Method and apparatus for integrating pressure and flow controller

Abstract

The invention provides a pressure control method and device. A pressure controller includes: a control valve configured to control a pressure of a fluid in a flow path; a flow restrictor provided in the flow path; and a distal pressure sensor and a proximal pressure sensor. A distal pressure sensor detects fluid pressure at the flow restrictor at a location remote from the control valve, and a proximal pressure sensor detects fluid pressure at the flow restrictor at a location proximate the control valve. The pressure controller further includes a controller configured to 1) control actuation of the control valve based on the pressure detected by the distal pressure sensor and a pressure setpoint, and 2) determine a mass flow rate based on the pressures detected by the distal pressure sensor and the proximal pressure sensor.

Description

Related applications

[0001] This application is a continuation-in-part of and claims priority to U.S. application No. 18 / 170,964, filed February 17, 2023. The entire teachings of the above application are incorporated herein by reference. Background Art

[0002] Pressure controllers with integrated mass flow meters are used in critical process applications, such as wafer backside cooling, to measure and control the pressure of a fluid while also monitoring the fluid's mass flow rate. Improved methods and apparatus for controlling and monitoring the pressure of certain process fluids are needed. Summary of the Invention

[0003] A pressure controller with an integrated mass flow meter is provided, which can be advantageously used to control and monitor the pressure of a reactive fluid, including a reactive gas, such as an O3 / O2 mixture.

[0004] A pressure controller includes: a control valve configured to control the pressure of a fluid in a flow path; a flow restrictor disposed in the flow path; and a distal pressure sensor and a proximal pressure sensor. The distal pressure sensor detects the pressure of the fluid in the flow path at a location distal to the control valve, and the proximal pressure sensor detects the pressure of the fluid in the flow path at a location proximal to the control valve. The flow restrictor is disposed between the distal location and the proximal location in the flow path. The pressure controller further includes a controller configured to control actuation of the control valve based on a pressure as detected by the distal pressure sensor and a pressure set point, and to determine a mass flow rate based on the pressures as detected by the distal pressure sensor and the proximal pressure sensor.

[0005] A pressure controller includes a control valve configured to control the pressure of a fluid in a flow path; a flow restrictor disposed in the flow path; and a distal pressure sensor and a proximal pressure sensor. The distal pressure sensor detects the pressure of the fluid at the flow restrictor at a location distal from the control valve, and the proximal pressure sensor detects the pressure of the fluid at the flow restrictor at a location proximal to the control valve. The pressure controller further includes a controller configured to control actuation of the control valve based on the pressure detected by the distal pressure sensor and a pressure set point. The controller is further configured to determine a mass flow rate based on the pressures detected by the distal pressure sensor and the proximal pressure sensor.

[0006] A method for controlling fluid pressure includes controlling actuation of a control valve based on a pressure detected by a distal pressure sensor and a pressure set point. The distal pressure sensor detects fluid pressure in a flow path at a location distal to the control valve. The method further includes determining a mass flow rate based on the pressures detected by the distal pressure sensor and a proximal pressure sensor, the proximal pressure sensor detecting fluid pressure in the flow path at a location proximal to the control valve. A flow restrictor is disposed between the distal and proximal locations in the flow path.

[0007] A method for controlling fluid pressure includes controlling actuation of a control valve based on a pressure detected by a distal pressure sensor and a pressure set point. The control valve controls the pressure of a fluid in a flow path, and a flow restrictor is disposed in the flow path. The distal pressure sensor detects fluid pressure at the flow restrictor at a location distal to the control valve. The method further includes determining a mass flow rate based on the pressures detected by the distal pressure sensor and a proximal pressure sensor, the proximal pressure sensor detecting fluid pressure at the flow restrictor at a location proximal to the control valve.

[0008] The proximal position and the distal position can be upstream of the control valve to provide upstream pressure control. Alternatively, the proximal position and the distal position can be downstream of the control valve to provide downstream pressure control.

[0009] Controlling actuation of the control valve may include closed-loop feedback control of the control valve based on the pressure as detected by the remote pressure sensor and a pressure set point.

