Flow through pressure sensor

Abstract

To remove the dead volume in the sensor assembly of a mass flow controller.SOLUTION: A sensor assembly 16 of a mass flow controller includes: a first reservoir 22 coupled to a first pressure transducer 18 and communicating with a sampling flow path 64; a second reservoir 24 coupled to a second pressure transducer 20, communicating with the first reservoir 22 via a flow-through path 26, and further communicating with another sampling flow path 66; and a restrictor 68 disposed in the flow-through path 26 so as to face the first and second pressure transducers 18 and 20.SELECTED DRAWING: Figure 2A

Description

[Background technology]

[0001] In many manufacturing operations where chemicals are used to produce electronic devices, mass flow controllers (MFCs) are used to measure and control the flow rate of fluids delivered to processing chambers. In semiconductor device manufacturing, there are over 50 gases used in etching and chemical vapor deposition (CVD) processes, and an additional 150 gases are known in the industry. On a production line, a semiconductor fabrication chamber may contain 9 to 16 MFCs, with each production line containing 1 to 6 chambers. The use of MFCs is widespread, and the gases they control range from inert to corrosive, pyrophoric, and / or highly toxic. Acceptable exposure limits are less than 1 ppm (parts per million). Therefore, the ability to completely remove residual gases from a failed MFC is a critical safety requirement when replacing a failed MFC with a new one.

[0002] Beyond the safety issues of replacing a failing MFC, there is a strong need to "dry down" the internal flow passages of a new MFC to remove atmospheric moisture adhering to the passage walls. All devices, including MFCs, exposed to air with any relative humidity will attract a level of H2O molecules to their surface walls. Given time, a stable temperature, and humidity, the amount of H2O molecules on the walls will reach equilibrium, with a balance between molecules adhering to the surface and molecules detaching from the surface. However, when initially introduced into a semiconductor processing tool, these atmospheric H2O molecules will still be on the internal surfaces and will begin to fall off the walls and bind to the dry process gas stream, resulting in corrosion or process chemical problems downstream of the MFC.

[0003] It is therefore common in the marketplace to quantify, as part of its performance documentation, the rate at which a new MFC will "dry down" when a dry, inert "purge" gas, usually nitrogen or argon, is circulated through the MFC to remove atmospheric molecules on its walls. A typical "dry down" test establishes a known level of moisture on the MFC's walls, and then records the changing moisture level at the device's exhaust over time as a specified flow rate of dry purge gas flows through the MFC until the detected moisture level reaches ppb (parts per billion) levels.

[0004] To remove moisture from the walls of new MFCs or to ensure removal of "nasty gases" from replacement MFCs, the industry has adopted the practice of "flow" and "cycle" purging MFCs and other devices with dry purge gas. Typically, a stream of purge gas is directed at the MFC, flows through the MFC, and then is diverted to a non-process location. After a period of flow purging, valve positions are changed and the device is repeatedly and alternately evacuated to a moderate vacuum level and then repressurized again with purge gas for several hundred cycles, ultimately achieving sub-ppm moisture levels or nasty gas concentrations.

[0005] Beyond the current "safety" and process "dry-down" benefits discussed above, additional benefits have emerged that rely on efficiently and quickly replacing one gas with another. These additional benefits stem from the continuing trend of reducing hardware costs and space by flowing multiple gas species through a single MFC. As an example, in a gas box containing 16 gas lines delivering gas to an etch process tool, each with its own dedicated MFC, two to four gases can flow simultaneously. Many of these 16 gases are from the same gas family and can be directed to and flow through the same MFC, provided that the old gas can be easily replaced by the new gas.

[0006] Flowing oxygen, nitrogen, or argon through the same MFC is one example. Similarly, multiple Freon gases can be grouped together. This approach offers significant opportunity to reduce hardware and associated costs by reducing the number of gas lines from 16 to perhaps 8. However, an efficient method of changing gas types is required, such as a flow purge or cycle purge, in which new gas is circulated through the MFC to replace the old gas. Without effective purging, a stagnant volume, called a dead head, can form in the small passages of the MFC. While purging is an effective practice when used with thermal MFCs, the practice is ineffective when used with pressure MFCs. Due to the natural flow created by the design of thermal MFCs, purging effectively removes dead gas volumes. However, pressure MFCs do not share the same or similar flows by design, and therefore purging is not an effective practice for removing dead gas volumes.

