Measuring and testing – Volume or rate of flow – Using differential pressure
Reexamination Certificate
2001-05-18
2003-09-16
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Using differential pressure
C702S098000
Reexamination Certificate
active
06619141
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A “MICROFISCH APPENDIX”
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention generally relates to measuring and testing and specifically to instrument proving and calibrating. In greater detail, the invention is applied to set point adjustment, e.g., zero correction. In a further aspect, the invention is applied to measuring and testing; to instrument proving and calibrating; to volume of flow, speed of flow, volume rate of flow, or mass rate of flow; with signal processing, set point adjustment, e.g., zero correction. The invention is a method and apparatus for the dynamic calibration of a pressure transducer passing a signal whose output strength is proportional to applied pressure or to pressure differential.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The flow rate of a fluid can be detected and monitored by measuring a parameter that varies in a corresponding way to the flow. Wind speed, for example, commonly is monitored by a cup anemometer, which rotates in a wind stream in proportion to wind velocity. The mechanical movement of the cups can be translated into a signal indicative of wind speed by known systems that employ conventional electrical, magnetic, and mechanical means. Because a cup anemometer relies upon moving parts, its accuracy and reliability are degraded over time by mechanical wear and corrosion. Adverse climates can damage a mechanical moving system quite rapidly, such as by sand abrasion in desert areas or icing in high mountainous regions. In addition, a moving mechanical system tends to have limited accuracy, especially at low fluid flow rates where mechanical friction becomes an increasingly large source of error.
In order to overcome the problems inherent in a mechanical system, various types of motionless anemometers have been developed. Some of these operate on the principal of thermal diffusion, by monitoring a heated wire in a wind stream. Such a wire has a known temperature coefficient of resistance. The rate of heat loss, which may correspond to the electrical energy needed in order to sustain a specified resistance, provides a measurable parameter corresponding to wind speed. However, thermal anemometers also face environmental deterioration due to deposits of dirt and other atmospheric pollutants. Over time, they are known to become less accurate in detecting wind speed, particularly at low wind velocity. Another variety of anemometer employs one or more pressure transducers for determining differential pressure between a static pressure and a measured pressure, which result can be correlated with wind speed. The directly measured parameter in the pressure cell is capacitance. The sensitivity of a pressure transducer also tends to decrease over time, typically due to aging, accumulations of dirt and deposit of other atmospheric contaminants. There are still other known systems for measuring wind velocity, although substantially all employ electronic components in order to obtain a sensor reading and interpretation. Loss of accuracy over time remains a persistent problem, particularly at low velocity readings. Both deposits on the sensors and aging of all circuit components contribute to loss of sensitivity and read-out error.
Transducers are calibrated at the time of manufacture so that one or more correction values that will be applied to the output of the transducer. These correction values deal with factors such as ambient temperature, which can cause zero point drift. Overall, the manufacturer attempts to fingerprint the transducer's performance over its operating range and then compensate for any discovered inaccuracy. Fingerprinting is conducted under controlled temperature conditions. A modem transducer is associated with a processor that is programmed to apply corresponding correction factors to the output signal. Although fingerprinting can add accuracy to the transducer's output performance based upon the transducer's characteristics at the time of manufacture, fingerprinting cannot correct for aging and for perhaps other factors. Thus, although it is known to correct the zero level by compensating for output levels as a function of temperature, in practice the data supporting the correction is created in a static, one-time event, such as at the time of manufacture. It would be possible to recalibrate a transducer during its life, such as by returning it to the manufacturer for adjustment. However, this is impractical because the transducer has been installed in an operating device deployed to the field. Further, it may not be evident that a transducer in field use is in need of recalibration. Various methods and circuitry used in field equipment attempt to preserve the accuracy of a transducer by taking into account common sources of inaccuracy. For example, differential pressure signals have been corrected in the field to compensate for inaccuracy in reading ambient temperature and static pressure.
In real time usage, a digital computer may receive the signal from one or more pressure sensors and process the signals to produce an improved resulting signal. U.S. Pat. No. 4,598,381 to Cucci uses a digital computer, serving as a correction circuit, to compare a reference pressure signal to a differential sensor signal. The computer adjusts the reference signal and provides an improved output signal as a function of the differential sensor signal and the adjusted reference signal.
Another correction method is shown in U.S. Pat. No. 5,623,101 to Freitag, which corrects for inaccuracy caused by “disturbance variables,” namely temperature and static pressure. Those variables influence absolute pressure and thereby reduce the accuracy of a corrected differential pressure signal. Freitag recursively calculates a corrected differential pressure signal from a measured differential pressure signal, a measured absolute-pressure signal and a temperature signal in combination with a plurality of lower degree correction polynomials. The corrected differential pressure signal may be further processed via a linearization polynomial to produce a linearization correction signal which, when combined with the corrected differential pressure signal, produces a linearization differential pressure signal.
Another such method is shown in U.S. Pat. No. 5,383,345 to Berard et al., which addresses inaccuracy caused by differences in temperature across a cell used to sense pressure. According to the method, a microprocessor calculates a dynamic temperature factor signal by multiplying a signal representative of temperature change across the cell by a coefficient based on measurements made of the effect of temperature change on the cell. The dynamic temperature factor signal is then subtracted from the signal representative of the differential pressure sensed by the cell to thereby provide the dynamic offset compensation.
U.S. Pat. No. 5,329,818 to Frick corrects the output signal of a differential pressure sensor for errors in static pressure due to changes in temperature.
It would be desirable to create an apparatus and method able to directly correct for aging and other environmental deterioration of a transducer. In particular, it would be desirable to provide a compensation algorithm capable of setting the zero level at a predetermined or ideal signal level. Further, it would be desirable to determine a contemporaneous correction factor as a function of temperature and then apply such factor to the output signal during active measurements of applied pressure.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the method and apparatus of this invention may comprise the following.
BRIEF SUMMARY OF THE INVENTION
Against the described background, it is therefore a general object of the invention to correct the output of a different
Dickens Charlene
Lefkowitz Edward
Rost Kyle W.
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