Apparatus and method for calibration of mass flow controller

• US 20030236643A1
• Filed: 06/24/2002
• Published: 12/25/2003
• Est. Priority Date: 06/24/2002
• Status: Active Grant

• Current Assignee: MKS Instruments Incorporated
• Original Assignee: MKS Instruments Incorporated
• Inventors: Shajii, Ali, Ambrosina, Jesse, Kottenstette, Nicholas
• Granted Patent: US 7,136,767 B2
• Time in Patent Office: Days
• US Class Current: 702/100
• CPC Class Codes: G01F 1/684 Structural arrangements; Mounting of elements, e.g. in relation to fluid flow
• CPC Class Codes: G01F 1/6965 comprising means to store calibration data for flow signal calculation or correction
• CPC Class Codes: G01F 1/86 Indirect mass flowmeters, e.g. measuring volume flow and density, temperature or pressure
• CPC Class Codes: G01F 25/15 specially adapted for gas meters G01F25/11 - G01F25/14, G01F25/17 take precedence
• CPC Class Codes: G01F 5/00 Measuring a proportion of the volume flow
• CPC Class Codes: G05D 7/0635 by action on throttling means G05D7/0688, G05D7/0694 take precedence

Abstract

A mass flow sensor calibrator employs a variable-flow fluid source, a receptacle of known volume, and a pressure differentiator. The variable-flow fluid source supplies gas at varying rates to the mass flow sensor being calibrated and at proportional rates to a receptacle of known volume. A pressure differentiator computes the time derivative of gas flow into the receptacle of known volume and, from that, the actual flow into the receptacle. Given the actual flow, the proportionate flow into the mass flow sensor may be determined and the flow signal from the mass flow sensor correlated to the actual flow.

Claims

1. A gas flow standard comprising:

a variable gas flow source;

a receptacle of predetermined volume configured to receive gas from the variable gas flow source; and

a pressure differentiator configured to produce an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume.

3. The standard of claim 1 wherein the differentiator includes:

a pressure transducer configured to produce an electronic signal representative of the pressure within the receptacle;

analog differentiator circuitry configured to produce an electronic signal that is representative of the time derivative of said electronic signal representative of the pressure within the receptacle; and

an analog to digital converter (ADC) configured to convert one or more values of the analog time derivative signal to digital samples of the time derivative.

4. The standard of claim 3 further comprising:

storage for storing one or more samples of the signal representative of the time derivative of gas pressure.

5. The standard of claim 1 wherein the differentiator includes:

a pressure transducer configured to produce an electronic signal representative of the pressure within the receptacle;

an analog to digital converter (ADC) configured to convert one or more values of the pressure transducer signal to digital form; and

a digital differentiator configured to produce a plurality of digital values representative of the time derivative of the pressure signal.

6. The standard of claim 5 further comprising:

storage for storing one or more samples of the signal representative of the time derivative of gas pressure.

7. The standard of claim 1 wherein the gas source is configured to provide an exponentially variable gas flow.

8. The standard of claim 7 wherein the gas source is configured to provide an exponentially increasing gas flow.

9. The standard of claim 7 wherein the gas source is configured to provide an exponentially decreasing gas flow.

2. A gas flow standard comprising:

a receptacle of predetermined volume configured to supply a variable gas flow; and

a pressure differentiator configured to produce an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume.

10. The standard of claim 2 wherein the gas source is configured to provide an exponentially variable gas flow.

11. The standard of claim 10 wherein the gas source is configured to provide an exponentially increasing gas flow.

12. The standard of claim 11 wherein the gas source is configured to provide an exponentially decreasing gas flow.

13. A method of producing a gas flow standard signal comprising the steps of:

(A) providing a variable gas flow to or from a source;

(B) flowing the variable gas flow into or out of a receptacle of predetermined volume at a variable rate; and

(C) producing an electronic signal that is representative of the derivative of the gas pressure within the receptacle of predetermined volume as the variable gas flows into or out of the receptacle.

14. The method of claim 13 wherein the step (C) comprises the steps of:

(C1) measuring the pressure within the receptacle of predetermined volume at least three times during the variable rate flow;

(C2) computing at least two pressure differences; and

(C3) dividing each pressure difference by the time between the two pressure measurements used to compute each respective pressure difference.