[0007] For a more complete understanding of the features and advantages of the present disclosure, reference is now made to the detailed description taken in conjunction with the accompanying drawings, in which corresponding numerals in the different figures refer to corresponding parts, and in which: [Brief explanation of the drawings]

[0008] [Figure 1A] FIG. 1 is an illustration of one of various MFC configurations having a sensor assembly designed to include a reservoir passage having a flow-through path and a select volume, according to certain example embodiments. [Figure 1B] FIG. 1 is an illustration of one of various MFC configurations having a sensor assembly designed to include a reservoir passage having a flow-through path and a select volume, according to certain example embodiments. [Figure 2A] 1A-1C are illustrations of a sensor assembly with flow-through paths and various MFC configurations, according to certain example embodiments. [Figure 2B] 1A-1C are illustrations of a sensor assembly with flow-through paths and various MFC configurations, according to certain example embodiments. [Figure 2C] 1A-1C are illustrations of a sensor assembly with flow-through paths and various MFC configurations, according to certain example embodiments. [Figure 2D] 1A-1C are illustrations of a sensor assembly with flow-through paths and various MFC configurations, according to certain example embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0009] While the making and use of various embodiments of the present disclosure are discussed in detail below, it should be understood that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely exemplary and do not delimit the scope of the present disclosure. In the interest of clarity, this disclosure may not describe all features of an actual implementation. Of course, in developing any such actual embodiment, many implementation-specific decisions must be made to achieve the developer's specific goals, including compliance with system-related and business-related constraints that vary from implementation to implementation. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

[0010] Pressure-based MFCs typically have a closed-loop design in which a pressure sensor assembly includes a port and a small reservoir so that fluid pressure can be sampled and measured from the primary flow path to determine fluid flow rate. In practice, the sensor assembly can include one or more absolute pressure transducers and one or more differential pressure transducers. Each transducer includes a port and a reservoir for sampling fluid pressure from the primary flow path. The primary flow path can include a distinctive flow restrictor, allowing fluid pressure to be sampled upstream and downstream from the flow restrictor. Because the reservoir for the transducer does not include a flow-through path, i.e., the reservoir is closed with respect to the primary flow path, dead heads—retained volumes where gas flow does not easily replace old gas—can form within the reservoir. Again, this configuration results in dead heads, which are volumes that are not efficiently cleared by gas flow. Dead volume, i.e., stagnant fluid, is not displaced from the reservoir as gas flows through the restrictor in the MFC base. Therefore, when new gas is circulated to replace the old gas, the reservoir is not purged. The effect of this dead volume on MFC operation is the introduction of undesirable mixing of gases, which can affect downstream processes.

[0011] Presented herein are systems, methods, and apparatus that direct gas flow to sweep through the volume of a sensor's reservoir, thus eliminating the stagnant nature of past designs and eliminating dead volume within the transducer assembly of an MFC. The transducer assembly includes a first reservoir coupled to a first pressure transducer, a second reservoir coupled to another pressure transducer, and a flow restrictor disposed within a flow-through passageway connecting the first reservoir to the second reservoir. The flow-through passageway provides a path for fluid flow between and through the two reservoirs. The effect of this passageway is for new gas to sweep / replace previous gas from the reservoir, thus preventing the slow bleeding of old gas into the main stream, which can result in an unknown mixture being delivered to the process for a significant period of time after initiating a gas change to an MFC, such as switching from one gas type to another in a semiconductor manufacturing operation.

[0012] 1A and 1B, various MFC configurations are shown having a sensor assembly designed to include a reservoir passage with a flow-through path and a select volume, according to certain example embodiments. The overall volume of the reservoir and the location of the flow-through path within the sensor assembly depend on the type of MFC application. For low flow applications, a reduced reservoir volume may be required. For high flow applications, an increased reservoir volume may be required. However, the reservoir volume may also be based on bleed-down time. Additionally, each reservoir within the sensor assembly may have a unique volume size.