[0010] The mass flow rate Q can be determined according to the function provided by the following formula: Q=f(R,P u ,P d ,T,mw,μ,γ) (1) Where R is the characteristic of the flow restrictor, P u is the pressure upstream of the flow restrictor detected by one of the distal pressure sensor and the proximal pressure sensor, P d is the pressure downstream of the flow restrictor detected by the other of the distal pressure sensor and the proximal pressure sensor, T is the temperature of the fluid, mw is the molecular weight of the fluid, µ is the viscosity of the fluid, and γ is the specific heat ratio of the fluid. The determined mass flow rate can be output.

[0011] The temperature of the fluid in the flow path can be detected. For example, the pressure controller can include a temperature sensor configured to detect the temperature of the fluid in the path. The temperature sensor can detect the temperature of the fluid at or near the flow restrictor in the flow path.

[0012] The fluid may be a reactive gas, such as an O3 / O2 gas mixture or a HBr / Cl2 gas mixture.

[0013] A pressure controller includes: a control valve configured to control the pressure of a fluid in a flow path; a flow restrictor disposed in the flow path; an upstream pressure sensor; and a downstream pressure sensor. The upstream pressure sensor detects the pressure of the fluid at the flow restrictor upstream of the flow restrictor, and the downstream pressure sensor detects the pressure of the fluid at the flow restrictor downstream of the flow restrictor. The pressure controller further includes a controller configured to control actuation of the control valve based on a pressure detected by one of the upstream pressure sensor and the downstream pressure sensor and a pressure set point. The controller is further configured to determine a mass flow rate based on the pressures detected by the upstream pressure sensor and the downstream pressure sensor.

[0014] A method for controlling the pressure of a fluid includes controlling actuation of a control valve based on a pressure setpoint and a pressure detected by one of an upstream pressure sensor and a downstream pressure sensor. The upstream pressure sensor detects the fluid pressure at the flow restrictor upstream of the flow restrictor, and the downstream pressure sensor detects the fluid pressure at the flow restrictor downstream of the flow restrictor. The control valve controls the pressure of the fluid in a flow path, and the flow restrictor is disposed in the flow path. The method further includes determining a mass flow rate based on the pressures detected by the upstream pressure sensor and the downstream pressure sensor.

[0015] The flow restrictor, the upstream pressure sensor, and the downstream pressure sensor may be disposed upstream of the control valve, wherein one of the upstream pressure sensor and the downstream pressure sensor serves as the pressure sensor for pressure control. Alternatively, the flow restrictor, the upstream pressure sensor, and the downstream pressure sensor may be disposed downstream of the control valve, wherein one of the upstream pressure sensor and the downstream pressure sensor serves as the pressure sensor for pressure control. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The foregoing will be apparent from the following more detailed description of example embodiments, as illustrated in the accompanying drawings, in which like reference numerals refer to like parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the embodiments.

[0017] FIG1 is a schematic diagram of a pressure controller integrated with a mass flow meter according to the prior art.

[0018] Figure 2 is a schematic diagram of an exemplary downstream pressure controller integrated with a mass flow meter.

[0019] Figure 3is a schematic diagram of an exemplary upstream pressure controller integrated with a mass flow meter.

[0020] Figure 4 This is a diagram illustrating a method of controlling the pressure and monitoring the mass flow rate of reactive gases. DETAILED DESCRIPTION

[0021] Pressure controllers with integrated mass flow meters can control the pressure of a fluid while simultaneously monitoring its mass flow rate for critical process applications, such as wafer backside cooling. Such pressure controllers offer both pressure control and mass flow rate monitoring in a compact package. Examples of pressure controllers include the General Purpose Downstream Pressure Controller with Mass Flow Meter (GPCMA) and the High Performance Downstream Pressure Controller with Mass Flow Meter (PPCMA), manufactured by MKS Instruments, Inc. (Andover, Massachusetts).

[0022] FIG1 shows an example of a prior art pressure controller integrated with a mass flow meter. The pressure controller 100 includes a thermal flow sensor 102, a control valve 110, and a pressure sensor 104. The control valve 110 controls the pressure of the fluid flowing through the flow path 130 of the device based on the pressure sensed by the pressure sensor 104. In particular, the controller 120 (e.g., a microprocessor) can control the actuation of the control valve 110 so that the monitored pressure (P u ) is regulated to the pressure set point. Thus, the pressure of the fluid leaving the device (P d ) is controlled to a pressure set point. Simultaneously, the thermal flow sensor 102 measures the mass flow rate of the fluid. The mass flow rate and, if necessary, the monitored pressure can be output by the device to report to the user.