15. The method of claim 13 wherein the step (C) of producing an electronic signal that is representative of the derivative of the gas pressure within the receptacle of predetermined volume includes the steps of:

(C4) a pressure transducer producing an electronic signal representative of the pressure within the receptacle;

(C5) analog differentiator circuitry producing an electronic signal that is representative of the time derivative of said electronic signal representative of the pressure within the receptacle.

16. The method of claim 15 wherein the step (C) of producing an electronic signal that is representative of the derivative of the gas pressure within the receptacle of predetermined volume includes the step of:

(C6) an analog to digital converter converting one or more values of the analog pressure time derivative signal to digital samples of the pressure time derivative.

17. The method of claim 16 further comprising the step of:

(D) storing one or more samples of the signal representative of the time derivative of gas pressure.

18. The method of claim 13 wherein the step (C) of producing an electronic signal that is representative of the derivative of the gas pressure within the receptacle of predetermined volume as the variable gas flows into or out of the receptacle includes the steps of:

(C7) a pressure transducer producing an electronic signal representative of the pressure within the receptacle;

(C8) an analog to digital converter converting one or more values of the pressure transducer signal to digital form; and

(C9) a digital differentiator producing a plurality of digital values representative of the time derivative of the pressure signal.

19. The method of claim 18 further comprising the step of:

(E) storing one or more samples of the signal representative of the time derivative of gas pressure.

20. The method of claim 13 wherein the gas source provides an exponentially variable gas flow.

21. The method of claim 20 wherein the gas source provides an exponentially increasing gas flow.

22. The method of claim 20 wherein the gas source provides an exponentially decreasing gas flow.

23. A mass flow calibrator for calibrating a mass flow sensor that produces a signal representative of mass flow, comprising:

a variable gas flow source;

a receptacle of predetermined volume configured to receive gas from the variable gas flow source, the variable gas flow source configured to provide proportionate flow to a mass flow sensor being calibrated and to the receptacle;

a pressure differentiator configured to produce an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume;

a comparator configured to compare the electronic signal from the differentiator to the signal from the mass flow sensor; and

a compensator configured to compute a correction factor from the difference between the signal from the differentiator and the flow sensor if such a difference exists.

24. The calibrator of claim 23 wherein the gas flow source is configured to supply the same gas at the same flow rate to both the mass flow sensor and the receptacle of predetermined volume.

25. The calibrator of claim 23 wherein the gas flow source is configured to supply gas to the mass flow sensor and to the receptacle of predetermined volume in parallel.

26. The calibrator of claim 23 wherein the gas flow source is configured to supply gas to the mass flow sensor and to the receptacle of predetermined volume in series.

27. A gas flow calibrator for calibrating a mass flow sensor that produces a signal representative of mass flow, comprising:

a receptacle of predetermined volume configured to supply a variable gas flow;

a pressure differentiator configured to produce an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume, the receptacle of predetermined volume configured to provide proportionate flow through a mass flow sensor being calibrated and from the receptacle;

a pressure differentiator configured to produce an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume;

a comparator configured to compare the electronic signal from the differentiator to the signal from the mass flow sensor; and

a compensator configured to compute a correction factor from the difference between the signal from the differentiator and the flow sensor if such a difference exists.

28. The gas flow calibrator of claim 27 wherein the differentiator includes:

a pressure transducer configured to produce an electronic signal representative of the pressure within the receptacle;

analog differentiator circuitry configured to produce an electronic signal that is representative of the time derivative of said electronic signal representative of the pressure within the receptacle; and

an analog to digital converter (ADC) configured to convert one or more values of the analog time derivative signal to digital samples of the time derivative.

29. The gas flow calibrator of claim 28 further comprising:

storage for storing one or more samples of the signal representative of the time derivative of gas pressure.

30. The gas flow calibrator of claim 27 wherein the differentiator includes:

a pressure transducer configured to produce an electronic signal representative of the pressure within the receptacle;

an analog to digital converter configured to convert one or more values of the pressure transducer signal to digital form; and

a digital differentiator configured to produce a plurality of digital values representative of the time derivative of the pressure signal.