[0013] 1A, two MFC configurations, generally designated 10A and 10B, are shown, each including a sensor assembly having at least one flow-through passage and a reservoir passage having a defined volume. MFC configuration 10A includes a valve assembly 12A, a base having a primary flow path 14A with a primary flow path obstruction 28A disposed therein, and a sensor assembly 16A. Sensor assembly 16A includes two absolute pressure transducers 18A and 20A, reservoirs 22A and 24A, at least one flow-through passage 26A, and a restrictor feature (not shown). MFC configuration 10B includes a valve assembly 12B, a base having a primary flow path 14B and a restrictor feature 28B disposed within the primary flow path 14B, and a sensor assembly 16B. The sensor assembly 16B includes two absolute pressure transducers 18B, 20B, reservoirs 22B, 24B, at least one flow-through passage 26B, and a restrictor feature (not shown).

[0014] MFC configuration 10A is configured to process fluids at low flow rates, e.g., 0.15 sccm (standard cubic centimeters per minute) to 500 sccm. The specific sccm depends on the gas velocity and the specifics of the target pressure drop across the restrictor and can vary depending on the specific design and application. MFC configuration 10B is configured to process fluids at high flow rates, e.g., greater than 500 sccm. Restrictors (not shown) and 28B can be characterized based on the required flow rate. In sensor assemblies 16A and 16B, the volumes of reservoirs 22A, 24A; 22B, 24B can be configured to have specified volumes based on the application. For example, for the lowest full-scale flow rate, the volume for 22A can be reduced to also reduce bleed-down time, i.e., the time it takes for the pressure in reservoir 22A to decay as it exits through restrictor 26, in response to the closure of an upstream valve, e.g., when switching gases. The bleed-down time constant is proportional to the reservoir volume divided by the full-scale flow rate of the MFC. Therefore, a larger full-scale MFC can have a larger reservoir volume required for higher flow rates without being adversely affected by a longer bleed-down time. A well-designed bleed-down time can also reduce or eliminate dead volume in the sensor assemblies 16A, 16B.

[0015] 1B, two additional MFC configurations, generally designated 30A and 30B, are shown having sensor assemblies with at least one flow-through passage and reservoir passage with volumes for certain MFC applications. MFC configuration 30A can include a valve assembly 32A, a base with a primary flow path 34A, and a sensor assembly 36A. Sensor assembly 36A can include two absolute pressure transducers or one absolute pressure transducer and one differential pressure transducer 38A, 40A, reservoirs 42A, 44A, at least one flow-through passage 46A, and a restrictor (not shown). MFC configuration 30A includes sensor assembly 36A with reservoirs 42A, 44A with volumes better suited to high-flow applications because the majority of the flow passes through larger passage 42A-1 and a smaller portion of the flow passes through 42A-2, sweeping across the sensor face. The MFC configuration 30B can include a valve assembly 32B, a base having a primary flow path 34B, and a sensor assembly 36B. The sensor assembly 36B can include two absolute pressure transducers or one absolute pressure transducer and one differential pressure transducer 38B, 40B, reservoirs 42B, 44B, at least one flow-through passage 46B, and a restrictor (not shown). The MFC configuration 30B includes a sensor assembly 36B with reservoirs 42B, 44B having reduced volume sizes and therefore better suited for low-flow applications due to bleed-down issues. As shown, the inner diameters, and therefore the overall volumes, of reservoirs 42A, 44A are larger than the volumes of reservoirs 42B, 44B. In MFC configuration 30A, the volumes of reservoirs 42A, 44A are fluidly coupled and connected in series with primary flow path 34A, and are considered internal flow paths within the structure of sensor assembly 36A; flow from 34B is split into two parallel paths, with the majority of the flow continuing straight through 42A-1, while a smaller flow is redirected to sweep across the face of transducer 38A via passage 42A-2.Flows 42A-1, 42A-2 combine at the passage inlet of passage 46A, pass through a restrictor feature, and exit into passage 44A, where the flow splits again into parallel paths 44A-1, 44A-2, which is redirected to sweep the sensor face, recombine with 44A-1, and exit into the main flow path within MFC base 34B. In MFC configuration 30B, low flow rates do not require splitting of flow paths 42B, 44B; the entire flow sweeps across sensor faces 38B, 40B. In this particular embodiment, the flow rate is low enough to route the entire flow through the sensor face without introducing velocity-related problems.

