Method and apparatus for detecting and controlling mass flow

• US 5,975,126 A
• Filed: 12/01/1998
• Issued: 11/02/1999
• Est. Priority Date: 10/04/1996
• Status: Expired due to Term

• Current Assignee: Brooks Instrument LLC (Illinois Tool Works Inc.)
• Original Assignee: Emerson Electric Company
• Inventors: White, William W., Dille, Joseph C., Campbell, Gary P., Bump, Scott S.
• Primary Examiner(s): Ferensic, Denise L.
• Assistant Examiner(s): Kim, Joanne Y.
• Application Number:
  US09/203,151
• Time in Patent Office: 336 Days
• Field of Search: 137/487.5, 137/486, 73/861, 73/204.15, 73/204.16, 73/3
• US Class Current: 137/487.5
• CPC Class Codes: G01F 1/6842 with means for influencing the fluid flow
• CPC Class Codes: G01F 1/6847 where sensing or heating elements are not disturbing the fluid flow, e.g. elements mounted outside the flow duct
• 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 15/024 involving digital counting
• CPC Class Codes: G01F 25/13 using a reference counter
• 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
• 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
• CPC Class Codes: Y10T 137/7759 Responsive to change in rate of fluid flow
• CPC Class Codes: Y10T 137/7761 Electrically actuated valve

Abstract

A process mass flow instrument for measuring the mass flow rate of a process fluid including a fluid inlet, a fluid outlet, a flow passage connecting the fluid inlet and the fluid outlet, a fluid flow sensor which provides a raw output signal corresponding to an unknown mass flow rate and a processor which contains an empirically derived conversion routine for converting the raw output signal to an adjusted signal indicating the mass flow rate of the process fluid. A method for determining an unknown mass flow using an empirically derived conversion function, the function being derived by correlating the output signals of a mass flow instrument sensing the flow of a calibration fluid with the output signals of the mass flow instrument sensing the flow of the process fluid.

Claims

1. A process mass flow instrument for measuring the mass flow rate of a process fluid comprising:

a fluid inlet;

a fluid outlet;

a flow passage connecting the fluid inlet and the fluid outlet;

a fluid flow sensor which provides a raw output signal corresponding to an unknown mass flow rate; and

a processor;

the processor containing a conversion routine for converting the raw output signal to an adjusted signal indicating the mass flow rate of the process fluid, wherein the conversion routine is empirically derived using at least one calibration mass flow instrument which is similar to the process mass flow instrument to correlate the output signals of the calibration mass flow instrument sensing the mass flow of a calibration fluid which is different than the process fluid with the output signals of the calibration mass flow instrument sensing the mass flow of the process fluid.

2. The process mass flow instrument of claim 1 further comprising a flow control portion for controlling the mass flow of the process fluid, the flow control portion being responsive to the adjusted output signal.

3. The process mass flow instrument of claim 2 wherein the flow control portion comprises a control unit, a valve drive and a valve.

4. The process mass flow instrument of claim 1 wherein the processor includes a memory and the conversion routine comprises a calibration table stored in the memory, the calibration table containing a plurality of adjusted signal values which indicate the mass flow of the process fluid, the processor using the raw output signal from the flow sensor to index the calibration table to convert the raw output signal to the adjusted signal indicating the mass flow rate of the process fluid.

5. The process mass flow instrument of claim 4 wherein the plurality of adjusted signal values comprises 25 adjusted signal values.

6. The process mass flow instrument of claim 4 wherein the processor further comprises an interpolation routine which performs a linear interpolation to determine the adjusted signal indicating mass flow of the process fluid.

7. The process mass flow instrument of claim 4 wherein a plurality of calibration tables are stored in the memory.

8. The process mass flow instrument of claim 1 wherein the conversion routine comprises a mathematical function which receives the raw output signal from the flow sensor as an input to convert the raw output signal to the adjusted signal which indicates the mass flow of the process fluid.

9. The process mass flow instrument of claim 8 wherein the mathematical function comprises a fourth-order polynomial equation.

10. The process mass flow instrument of claim 8 wherein a plurality of mathematical functions are stored in the memory.

11. The process mass flow instrument of claim 1 wherein the raw output signal is expressed as digital counts which are proportional to the raw signal.

12. The process mass flow instrument of claim 1 wherein the fluid flow sensor comprises

a sensor tube through which a portion of the process fluid flow is directed;

a heater coupled to the midpoint of the sensor tube which transfers heat to the process fluid flowing through the sensor tube;

first and second temperature measuring elements mounted on the sensor tube on opposite sides of the heater for measuring the temperature of the process fluid flowing through the sensor tube, the first and second temperature sensing elements being equidistant from the heater; and

means for detecting the temperature difference between the first and second temperature sensing elements and outputting a signal representing the temperature difference, wherein the temperature difference between the first and second temperature sensing elements represents the mass fluid flow through the sensor tube.

13. A process mass flow instrument for measuring the mass flow rate of a process fluid comprising:

a fluid inlet;

a fluid outlet;

means for sensing and indicating an unknown mass flow rate of the process fluid through the process mass flow instrument;

means for converting the indication of the unknown mass flow rate to indicating the actual mass flow rate of the process fluid, the conversion means being empirically derived by comparing the mass flow indications for a calibration fluid, which is different than the process fluid, flowing through at least 1 calibration mass flow instrument, which is similar to the process mass flow instrument, with the mass flow indications for the process fluid flowing through the calibration mass flow instrument.

14. The process mass flow instrument of claim 13 further comprising means for controlling flow.