[0023] Pressure controllers, such as pressure controller 100 in FIG. 1 , are commonly used to monitor process gases; however, such pressure controllers can be unreliable and unsuitable for use with reactive gases. As used herein, a "reactive gas" is a gas or gas mixture that is unstable and / or exhibits high chemical reactivity. Examples of reactive gases include ozone (O 3 ), ozone-oxygen mixtures (O 3 / O 2 ), and hydrogen bromide-chlorine mixtures (HBr / Cl 2 ).

[0024] Thermal flow sensors typically include a heat source through which the gas being measured passes and operate based on the temperature measurements obtained of the gas. For example, a thermal flow sensor may include a sensor tube having a thermal element disposed thereon. For example, the thermal element may be a coiled resistor that is wound around the sensor tube and heated to a temperature above the ambient temperature. As the gas flows through the sensor tube, the gas, which is typically at ambient temperature, has a cooling effect on the coil and reduces its temperature according to the mass flow rate. The flowing gas cools the upstream coil more than the downstream coil, and therefore, the mass flow rate of the gas can be determined based on the measured temperature difference between the coils (as indicated by the measured resistance difference between the coils). U.S. Patent No. 5,461,913 further describes an example of a thermal flow sensor.

[0025] The application of heat to reactive gases and / or the generation of heat by exothermic reactions involving reactive gases can interfere with thermal flow measurements. For example, for process gases including ozone (O3), the mass flow measurement derived by thermal flow sensor 102 may be inaccurate because ozone is unstable and releases heat as it decomposes. Furthermore, the application of heat to gases including ozone may cause ozone degradation. Consequently, the signal from a thermal flow sensor is susceptible to saturation when used with ozone and other reactive gases. Furthermore, thermal flow sensors are susceptible to damage when used with reactive gases.

[0026] A description of exemplary implementations follows.

[0027] The present invention provides a pressure control device and method suitable for use with reactive gases. The pressure control device can further perform both pressure control and mass flow rate monitoring in a compact form.

[0028] Figure 2An exemplary pressure controller is shown. The pressure controller 200 includes a control valve 210 configured to control the pressure of a fluid in a flow path 230. The flow path 230 may be defined by a body 232 of the device. A flow restrictor 208 is provided in the flow path 230. The device further includes pressure sensors 204, 206. The pressure sensor 204 is a proximal pressure sensor that detects the pressure of the fluid in the flow path at a position 214 near the control valve. The pressure sensor 206 is a distal pressure sensor that detects the pressure of the fluid in the flow path at a position 216 distal to the control valve. The controller 220 controls the actuation of the control valve 210 based on the pressure as detected by the distal pressure sensor 206 and the pressure set point. The controller is further configured to control the pressure (P) as detected by the distal pressure sensor 206 and the proximal pressure sensor 204. u and P d ) to determine the mass flow rate. The pressure controller 200 may further include a temperature sensor 202. The temperature sensor 202 may be configured to detect the temperature of the fluid at or near the flow restrictor 208 for determining the mass flow rate of the fluid.

[0029] Pressure sensors 204, 206 can be configured to detect pressure at the flow restrictor 208. As used herein, "at the flow restrictor" means at the inlet or outlet of the flow restrictor, including a location in the flow path adjacent to and sufficiently close to the inlet or outlet of the flow restrictor to provide a pressure measurement that can be used to determine mass flow rate. For example, the pressure sensor or a portion thereof can be positioned in the flow path so that it is adjacent to the flow restrictor in the flow path.

[0030] like Figure 2 As shown, the pressure controller 200 is a downstream pressure controller that is based on a pressure measurement (P d ) to control the pressure of the fluid flowing through the device. Thus, the pressure of the fluid leaving the device at outlet 236 is controlled to the pressure set point. In this configuration, the pressure sensor (pressure sensor 204) near the control valve is the upstream pressure sensor that obtains the pressure reading (P u ), and the pressure sensor (pressure sensor 206) away from the control valve is a downstream pressure sensor that obtains a pressure reading (P d ).

