Measuring and testing – Volume or rate of flow – Thermal type
Patent
1991-09-20
1994-08-23
Chilcot, Jr., Richard
Measuring and testing
Volume or rate of flow
Thermal type
7320427, G01F 100
Patent
active
053396871
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a flowmeter.
There are many well established techniques which are used to measure the flow rate of a fluid in a closed pipe and which rely upon the thermal properties of the fluid. A number of these techniques are described below, the described techniques being used in commercially available equipment. All the available equipment is constructed and optimised in such a way so as to use only one of the established techniques.
The basic equation of heat flow from a cylindrical transducer placed in a fluid cross flow can be expressed in the general form:
There are in fact extra terms that have been ignored in the above equation that contain dependencies on other fluid properties such as thermal conductivity. These terms can be significant under certain conditions as indicated below.
In a first known technique for measuring the flow rate of a fluid, a transducer comprising a heating element and a temperature sensor is mounted so as to have direct (inside a fluid carrying pipe) or indirect (via the pipe wall) contact with the fluid flow. The heating element has power applied to it and the resultant transducer temperature is measured by the sensor. The power input is controlled externally such that the temperature of the transducer is maintained at a fixed value. The moving fluid stream will carry heat from the heating element as it moves past the transducer and hence the power supplied to the heating element to maintain a constant temperature will be proportional to the fluid flow. This technique has the following limitation and/or disadvantages: properties do not appreciably change with temperature or composition. the ambient temperature changes. can be heated to a temperature considerably higher than that of the fluid. Sensitivity then becomes a compromise however as the higher the temperature differential the less sensitive the sensor becomes. This also requires high powers at high flow rate, and so its applications are limited.
This technique does find applications as it provides results that in some circumstances are sufficiently accurate and it is cheap to implement as the transducer construction particularly simple.
In a second known technique using the same transducer type as the first, a constant differential temperature maintained between the sensor and the fluid. This is possibly the most commonly used technique, but requires a second temperature measuring sensor to monitor the fluid temperature. An external control circuit is provided to control the power supplied to the heating element to maintain a constant temperature difference. The power supplied is thus a function of the fluid flow rate. This second technique has the following limitations, and/or disadvantages: caused by changes in the thermal properties of the process fluid with temperature. If high accuracy is to be achieved this demands a thorough knowledge of the fluid type and operating conditions prior to calibration of the device. heat and/or thermal conductivity changes. temperature difference must be precise and stable and can thus be relatively complex. differential temperature can be excessive. For any particular maximum transducer power setting the dynamic range of measurable flow rate varies with the required differential temperature so for a high differential temperature the dynamic range is severely limited.
This second technique is widely used nevertheless as the components are relatively simple to construct, and, when the fluid properties and operating conditions are known, the device can be calibrated very accurately. In addition, the power consumption is relatively small, at small temperature differentials the device is very sensitive to small flow rate changes and/or low flow rates, at high temperature differentials the same device can handle wide flow rate changes (at the expense of sensitivity), and since the fluid temperature is continually monitored the required sensitivity can be maintained.
In a third known technique a combined heating element/temperature sensor transducer is again used
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Gimson Christopher J.
Pasdari Mehran
Biegel R.
Chilcot, Jr. Richard
Endress & Hauser Limited
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