High accuracy mass flow verifier with multiple inlets

• US 7,757,554 B2
• Filed: 03/18/2008
• Issued: 07/20/2010
• Est. Priority Date: 03/25/2005
• Status: Active Grant

• Current Assignee: MKS Instruments Incorporated
• Original Assignee: MKS Instruments Incorporated
• Inventors: Zarkar, Kaveh, Ding, Junhua
• Primary Examiner(s): THOMPSON, JEWEL VERGIE
• Application Number:
  US12/050,597
• Publication Number:
  US 20090112504A1
• Time in Patent Office: 854 Days
• Field of Search: 732/39, 738/65, 738/61, 732/042.2, 37/13.5, 137/2, 137/486, 702/45
• US Class Current: 73/239
• CPC Class Codes: G01F 22/02 involving measurement of pressure
• CPC Class Codes: G01F 25/15 specially adapted for gas meters G01F25/11 - G01F25/14, G01F25/17 take precedence
• CPC Class Codes: G01F 25/17 using calibrated reservoirs

Abstract

A high accuracy mass flow verifier (HAMFV) which provides high measurement accuracy over a wide flow verification range with low inlet pressures is disclosed for verifying flow measurement by a fluid delivery device. The HAMFV includes a chamber defining a plurality N of inlets with upstream valves, an outlet with a downstream valve, a pressure sensor and a temperature sensor configured to measure the pressure and the temperature of the fluid within the chamber, respectively. A plurality N of critical flow nozzles is located adjacent to the corresponding upstream valve. The HAMFV further includes a controller configured to activate one of the plurality N of critical flow nozzles based on the desired flow verification range and the fluid type by opening the corresponding upstream valve and closing all other upstream valves. At least two of the plurality N of critical flow nozzles have different cross-sectional areas.

Claims

1. A mass flow verifier for verifying flow measurement of a fluid by a fluid delivery device, the mass flow verifier comprising:

a chamber defining a plurality N of inlets, the chamber configured to receive at each of the inlets a flow of the fluid from the device;

a pressure sensor configured to measure pressure of the fluid within the chamber;

a plurality N of critical flow nozzles, each critical flow nozzle located adjacent to a corresponding one of the inlets, each critical flow nozzle located upstream of the chamber along a flow path of the fluid from the device to the chamber through the corresponding inlet;

wherein for all i (i=1, . . . , N), the i-th critical flow nozzle is configured to maintain, during a critical flow time period t_cf,i (i=1, . . . , N) for the i-th critical flow nozzle, the flow rate of the fluid through the i-th critical flow nozzle substantially constant and substantially insensitive to variation in pressure within the chamber.

2. The mass flow verifier of claim 1, further comprising a controller configured to selectively activate one of the plurality N of critical flow nozzles for a flow verification operation by the mass flow verifier.

3. The mass flow verifier of claim 2,

wherein the controller is further configured to selectively activate said one of the plurality N of critical flow nozzles based on a desired flow rate verification range for the mass flow verifier, and based on fluid type of the fluid.

4. The mass flow verifier of claim 3,

further comprising a plurality N of upstream valves, each upstream valve associated with a corresponding one of the critical flow nozzles;

wherein each one of the critical flow nozzles can be activated by opening the corresponding upstream valve and closing all other upstream valves.

5. The mass flow verifier of claim 4, wherein each critical flow nozzle is located before or after the corresponding upstream valve.

6. The mass flow verifier of claim 4, wherein each critical flow nozzle is located adjacent to the corresponding upstream valve.

7. The mass flow verifier of claim 4, further comprising a downstream valve configured to turn on or shut off flow of the fluid from an outlet of the chamber.

8. The mass flow verifier of claim 3,

wherein for all i (i=1, . . . , N), the critical flow time period t_cf,i (i=1, . . . , N) for the i-th critical flow nozzle is inversely proportional to the cross-sectional area A_i of said i-th critical flow nozzle;

and

wherein at least two of the plurality N of critical flow nozzles have different cross-sectional areas.

9. The mass flow verifier of claim 8, wherein when the desired flow rate verification range is relatively high, critical flow nozzles having a relatively large size are configured to provide a relatively low inlet pressure to the MFV.