[0031] The pressure controller can alternatively be configured as an upstream pressure controller, such as Figure 3As shown. The pressure controller 300 includes elements similar to those shown with respect to the pressure controller 200, except that the pressure sensors 304, 306 are disposed upstream of the control valve 210. In this configuration, the pressure sensor (pressure sensor 306) distal to the control valve is an upstream pressure sensor that detects the fluid pressure in the flow path at a location 316 distal to the control valve, and the pressure sensor (pressure sensor 304) proximal to the control valve is a downstream pressure sensor that detects the fluid pressure in the flow path at a location 314 proximal to the control valve. Based on the pressure (P u ), the controller 220 controls the pressure of the fluid entering the device at the inlet 234 to a pressure set point. The controller 320 is further configured to control the pressure of the fluid entering the device at the inlet 234 to a pressure set point based on the pressure (P u and P d ) to determine the mass flow rate.

[0032] In both the upstream and downstream configurations (200, 300), the pressure sensor (pressure sensor 206, 306) remote from the control valve advantageously serves a dual purpose. Specifically, the pressure sensed by the remote pressure sensor is selected as the target for pressure control, and the controller (220, 320) uses this target to control actuation of the control valve 210 to control the downstream port ( Figure 2 ) or upstream port ( Figure 3 ). Furthermore, the controller (220, 320) uses the pressure sensed by the distal pressure sensor in combination with the pressure sensed by the proximal pressure sensor (pressure sensors 204, 304) to determine the mass flow rate of the fluid. Thus, the distal pressure measurement enables both pressure control and mass flow rate determination.

[0033] The control of the control valve 210 can be closed-loop. In particular, the controller (220, 320) can be configured to perform closed-loop feedback control of the control valve 210 based on the pressure detected by the remote pressure sensor and the pressure set point. The actuation of the control valve can be directly based on the detected pressure. The detected pressure can be compared with the pressure set point, and the opening or closing of the valve can be adjusted accordingly so that the remote pressure is equal to the pressure set point or within an acceptable tolerance range of the pressure set point. The remote pressure can be monitored throughout the pressure control process, and the actuation of the control valve can be adjusted accordingly to maintain the upstream or downstream pressure control to the pressure set point.

[0034] As used herein, the term "control valve" refers to a valve that can provide a controllable range of opening states (possibly between an open state and a closed state) and does not include on / off type valves. The openness of an adjustable control valve can be controlled in response to a control signal and can control the flow rate or pressure of a fluid traveling through the valve. Adjustable control valves include proportional control valves. Examples of suitable control valves for use as adjustable control valves in the provided devices include solenoid valves, piezoelectric valves, and stepper motor valves.

[0035] The mass flow rate (Q) of the fluid can be determined according to the function provided by the following equation: Q=f(R,P u ,P d ,T,mw,μ,γ) (1) Where R is the characteristic of the flow restrictor, P u is the pressure upstream of the flow restrictor detected by one of the distal pressure sensor and the proximal pressure sensor, P d is the pressure downstream of the flow restrictor sensed by the other of the distal pressure sensor and the proximal pressure sensor, T is the temperature of the fluid, mw is the molecular weight of the fluid, µ is the viscosity of the fluid, and γ is the specific heat ratio of the fluid. The characteristic (R) of the flow restrictor may be, for example, the orifice size of the flow restrictor. When determining the mass flow rate according to Equation 1, one or more flow restrictor characteristics may be considered (e.g., R may represent or include more than one physical parameter of the flow restrictor). The properties of the fluid (including molecular weight, viscosity, and specific heat ratio) may be known values.

[0036] Methods for determining the mass flow rate of a fluid based on pressures sensed upstream and downstream of a flow restrictor are common knowledge in the art. The flow restrictor can be of any suitable type for restricting the flow of a fluid, including, for example, a critical flow nozzle, a laminar flow element, a porous media flow restrictor, an orifice, a valve, or a tube.

[0037] The controller 220, 320 can be configured to output the determined mass flow rate. For example, the pressure controller 200, 300 can include an output 240 for reporting the determined mass flow rate to another device, or the output 240 can be a display. Although the pressure controller is configured to control the pressure of the fluid (e.g., not to control the mass flow rate to a mass flow setpoint), the monitored mass flow rate is valuable information for process monitoring and can be reported and / or used by other process equipment. Optionally, the monitored pressure (e.g., the pressure detected by the remote pressure sensor 206, 306) can also be output.

