Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1999-01-15
2001-11-20
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
C073S204160, C340S515000
Reexamination Certificate
active
06318168
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a thermal dispersion probe, and more particularly to thermal dispersion probes used as flow rate sensors in process control.
BACKGROUND OF THE INVENTION
A thermal dispersion probe typically includes two thermowell-protected RTD's (Resistance Temperature Detectors) which are placed into a medium (air, gas, liquids, slurries or solids) to be monitored. One RTD is preferentially heated while the other RTD senses the temperature of the medium, the temperature differential of the two RTD's is related to the medium flow rate as well as the properties of the medium. The principle of operation of the probe is based on the rate of dispersion of thermal energy from the heated RTD by the medium. As the flow-rate of the medium increases, more of the heat created by the heater by the heater is carried away resulting in a reduction of the temperature differential between the sensors. Using a well-known mathematical formula, the device uses the temperature differential between the RTD's to determine the flow rate of a particular medium or, given a constant flow rate, can determine the type of medium being measured. This data is then processed by devices such as a computer to effect control systems. The device may be utilized in virtually any condition as it may be paired with external software controls which can be downloaded into the device.
Current designs offer a single heater setting for the entire range of the RTD's, these designs cannot intelligently allocate the proper amount of thermal energy required in all necessary instances as it is either ‘full on’ or ‘full off’. ‘Full on’ results in wasted energy when the sensor is located in a medium of low specific gravity and additionally results in very slow response times to major changes in the medium movement or composition. Additionally, when physical jumpers are utilized to select heater power for specific sections of the flow spectrum, it unwittingly restricts the spectrum of the sensors' range. Any significant change in medium will require operator intervention.
In other words, current flow rate measurements may not be as accurate as necessary if the flow rate is either very high or very low. The heat source in the probe is designed to operate for all rates of flow. If the flow-rate is very high, most of the heat created by the heat source will be removed by the first flowing fluid before the thermistor has a chance to measure it. Therefore, small changes in the flow-rate at this end of the spectrum may not be noticed. Similarly, if the flow-rate is very low, most of the heat generated by the heat source will be measured by the thermistor. Too much heat has the same effect on the results as too little heat in that the smaller changes is flow may go unnoticed. Accordingly, where large fluctuations in flow rate are encountered, accurate measurements over the whole range is difficult.
Another shortcoming of present thermal dispersion switches is the lack of appropriate methods to test the switch to ensure it is operating properly. Even those switches that do provide a self-test still require some operator intervention. Therefore, a malfunction of the switch can still go undetected until the next scheduled operator test.
It is an object of the present invention to obviate or mitigate at least some of the above disadvantages.
SUMMARY OF THE INVENTION
In general terms, the present invention provides a thermal dispersion switch in which a heat source is controlled by a switch having a variable duty cycle to provide a variable heating effect.
The variable heat source that is designed to self-adjust for all level and interface applications and all rates of flow. By self-regulating the energy used by the heater in this invention, the microcomputer within the device optimizes the heater settings for all the different fluids and gases found in flow and level applications. When the heater is provided with only the appropriate amount of energy needed to yield the required differential, the sensitivity and response rate of the switch is optimized and maintained without operator intervention. In addition by reducing the amount of energy drawn form the power source in low flow, and level applications, the switch uses less power and is more environmentally sensitive.
In accordance with this invention there is provided A thermal dispersion probe for measuring the flowrate of a medium comprising; heater for heating said medium at a predetermined power; temperature sensor for producing a temperature signal indicative of a temperature difference between an active sensor and a reference sensor; processor for varying said heater power to maintain the temperature differential between the active sensor and the reference sensor within a predetermined range, whereby the predetermined range provides an optimal sensitivity for the probe.
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Chidley Paul Edward
McClelland Brian Leslie
Hayes, Soloway, Hennessey Grossman & Hage, P.C.
Kayden Instruments Inc.
Patel Harshad
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