Principle:
Thermal mass flow meters are most commonly used for measuring the flow of gases, which is operated by the cooling effect of the gases stream.
The operating principle is based on conductive and convective heat transfer. Based on King’s law that reveals how a heated wire immersed in a fluid flow measure the mass flow velocity at a point in the flow.
Construction & Working :
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The thermal mass flow meter consists of two temperature sensing probes attached to the measurement system. And there is a heater in the middle of both thermometers.
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The heater at the middle heats up the fluid. The temperature before the heater and after the heater is measured using the thermometer.
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The temperature difference is computed using controllers attached.
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To maintain the temperature at a fixed level, the heater is activated. The power required to increase the heat is measured and it is proportional to the flow rate of the stream.
There are single probe flowmeters available which consist of both the sensor and transmitter attached to it.
Advantages:
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Thermal flowmeters are used to measure gas with low pressure
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Used measure low flow
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These flowmeters are frequently employed for monitoring and controlling of mass-related processes like chemical reactions.
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They offer good rangeability
Disadvantages:
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Practically for gas only.
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Power requirement execessive in large pipelines.
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Accurate field calibration is difficult.