[0038] By using proximal and distal pressure sensors, a pressure controller with integrated mass flow rate monitoring can be provided that is particularly suitable for use with reactive gases. The device's pressure sensors can accurately report the pressure of reactive gases, information that can then be used to calculate mass flow rate, whereas thermal flow sensors can be inaccurate for such gases. Examples of suitable pressure sensors include Baratron® manometers (MKS Instruments Ltd.) and pressure transducers.

[0039] Figure 4 Displays the description Figure 2 or Figure 3 Flowchart of the operation of a pressure controller. Process 400 begins by receiving a pressure set point (Psp) from a host (402). The upstream pressure (Pu), downstream pressure (Pd) and gas temperature (T) are measured (404). The flow rate (Q) is calculated based on the measured values ​​(406). The pressure set point (Psp) and the measured distal pressure (Pu or Pd, depending on whether the flow controller is upstream or downstream of the pressure control configuration) are provided to a feedback controller (K) (408) which determines the control command (I) for the control valve. The control valve is adjusted by the control command (I) to control the distal pressure to the pressure set point (410). The flow rate (Q) and, if necessary, the monitored target pressure (Pu or Pd) and / or any other information (e.g., control command (I), proximal pressure (Pu or Pd), etc.) are reported to the host (412). The process can be repeated, with pressure being monitored continuously or periodically for a given pressure set point and / or after a new pressure set point is received from the host. Although in Figure 4 Process 400 is shown as the flow rate calculation (406) being performed before providing the measured pressure to the feedback controller (408) and regulating the control valve (410), but it should be understood that the flow rate calculation (406) can alternatively be performed simultaneously with or after providing the pressure feedback control (408, 410).

[0040] The provided methods and apparatus offer several improvements over existing methods and apparatus for providing pressure control integrated with mass flow measurement. The pressure-based flow measurement provided by the example apparatus and method can be more reliable than prior art devices that rely on thermal flow measurement, particularly in reactive gas environments (e.g., O3 / O2 mixtures). The provided apparatus can be more durable than prior art devices. By utilizing one of the two pressure sensors (i.e., the pressure sensor remote from the control valve) for dual use, a compact and cost-effective pressure controller can be provided.

[0041] like Figure 2 and Figure 3 The pressure sensors shown and the locations within the flow path where they obtain pressure measurements are generally described as "proximal" and "distal" based on their proximity to the device's control valve. A "distal pressure sensor" and / or "distal location" within the flow path can be on the side of the flow restrictor farthest from the control valve. A "proximal pressure sensor" and / or "proximal location" within the flow path can be on the side of the flow restrictor closest to the control valve.

[0042] The teachings of all patents, published applications, and references cited herein are incorporated by reference in their entirety.

[0043] While exemplary embodiments have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the embodiments as encompassed by the appended claims.

Claims

1. A pressure controller comprising: a control valve configured to control the pressure of the fluid in the flow path; a flow restrictor disposed in the flow path; a remote pressure sensor that detects fluid pressure at the flow restrictor at a location remote from the control valve; a proximal pressure sensor that detects fluid pressure at the flow restrictor at a location proximate to the control valve; as well as A controller is configured to control actuation of the control valve based on the pressure detected by the distal pressure sensor and a pressure set point, and to determine a mass flow rate based on the pressures detected by the distal pressure sensor and the proximal pressure sensor.

2. The pressure controller according to claim 1, wherein: The proximal position and the distal position are located upstream of the control valve.

3. The pressure controller according to claim 1, wherein: The proximal position and the distal position are located downstream of the control valve.

4. The pressure controller according to claim 1, wherein: The controller is configured to perform closed-loop feedback control of the control valve based on the pressure detected by the remote pressure sensor and the pressure set point.

5. The pressure controller according to claim 1, wherein: The controller is configured to determine the mass flow rate Q according to the function provided by the following formula: Q=f(R,P u ,P d ,T,mw,μ,c) Where R is the characteristic of the flow restrictor, P u is the pressure upstream of the flow restrictor detected by one of the distal pressure sensor and the proximal pressure sensor, P d is the pressure downstream of the flow restrictor sensed by the other of the distal pressure sensor and the proximal pressure sensor, T is the temperature of the fluid, mw is the molecular weight of the fluid, µ is the viscosity of the fluid, and γ is the specific heat ratio of the fluid.