[0016] It should also be understood that any of the primary flow paths 14 can have a flow restrictor feature disposed therein. It should also be understood that the sensor assemblies 16, 36 can include a combination of absolute and differential pressure transducers. It should also be understood that the pressure transducers 18, 20, 38, 44 described herein can include a semiconductor-based transducer, an oil bath, and an isolation diaphragm, where fluid passing through the diaphragm generates a force on the diaphragm that is transmitted through the oil bath to the transducer for measurement.

[0017] 2A-2D, block diagrams of sensor assemblies 16, 36 with flow-through channels and various MFC configurations are shown. In FIG. 2A, sensor assembly 16 includes pressure transducers 18, 20. In this particular embodiment, transducers 18, 20 are absolute pressure transducers. Reservoirs 22, 24 include ports 60, 62 for receiving fluid through sampling channels 64, 66 to be sampled from primary flow path 14. In this particular embodiment, flow-through channel 26 is disposed between reservoirs 22, 24, and a distinctive restrictor 68 is disposed within flow-through channel 26. In FIG. 2B, sensor assembly 36 includes pressure transducers 38, 40, flow-through channel 46, and ports 60A, 60B; 62A, 62B. The pressure transducers 38, 40 may be an absolute pressure transducer 38 and a differential pressure transducer 40, or two absolute pressure transducers, with the pressure-sensing face of 40 facing 44. A flow-through passage 46 fluidly couples a port 60B on one end of the reservoir 42 to another port 62B on the other end of the reservoir 44. A restrictor feature 68 is disposed within the flow-through passage 46. In both embodiments, a restrictor feature 70 is disposed within the primary flow path 14, 34 and carries the bulk of the flow. However, in FIGS. 2C and 2D, the primary flow path 14, 34 does not include the restrictor 70. In certain applications, both restrictors 68, 70 are not required. In MFC applications where low flow rates do not pose gas velocity issues, the restrictor 70 is not required. Therefore, the MFC design is less expensive. In this particular embodiment, the upstream portion of the primary flow path 14, 34 continues through the sampling flow path 64, the flow-through passages 26, 46, the sampling flow path 66, and back to the downstream portion of the primary flow path 14, 34. The restrictor 68 can be characterized in this case as needed for the particular application. Sampling flow path 64, 66, as used herein, refers to a flow path carrying the fluid to be measured, which is in communication with the pressure transducer 38, 40 and passes through the transducer ports 60, 62.While the sensor assemblies 16, 36 have been described with certain transducer configurations, it should be understood that other configurations are possible. While the primary flow paths 14, 34 have been shown and described as flowing from upstream to downstream, it should be understood that fluid flow along the primary flow paths 14, 34 can be from downstream to upstream. This clearly means that fluid passes through the reservoirs 22, 24 and flow-through paths 26, 46 in the opposite direction.

[0018] The above disclosed embodiments are presented for purposes of illustration and to enable those skilled in the art to practice the disclosure, but the disclosure is not intended to be exhaustive or limited to the disclosed forms. Many insubstantial modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the disclosure. The claims are intended to broadly cover the disclosed embodiments and any such modifications. Furthermore, the following clauses represent further embodiments of the present disclosure and should be considered to be within the scope of the present disclosure:

[0019] Clause 1. An apparatus for eliminating dead volume in a sensor assembly of a mass flow controller, comprising: a first reservoir coupled to a first pressure transducer and having a port communicating with a predetermined location in a sampling flow path; a second reservoir coupled to a second pressure transducer and fluidly coupled to the first reservoir via a flow-through path and having another port communicating with another location in another sampling flow path; and a restrictor disposed in the flow-through path.

[0020] Clause 2. The apparatus of clause 1, wherein the first pressure transducer is an absolute pressure transducer and the second pressure transducer is one of a transducer selected from the group including other absolute pressure transducers and differential pressure transducers.

[0021] Clause 3. The apparatus of clause 1, wherein the second pressure transducer is a differential pressure transducer, the second pressure transducer being one of a transducer selected from the group including absolute pressure transducers and other differential pressure transducers.

[0022] Clause 4. The apparatus of clause 1, wherein the flow-through passage is disposed between the first reservoir and the second reservoir.

[0023] Clause 5. The apparatus of clause 1, wherein the flow-through passage fluidly couples an end of the first reservoir to the other end of the second reservoir.

