Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1997-12-31
2001-05-22
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06234016
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the measurement of fluid properties and, more particularly, to the determination of the velocity of a fluid of interest.
2. Description of the Prior Art
A number of techniques have been devised to measure the velocity of a fluid within a lumen. One approach is the “time of flight” approach, which generally includes determining the time require for a thermal wave to flow from a source heater element to a destination sensor element. By knowing the distance between the heater and sensor, the velocity of the fluid can thus be calculated.
U.S. Pat. No. 4,576,050 to Lambert discloses one such “time of flight approach. Lambert suggests energizing a heater strip with an oscillating heater input signal to emit thermal waves in the fluid. The thermal waves propagate through the fluid at a rate that is dependent on the fluid velocity that flows perpendicular to the heater strip. A thermo-electric detector, spaced from one or both side of the heater, senses the thermal wave and provides a corresponding detector output signal. The velocity of the fluid can be determined, at least to first order, from the time differential between the heater input signal and the detector output signal.
A limitation of the Lambert approach is that the measured time or phase differential between the heater input signal and the detector output signal depends on a number of physical properties of the fluid, including, for example, temperature, pressure, thermal conductivity, and thermal diffusivity. To compensate for some of these parameters, Lambert suggests making a reference measurement with a second sensor that is exposed to still fluid or fluid having a velocity component that is perpendicular to the source of the second sensor. By comparing the output of the second sensor with that of the first sensor, Lambert suggests that a phase difference can be computed that is independent of at least some of the fluid properties listed above. As can be seen, the second sensor of Lambert is not used to measure the fluid velocity, but rather is used to compensate for selected physical properties of the fluid.
Another limitation of the Lambert approach is that several potential error sources in the phase differential measurement are neglected, thereby reducing the accuracy of the measurement. One such error source is the non-zero heater time lag that typically exists between the heater input signal and the elevated temperature response of the heater element (and thus the fluid). The heater time lag is typically dominated by the thermal conductivity, k, of the fluid of interest, at least for microbridge structures as contemplated by a preferred embodiment of the present invention.
Another error source is the non-zero sensor time lag that typically exists between the arrival of the temperature disturbance at the sensor element and the corresponding response of the sensor element. The temperature of the sensor element typically does not react instantaneously to a temperature change in the fluid, primarily due to the sensors non-zero thermal mass.
It would be desirable, therefore, to provide an method and apparatus whereby the fluid velocity can be determined relatively independently of the physical properties of the fluid. It would also be desirable to provide a flow sensor that accounts for the non-zero heater time lag and the non-zero sensor time lag for increased accuracy.
SUMMARY OF THE INVENTION
The present invention overcomes many of the disadvantages associated with the prior art by providing a method and apparatus whereby the fluid velocity can be determined relatively independently of the physical properties of the fluid, and minimizes the effects of the non-zero heater time lag and/or the non-zero sensor time lag for increased accuracy.
In a first embodiment of the present invention, a heater element and at least two spaced sensor elements are provided, whereby the sensor elements are spaced at different distances from the heater element. The heater and sensor elements are disposed in and closely coupled to a fluid medium (fluid or gas) of interest. A time-varying input signal is provided to the heater element, which heats the surrounding fluid. Of interest is the transit times for the temperature disturbance to travel from the heater element to selected sensor elements. Because the sensor elements are spaced at different distances from the heater element, and as more fully described below, the fluid velocity can be calculated from the measured transit time values relatively independently of the fluid properties.
To reduce the effects of the non-zero heater and sensor time lags on the desired transit times, a first embodiment of the present invention contemplates measuring the heater and sensor time lags during a calibration procedure. The heater and sensor time lags can be directly measured by providing a heater input signal to the heater element, and a sensor input signal to each of the sensor elements. By monitoring the transient temperature responses of the heater and sensor elements via heater and sensor output signals, respectively, the heater and sensor time lags can be determined. To obtain more accurate transit times for the temperature disturbance in the fluid, the heater time lag and/or the sensor time lags may be subtracted from the overall time lags measured between the heater input signal and each of the sensor output signals.
Another approach to reduce the effects of the non-zero heater and sensor time lags is to provide heat to the sensor elements via the corresponding sensor input signal during operation. The sensor input signals are preferably controlled to provide a frequency, phase and amplitude that produce a resulting temperature response in the corresponding sensor element that tracks the temperature disturbance in the fluid. When this condition is satisfied, substantially zero heat is transferred from the fluid to the sensor elements, and the effect of the sensor time lags are minimized.
The proper phase and amplitude of the sensor input signals can be determined during a calibration procedure. In one embodiment, the sensor elements are first subjected to a vacuum condition, and a sensor input signal is provided to each of the sensor elements. Because no fluid surrounds the sensor elements, substantially no heat is transferred from the sensor elements to the fluid. A sensor time lag between the sensor input signal and the resulting temperature response of each sensor element is measured and stored. A power/resistance ratio of each sensor element is also measured and stored.
The sensor elements are then subjected to a fluid of interest. During operation, the phase of each of the sensor input signals is adjusted so that the resulting sensor time lag equals the sensor time lag measured under vacuum conditions. Likewise, the amplitude of each of the sensor input signals is adjusted so that the resulting power/resistance ratio equals the power/resistance ratio measured under vacuum conditions. Under these conditions, the sensor time lags may be reduced or eliminated as a factor when determining the transit times of the temperature disturbance.
The effect of the heater lag time may be reduced or eliminated by beginning the time lag measurement when a heater output signal crosses a predetermined threshold, where the heater output signal is proportional to the resistance (temperature) of the heater element. Alternatively, the effect of the heater time lag may be reduced or eliminated by starting the time lag measurement when the heater input signal crosses a predetermined threshold, and then subtracting the heater time lag from the resulting overall time lag value.
Rather than measuring the temperature disturbance with a temperature sensitive resistor, it is contemplated that the velocity of the fluid may be determined by using optical sensors. In an illustrative embodiment, a time lag is determined between a first location and a second location for a transient elevated temperature condition in the f
Bonne Ulrich
Kubisiak David
Matthys Robert J.
Schuldt Spencer B.
Fuller Benjamin R.
Honeywell International , Inc.
Shudy, Jr. John G.
Thompson Jewel V.
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