6. The pressure controller according to claim 1, wherein: The controller is further configured to output the determined mass flow rate. 7 . The pressure controller according to claim 1 , further comprising a temperature sensor that detects a temperature of the fluid in the flow path.

8. The pressure controller according to claim 1, wherein: The fluid is a reactive gas.

9. A method for controlling the pressure of a fluid, comprising: controlling actuation of a control valve that controls the pressure of a fluid in a flow path based on a pressure detected by a remote pressure sensor and a pressure set point, the remote pressure sensor detecting the pressure of the fluid at a flow restrictor at a location remote from the control valve, the flow restrictor being disposed in the flow path; as well as The mass flow rate is determined based on the pressures detected by the distal pressure sensor and a proximal pressure sensor, the proximal pressure sensor detecting fluid pressure at the flow restrictor at a location proximate to the control valve.

10. The method according to claim 9, wherein: The proximal position and the distal position are located upstream of the control valve.

11. The method according to claim 9, wherein The proximal position and the distal position are located downstream of the control valve.

12. The method according to claim 9, wherein Controlling actuation of the control valve includes closed-loop feedback control of the control valve based on the pressure detected by the remote pressure sensor and the pressure set point.

13. The method according to claim 9, wherein: The mass flow rate Q is determined according to the function provided below: Q=f(R,P u ,P d ,T,mw,μ,c) Where R is the characteristic of the flow restrictor, P u is the pressure upstream of the flow restrictor detected by one of the distal pressure sensor and the proximal pressure sensor, P d is the pressure downstream of the flow restrictor sensed by the other of the distal pressure sensor and the proximal pressure sensor, T is the temperature of the fluid, mw is the molecular weight of the fluid, µ is the viscosity of the fluid, and γ is the specific heat ratio of the fluid.

14. The method of claim 9, further comprising outputting the determined mass flow rate.

15. The method of claim 9, further comprising sensing a temperature of the fluid in the flow path.

16. The method according to claim 9, wherein The fluid is a reactive gas.

17. A pressure controller comprising: a control valve configured to control the pressure of the fluid in the flow path; a flow restrictor disposed in the flow path; an upstream pressure sensor that detects fluid pressure at the flow restrictor upstream of the flow restrictor; a downstream pressure sensor that detects fluid pressure at the flow restrictor downstream of the flow restrictor; as well as A controller configured to: controlling actuation of the control valve based on a pressure detected by one of the upstream pressure sensor and the downstream pressure sensor and a pressure set point, and A mass flow rate is determined based on the pressures detected by the upstream pressure sensor and the downstream pressure sensor.

18. The pressure controller according to claim 17, wherein: The flow restrictor, the upstream pressure sensor, and the downstream pressure sensor are disposed upstream of the control valve, one of the upstream pressure sensor and the downstream pressure sensor being the upstream pressure sensor.

19. The pressure controller according to claim 17, wherein: The flow restrictor, the upstream pressure sensor, and the downstream pressure sensor are disposed downstream of the control valve, and one of the upstream pressure sensor and the downstream pressure sensor is the downstream pressure sensor.

20. A method of controlling the pressure of a fluid, comprising: controlling actuation of a control valve based on a pressure set point and a pressure detected by one of an upstream pressure sensor that detects a fluid pressure at a flow restrictor upstream of the flow restrictor and a downstream pressure sensor that detects a fluid pressure at the flow restrictor downstream of the flow restrictor, the control valve controlling the pressure of a fluid in a flow path provided in the flow path; as well as A mass flow rate is determined based on the pressures detected by the upstream pressure sensor and the downstream pressure sensor.

21. The method according to claim 20, wherein The flow restrictor, the upstream pressure sensor, and the downstream pressure sensor are disposed upstream of the control valve, one of the upstream pressure sensor and the downstream pressure sensor being the upstream pressure sensor.

22. The method according to claim 21, wherein The flow restrictor, the upstream pressure sensor, and the downstream pressure sensor are disposed downstream of the control valve, and one of the upstream pressure sensor and the downstream pressure sensor is the downstream pressure sensor.

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