[0024] Clause 6. The apparatus of clause 1, wherein the volume of the first reservoir is determined based on a bleed down time, which is determined by dividing the volume in each reservoir by the full-scale flow rate of the MFC.

[0025] Clause 7. The apparatus of clause 1, wherein the volume of the second reservoir is determined based on a bleed down time, which is determined by dividing the volume in each reservoir by the full scale flow rate of the MFC.

[0026] Clause 8. A system for eliminating dead volume in a sensor assembly of a mass flow controller, comprising: a valve assembly in communication with a fluid in a primary flow path; and a pressure sensor assembly in communication with the fluid in the primary flow path, the pressure sensor assembly comprising: a first reservoir coupled to a first pressure transducer and having a port in communication with the first reservoir at a predetermined location in the sampling flow path; and a second reservoir coupled to a second pressure transducer, fluidly coupled to the first reservoir via a flow-through path, and having another port in communication with another location in the sampling flow path; the first pressure transducer and the second pressure transducer being communicatively coupled to the valve assembly.

[0027] Clause 9. The system of clause 8, wherein the first pressure transducer is an absolute pressure transducer and the second pressure transducer is one of a transducer selected from the group including other absolute pressure transducers and differential pressure transducers.

[0028] Clause 10. The system of clause 8, wherein the second pressure transducer is a differential pressure transducer, the second pressure transducer being one of a transducer selected from the group including absolute pressure transducers and other differential pressure transducers.

[0029] Clause 11. The system of clause 8, wherein the flow-through path is disposed between the first reservoir and the second reservoir.

[0030] Clause 12. The system of clause 11, wherein the flow-through passage fluidly couples an end of the first reservoir to the other end of the second reservoir.

[0031] Clause 13. The system of clause 8, wherein the volume of the first reservoir is determined based on a bleed down time, which is determined by dividing the volume in each reservoir by the full scale flow rate of the MFC.

[0032] Clause 14. The system of clause 13, wherein the volume of the second reservoir is determined based on a bleed down time, which is determined by dividing the volume in each reservoir by the full scale flow rate of the MFC.

[0033] Clause 15. A method of eliminating dead volume in a sensor assembly of a mass flow controller, the method comprising: pumping fluid from a primary flow path through a port into a first reservoir coupled to a first pressure transducer; and pumping fluid from the first reservoir through a flow-through path and another port into a second reservoir coupled to a second pressure transducer.

[0034] Clause 16. The method of clause 15, wherein the first pressure transducer is an absolute pressure transducer and the second pressure transducer is one of a transducer selected from the group comprising another absolute pressure transducer and a differential pressure transducer.

[0035] Clause 17. The method of clause 15, wherein the second pressure transducer is a differential pressure transducer, wherein the second pressure transducer is one of a transducer selected from the group comprising absolute pressure transducers and other differential pressure transducers.

[0036] Clause 18. The method of clause 15, further comprising pumping fluid from a first reservoir through a flow-through passageway, the flow-through passageway being disposed between the first reservoir and the second reservoir.

[0037] Clause 19. The method of clause 18, further comprising pumping fluid from the first reservoir through a flow-through passageway, the flow-through passageway fluidly coupling an end of the first reservoir to another end of the second reservoir.

[0038] Clause 20. The method of clause 15, wherein the volume of the first reservoir is determined based on a bleed down time, which is determined by dividing the volume in each reservoir by the full-scale flow rate of the MFC. [Explanation of symbols]

[0039] 10A MFC configuration 10B MFC configuration 12A valve assembly 12B Valve assembly 14 Primary flow path 16 Sensor assembly 18 Pressure Transducer 20 Pressure Transducer 22 Reservoir 24 reservoir 26 Penetration Route 26A Penetration Path 26B Penetration Route 28A Primary flow path blockage part 28B Limiter 30A MFC configuration 30B MFC configuration 32A Valve assembly 32B Valve assembly 34th flow path 34A Primary flow path 34B MFC base 36 Sensor assembly 38 Absolute Pressure Transducer 40 Differential Pressure Transducer 42 Reservoir 44 Pressure Transducer 46 Penetration Route 60 Transducer Port 62 Transducer Port 64 Sampling channel 66 Sampling channel 68 Restrictor 70 Limiter

Claims

1. 1. An apparatus for eliminating dead volume in a sensor assembly of a mass flow controller, comprising: a first reservoir coupled to the first pressure transducer and having a port in communication with a predetermined location within the sampling flow path; a second reservoir coupled to a second pressure transducer, fluidly coupled to the first reservoir via a flow-through passage, and having another port communicating with another location in another sampling flow path; a restrictor disposed in said flow-through passageway facing said first and second pressure transducers.

