upper side of the auxiliary printed circuit board 140 is exposed to the atmospheric pressure. This means that the auxiliary printed circuit board should be configured to withstand the difference between atmospheric pressure and the elevated pressure in the pressure compensation chamber 142 during operation. However, it also means that the components of the mass flow controller that are above the secondary printed circuit board 140 (such as the primary printed circuit board 106) are at atmospheric pressure and need not be configured to withstand elevated pressures. The first fluid conduit 132, the second fluid conduit 134, and the pressure compensating conduit 144 extend to the bottom of the cavity 145. The pressure compensation conduit opens into a pressure compensation chamber. The first and second fluid conduits 132, 134 each have a cup-shaped reservoir 1321, 1341 at their cavity end and a seal seat 1322, 1324 around the cup-shaped reservoir 1321, 1341 in which an O-ring around each sensor port is sealed to isolate the first and second fluid conduits from the pressure compensation chamber.

In this example, the third location 143 from which the pressure compensating conduit 144 extends is downstream of both the first and second locations 131, 133 from which the first and second fluid conduits 132, 134 extend. However, in other examples, the pressure compensating conduit 144 may be connected to a different location along the fluid flow channel 121, such as a location upstream of one or both of the first and second locations 131, 133.

To improve flow sensing accuracy, the fluid sensing device 120 further includes an optional laminar flow element 150 positioned in the fluid flow channel 121. The laminar flow element 150 includes a flow stabilizer bar 151 and a support 152 by which the flow stabilizer bar 121 is centrally mounted in the fluid flow channel 121. The flow stabilizer bar 151 extends at least from the first position 131 along the fluid flow channel 121 through the flow restrictor 125 to the second position 133 to promote laminar flow and inhibit turbulent flow in the fluid flow channel 121. In this example, the flow stabilizer bar 151 extends from a position upstream of the first position 131 to a position downstream of the second position 133. The support 152 of the laminar flow element 150 is secured within the fluid flow channel 121 at the upstream end of the flow stabilizer bar 151. The support 152 may be fixedly secured in the flow channel 151 or may be removably secured. In this example, the support 152 includes threads 156 on an outer surface 154 thereof that engage corresponding threads on the outer wall 122 of the flow channel 121. Thus, the support 152 is removably secured within the flow channel 121 by a threaded connection at a location upstream of the first location 131.

The support 152 of the laminar flow element 150 includes a plurality of fluid flow apertures 155 spaced at regular intervals around the circumference of the support 152 and spaced from the outer surface 154 of the support. The fluid flow apertures 155 allow fluid to pass through the support 152 and promote a more uniform flow of fluid along the fluid flow channel 121. The support 152 may have any suitable shape. In this example, the outer surface 154 of the support 152 has a shape corresponding to the shape of the outer wall 122 of the flow channel 121 and is fastened by a threaded connection. This prevents or reduces the amount of fluid that can pass between the outer surface 154 of the support 152 and the outer wall 122 of the flow passage 121. Because the shape of the outer surface 154 corresponds to the shape of the outer wall 122 of the flow channel 121, substantially all of the fluid flowing along the fluid flow channel flows through the fluid flow aperture 155. In other examples, one or more outer apertures may be formed between the outer surface of the support and the outer wall of the flow channel such that fluid may pass around the outer surface of the support.

As best seen in fig. 6, the outer wall 122 of the fluid flow channel 121 and the outer surface 153 of the flow stabilizer bar 151 together define an annular portion 126 of the fluid flow channel 121 through which fluid flows. The outer surface 153 of the flow stabilizer bar 151 is substantially continuous. That is, the outer surface 153 of the flow stabilizer bar 151 is substantially free of any grooves, protrusions, or other surface features that may otherwise impede flow attachment. The outer surface 153 of the flow stabilizer bar 151 may be smooth. In this example, the diameter of the outer surface 153 is substantially constant along the entire length of the flow stabilizer bar 151. Thus, the cross-sectional area of the annular portion 126, which varies with the radial distance between the outer wall 122 of the fluid flow channel 121 and the outer surface 153 of the flow stabilizer 151, decreases at the flow restriction purely as the diameter of the outer wall 122 of the flow channel 121 decreases. In other examples, the diameter of the outer surface 153 may vary along the length of the stabilizer bar 151. The diameter of the outer surface 153 may increase or decrease along its length, provided that the radial distance between the outer wall 122 of the fluid flow channel 121 and the outer surface 153 of the flow stabilizer 151 decreases to define a flow restriction. In examples where the diameter of the outer surface of the flow stabilizer bar increases, the diameter of the outer wall of the flow passage may decrease, remain the same, or increase in the area of the flow restrictor, provided that the increase in the diameter of the outer surface of the flow stabilizer bar is sufficient to cause the radial distance between the outer wall of the fluid flow passage and the outer surface of the flow stabilizer bar to still decrease across the flow restrictor.