10. The mass flow verifier of claim 8, wherein when the desired flow rate verification range is relatively low, critical flow nozzles having a relatively small size are configured to increase the measurement time, thereby increasing accuracy of the measurement.

11. The mass flow verifier of claim 1, further comprising a temperature sensor configured to measure temperature of the fluid within the chamber.

12. The mass flow verifier of claim 1, wherein the i-th critical flow nozzle (i=1, . . . , N) is configured to allow the fluid flowing therethrough to satisfy a critical flow condition during its critical flow time period t_cf,i, and wherein the critical flow condition for each one of the N critical flow nozzles is given mathematically by:

$\frac{P_d}{P_u}\le <br><br>a_{\mathrm{pc}}={\left(\frac{2}{\mathrm{\gamma <br><br>}+1}\right)}^{\frac{\mathrm{\gamma <br><br>}}{\mathrm{\gamma <br><br>}-1}}$

where

P_d is pressure of the fluid within the chamber and downstream of the i-th critical flow nozzle,

P_u is pressure of the fluid upstream of the i-th critical flow nozzle,

γ

is the ratio of specific heats of the fluid defined as γ

=C_p/C_v, where C_p is heat capacity of the fluid at constant pressure, and C_v is heat capacity of the fluid at constant volume, and

α

_pc is critical pressure ratio representing the maximum allowable ratio between P_d and P_u for which flow of the fluid across each one of the N critical nozzles will remain substantially constant and substantially insensitive to any variation in pressure within the chamber.

13. The mass flow verifier of claim 7,

wherein the controller is further configured to control the downstream valve, the upstream valves, the pressure sensor, and the temperature sensor;

and

wherein the controller is further configured to measure a rate of rise in pressure of the fluid within the chamber after the downstream valve is closed, using the measured rate of rise to calculate the flow rate of the fluid from the device into the chamber, thereby verifying measurement by the device.

14. The mass flow verifier of claim 13, wherein the critical flow time period t_cf,i for the i-th critical flow nozzle is defined between a point in time when the downstream valve is shut, to a point in time when a ratio between P_d and P_u exceeds the critical pressure ratio limit, α

_pc, as given by;

$t_{\mathrm{cf},i}=\frac{V_c·<br><br>\left(a_{\mathrm{pc}}-a_{p<br><br><br><br>0}\right)}{C_i·<br><br>A_i·<br><br>f_g<br><br>\left(M,\mathrm{\gamma <br><br>},T\right)}$

where

V_c is the volume of the chamber in the mass flow verifier,

a_p0 is the initial pressure ratio between the upstream and the downstream of the nozzle at the moment before the downstream valve is shut,

C_i is the discharge coefficient for the i-th (i=1, . . . , N) critical flow nozzle,

A_i (i=1, . . . , N) is the cross-sectional area of the nozzle throat for the i-th nozzle,

f_g(M, γ

, T) is a function of gas properties defined as

$f_g<br><br>\left(M,\mathrm{\gamma <br><br>},T\right)={\left(\frac{\mathrm{RT}}{M}<br><br>\frac{2<br><br>\mathrm{\gamma <br><br>}}{\mathrm{\gamma <br><br>}+1}\right)}^{1/2}<br><br>{\left(\frac{2}{\mathrm{\gamma <br><br>}+1}\right)}^{1/\left(\mathrm{\gamma <br><br>}-1\right)}$

where

M is the molecular weight of the fluid,

R is the universal gas constant,

T is the temperature of the fluid, and

γ

is the ratio of specific heats of the fluid.

15. The mass flow verifier of claim 14, wherein upon activation of the i-th critical flow nozzle by the controller, the pressure sensor and the temperature sensor are configured to make measurements within the critical time period tⁱ_cf for the i-th critical flow nozzle, so that flow verification by the mass flow verifier is substantially independent of the varying chamber pressure and the external volume between the mass flow verifier and the DUT.