31. The gas flow calibrator of claim 30 further comprising:

storage for storing one or more samples of the signal representative of the time derivative of gas pressure.

32. The gas flow calibrator of claim 27 wherein the gas source is configured to provide an exponentially variable gas flow.

33. The gas flow calibrator of claim 32 wherein the gas source is configured to provide an exponentially increasing gas flow.

34. The gas flow calibrator of claim 32 wherein the gas source is configured to provide an exponentially decreasing gas flow.

35. The gas flow calibrator of claim 27 wherein the gas source is configured to provide an exponentially variable gas flow.

36. The gas flow calibrator of claim 35 wherein the gas source is configured to provide an exponentially increasing gas flow.

37. The gas flow calibrator of claim 35 wherein the gas source is configured to provide an exponentially decreasing gas flow.

38. A method of calibrating a mass flow sensor comprising the steps of:

(A) providing a variable gas flow to or from a source;

(B) flowing the variable gas flow into or out of a receptacle of predetermined volume at a variable rate;

(C) producing an electronic signal that is representative of the derivative of the gas pressure within the receptacle of predetermined volume as the variable gas flows into or out of the receptacle;

(D) flowing gas into or out of the mass flow sensor at a rate proportionate to that into or out of the receptacle;

(E) comparing the electronic signal from the differentiator to the signal from the mass flow sensor; and

(F) computing a correction factor from the difference between the signal from the differentiator and the flow sensor if such a difference exists.

39. The method of claim 38 wherein the step D of flowing gas into or out of the mass flow sensor at a rate proportional to that into or out of the receptacle comprises flowing gas at the same rate to both the mass flow sensor and the receptacle of predetermined volume.

40. The method of claim 38 wherein the gas flow source supplies gas to the mass flow sensor and to the receptacle of predetermined volume in parallel.

41. The method of claim 38 wherein the gas flow source supplies gas to the mass flow sensor and to the receptacle of predetermined volume in series.

43. The method of claim 41 wherein the step of the differentiator producing an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume includes the steps of:

a pressure transducer producing an electronic signal representative of the pressure within the receptacle;

analog differentiator circuitry producing an electronic signal that is representative of the time derivative of said electronic signal representative of the pressure within the receptacle; and

an analog to digital converter converting one or more values of the analog time derivative signal to digital samples of the time derivative.

44. The method of claim 43 further comprising the step of:

storing one or more samples of the signal representative of the time derivative of gas pressure.

45. The method of claim 41 wherein the step of the differentiator producing an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume includes the steps of:

a pressure transducer producing an electronic signal representative of the pressure within the receptacle;

an analog to digital converter converting one or more values of the pressure transducer signal to digital form; and

a digital differentiator producing a plurality of digital values representative of the time derivative of the pressure signal.

46. The method of claim 45 further comprising the step of:

storing one or more samples of the signal representative of the time derivative of gas pressure.

47. The method of claim 41 wherein the gas source provides an exponentially variable gas flow.

48. The method of claim 47 wherein the gas source provides an exponentially increasing gas flow.

49. The method of claim 47 wherein the gas source provides an exponentially decreasing gas flow.

53. The method of claim 38 wherein gas flow is restricted in order to differentiate the pressure over a greater period of time than would be available with unrestricted flow.

42. A method of calibrating a mass flow sensor comprising the steps of

a receptacle of predetermined volume supplying a variable gas flow;

a pressure differentiator producing an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume, the receptacle of predetermined volume providing proportionate flow through a mass flow sensor being calibrated and from the receptacle;

a pressure differentiator producing an electronic signal representative of the time derivative of gas pressure within the receptacle of predetermined volume;

a comparator comparing the electronic signal from the differentiator to the signal from the mass flow sensor; and

a compensator computing a correction factor from the difference between the signal from the differentiator and the flow sensor if such a difference exists.

50. The method of claim 42 wherein the gas source provides an exponentially variable gas flow.

54. The method of claim 42 wherein gas flow is restricted in order to differentiate the pressure over a greater period of time than would be available with unrestricted flow.

51. The gas flow calibrator of claim 51 wherein the gas source is configured to provide an exponentially increasing gas flow.

52. The gas flow calibrator of claim 52 wherein the gas source is configured to provide an exponentially decreasing gas flow.