During operation of mass flow controller 100, fluid enters housing 101 through fluid inlet 102 and enters fluid flow channel 121 via channel inlet 123. When the fluid reaches the laminar flow element 150, the fluid passes through the plurality of fluid flow apertures 155 in the support 152 and into the annular portion 126 of the fluid flow channel 121 defined between the flow stabilizer bar 151 and the outer wall 122 of the fluid flow channel 121, wherein the fluid travels along the length of the flow stabilizer bar 151, through the flow restrictor 125, and exits the fluid flow channel 121 at the channel outlet 124. Fluid enters the housing 137 of the fluid sensor 130 via the first fluid conduit 132 and the second fluid conduit 134. The fluid sensor 130 monitors the first fluid pressure P1 at the first location 131 and the second fluid pressure P2 at the second location 133, and outputs sensed P1 and P2 values as sensor signals to the control electronics 105. The control electronics 105 determines the voltage drop Δ P across the restriction 125 by subtracting P2 from P1. The control electronics 105 calculates the flow rate through the fluid flow channel 121 based on the pressure drop Δ P and compares it to the desired flow rate in a conventional manner. If the calculated flow rate is greater than or less than the desired flow rate, the control electronics 105 then controls the fluid control valve 104 to adjust the flow rate as needed. Because the pressure compensation chamber 142 is in fluid communication with the fluid flow passageway via the pressure compensation conduit 144, the pressure compensation chamber 142 is filled with fluid at the same pressure P3 as the third location 143 along the fluid flow passageway 121. With this arrangement, the outer surface of the housing 137 of the fluid sensor 130 is exposed to an elevated pressure that varies with the pressure P3 of the fluid flowing along the flow channel 121 at the third location 143. At the same time, the inner surface of the housing 137 is exposed to an elevated pressure that varies with the pressure P1 at the first location 131 and the pressure P2 at the second location 133 of the fluid flowing along the flow channel 121. This means that the housing of the fluid sensor only needs to resist the relatively small difference between the pressure P3 at the third position and the first pressure P1 at the first position and the second pressure P2 at the second position, rather than the complete difference between atmospheric pressure and the first and second fluid pressures P1 and P2.

By enclosing the housing 137 of the fluid sensor 130 within the pressure compensation chamber 142, the outer surface of the housing 137 is exposed to a fluid pressure comparable to the fluid pressure inside the housing 137. Therefore, the pressure difference across the housing 137 is small. This means that the housing 137 need not be configured to withstand large internal pressures as these internal pressures will match large external pressures. Thus, the complexity, size, and weight of the fluid sensor may be reduced relative to conventional fluid sensing devices. Indeed, with the arrangement of the present invention, a fluid sensor having a housing capable of withstanding only a pressure differential of 1 bar or less may be used.

Fig. 7 shows a second embodiment of a mass flow controller 200 comprising a fluid sensing device 220 according to a second embodiment of the invention. The mass flow controller 200 has a similar structure and function to the mass flow controller 100 of the first embodiment, and like reference numerals are used to denote like features. As with the first embodiment, the fluid sensing device 220 includes a laminar flow element 250 having a flow stabilizer bar 251 centrally positioned in the fluid flow channel 221 and extending from a position upstream of the first position 231 to a position downstream of the second position 233. Mass flow controller 200 also includes control electronics 205, which are mounted on PCB 206. However, unlike the first embodiment, the fluid sensor 230 is mounted directly on the main PCB206 along with the control electronics 205. Thus, main PCB206 is the only PCB in mass flow controller 200. In the absence of a secondary printed circuit board, the primary PCB206 forms the upper wall of the pressure compensation chamber 242, is held in place by a pair of screws 209 extending into threaded holes in the solid body 207, and is sealed against the solid body 207 by a resilient seal (not shown). Thus, the lower side of the main PCB206 is exposed to the elevated pressure in the pressure compensation chamber 242, while the upper side of the main PCB206 is exposed to atmospheric pressure. This means that the main PCB206 should be constructed to withstand the difference between atmospheric pressure and the elevated pressure in the pressure compensation chamber 242 during operation.

Further, in the mass flow controller 200 of the second embodiment, the third location 243 from which the pressure compensating conduit 244 extends is located upstream rather than downstream of both the first location 231 and the second location 233.

Although the invention has been described above with reference to one or more preferred embodiments, it should be understood that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.