16. The mass flow verifier of claim 14, wherein upon activation of the i-th critical flow nozzle (i=1, . . . , N), the controller is configured to verify the measurement of the device by:

a) opening the downstream valve and activating the i-th upstream valve based on the desired flow verification range and the fluid type received from the host controller or the user;

b) giving a flow set point to the DUT by the host controller or the user;

c) waiting until pressure within the chamber reaches a steady state and stabilizes;

d) start to measure the pressure and the temperature of the fluid within the chamber for flow verification calculation;

e) shut the downstream valve, so that the pressure within the chamber rises;

f) keep measuring the pressure and the temperature of the fluid within the chamber for a period less than the critical flow period t_cf,i for the i-the critical flow nozzle, for flow verification;

g) open the downstream valve within a critical time period as measured from when the downstream valve was shut; and

h) compute the flow rate of the fluid into the chamber using;

$Q_{\mathrm{in}}=\frac{k_0·<br><br>T_{\mathrm{stp}}·<br><br>V_c}{P_{\mathrm{stp}}}<br><br>\frac{d<br><br>}{d<br><br>t}<br><br>\left(\frac{P}{T}\right)$

where V_c is the chamber volume in the mass flow verifier,

T_stp is the standard gas temperature which is 273.15K,

P_stp is the standard gas pressure which is 1.01325e5 P_a,

k₀ is about 6×

10⁷ in SCCM units and 6×

10⁴ in SLM units

P is the pressure of the fluid measured by the pressure sensor

T is the temperature of the fluid measured by the temperature sensor.

17. The mass flow verifier of claim 1, wherein the device comprises one of:

a mass flow meter;

a mass flow controller; and

a flow ratio controller.

18. A method of verifying measurement of a flow of a fluid by a device, comprising:

placing a plurality N of critical flow nozzles along the flow path of the fluid between a chamber in a mass flow verifier and the device;

activating an i-th one of the N critical flow nozzles (i=1, . . . , N), so that during a critical flow time period t_cf,i (i=1, . . . , N) defined for said i-th nozzle, flow of the fluid across the i-th nozzle and pressure of the fluid upstream of the i-th nozzle remains substantially constant and substantially insensitive to the rise of the pressure within the chamber;

causing the fluid to flow along a flow path from the device though the i-th inlet into the chamber, while an inlet valve upstream of the chamber and an outlet valve downstream of the chamber are kept open;

allowing a flow rate of the fluid into the chamber and a pressure of the fluid within the chamber to reach a steady state;

closing the outlet valve downstream of the chamber so that pressure of the fluid begins to rise within the chamber; and

making measurements of fluid pressure and fluid temperature within the critical flow time period t_cf,i to measure a rate of rise of pressure of the fluid within the chamber and using the measured rate of rise of pressure to compute the flow rate of the fluid along with the measurements of fluid temperature.

19. The method of claim 18, wherein each one of the N critical flow nozzles is constructed and arranged so that for the i-th nozzle, flow of fluid through that nozzle is restricted during a critical flow time period t_cf,i (i=1, . . . , N) for that nozzle, so that a critical flow condition is satisfied during t_cf,i (i=1, . . . , N).

20. The method of claim 18, wherein the act of computing the flow rate of the fluid comprises using a mathematical formula given by:

$Q_{\mathrm{in}}=\frac{k_0·<br><br>T_{\mathrm{stp}}·<br><br>V_c}{P_{\mathrm{stp}}}<br><br>\frac{d<br><br>}{d<br><br>t}<br><br>\left(\frac{P}{T}\right)$

where P and T are measured by the pressure sensor and the temperature sensor during the verification period which is within the critical flow period such that the varying chamber pressure has not impact on the downstream pressure of the DUT.

21. A method of increasing accuracy and operational flow range of a mass flow verifier that verifies measurement by a flow measurement device of the flow rate of the fluid, the mass flow verifier including a chamber configured to receive flow of the fluid from the flow measurement device, a pressure sensor configured to measure pressure of the fluid within the chamber, and a temperature sensor configured to measure temperature of the fluid within the chamber, the method comprising:

providing a plurality N of critical flow nozzles between the flow measurement device and the chamber; and

selectively activating one of the N critical flow nozzles by opening a corresponding upstream valve and closing all other upstream valves.

22. The method of claim 21, further comprising selecting an optimal orifice size for the critical flow nozzle, based on the desired flow verification range, the fluid type and the maximum inlet pressure requirement for MFVs.