2. 10. The apparatus of claim 1, wherein the first pressure transducer is an absolute pressure transducer and the second pressure transducer is one of a transducer selected from the group consisting of another absolute pressure transducer and a differential pressure transducer.

3. 10. The apparatus of claim 1, wherein the second pressure transducer is a differential pressure transducer, the second pressure transducer being one of a transducer selected from the group including absolute pressure transducers and other differential pressure transducers.

4. 2. The device of claim 1, wherein the flow-through passage is disposed between the first reservoir and the second reservoir.

5. 10. The device of claim 1, wherein the flow-through passage fluidly couples one end of the first reservoir to the other end of the second reservoir.

6. 10. The apparatus of claim 1, wherein the volume of the first reservoir is determined based on a bleed down time, which is determined by dividing the volume in the respective reservoir by a full-scale flow rate of the MFC.

7. 10. The apparatus of claim 1, wherein the volume of the second reservoir is determined based on a bleed down time, which is determined by dividing the volume in the respective reservoir by the full-scale flow rate of the MFC.

8. 1. A system for eliminating dead volume in a sensor assembly of a mass flow controller, comprising: a valve assembly in fluid communication with the primary flow path; a pressure sensor assembly in communication with fluid in the primary flow path; The pressure sensor assembly includes: a first reservoir coupled to the first pressure transducer and having a port in communication with the sampling flow path at a predetermined location; a second reservoir coupled to a second pressure transducer, fluidly coupled to the first reservoir via a flow-through passage, and having another port communicating with another location in another sampling flow path; a restrictor disposed in the flow-through path facing the first and second pressure transducers; The system wherein the first pressure transducer and the second pressure transducer are communicatively coupled to the valve assembly.

9. 9. The system of claim 8, wherein the first pressure transducer is an absolute pressure transducer and the second pressure transducer is one of a transducer selected from the group consisting of another absolute pressure transducer and a differential pressure transducer.

10. 9. The system of claim 8, wherein the second pressure transducer is a differential pressure transducer, the second pressure transducer being one of a transducer selected from the group consisting of absolute pressure transducers and other differential pressure transducers.

11. The system of claim 8 , wherein the flow-through path is disposed between the first reservoir and the second reservoir.

12. The system of claim 8 , wherein the flow-through passage fluidly couples an end of the first reservoir to the other end of the second reservoir.

13. 9. The system of claim 8, wherein the volume of the first reservoir is determined based on a bleed down time determined by dividing the volume in the respective reservoir by a full scale flow rate of the MFC.

14. 9. The system of claim 8, wherein the volume of the second reservoir is determined based on a bleed down time determined by dividing the volume in the respective reservoir by a full scale flow rate of the MFC.

15. 1. A method for eliminating dead volume in a sensor assembly of a mass flow controller, comprising: pumping fluid from the primary flow path through a port and into a first reservoir coupled to a first pressure transducer; and pumping fluid from the first reservoir through a flow-through passage and another port into a second reservoir coupled to a second pressure transducer.

16. 16. The method of claim 15, wherein the first pressure transducer is an absolute pressure transducer and the second pressure transducer is one of a transducer selected from the group consisting of another absolute pressure transducer and a differential pressure transducer.

17. 16. The method of claim 15, wherein the second pressure transducer is a differential pressure transducer, the second pressure transducer being one of a transducer selected from the group consisting of absolute pressure transducers and other differential pressure transducers.

18. 16. The method of claim 15, further comprising pumping fluid from the first reservoir through the flow-through passage, the flow-through passage being disposed between the first reservoir and the second reservoir.

19. 16. The method of claim 15, further comprising pumping fluid from the first reservoir through the flow-through passage, the flow-through passage fluidly coupling one end of the first reservoir to another end of the second reservoir.

20. 16. The method of claim 15, wherein the volume of the first reservoir is determined based on a bleed down time, which is determined by dividing the volume in the respective reservoir by a full-scale flow rate of the MFC.

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