Working principle of thermal mass flow meters
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Could any expert help me by explaining the various types of flow meters used for measuring gas flow rates, as well as their advantages and disadvantages? This post was last edited by Chifeng on 2009-2-20 at 10:09
The thermal mass flow meter features a sensor composed of 2 RTDs, one of which serves as the reference terminal to measure the temperature of the medium in question； The other RTD is equipped with a separate heater as the measurement terminal. When the sensor is placed in a medium with no flow, a temperature difference will be created between the two RTDs due to the action of the heater. As the medium flows, according to the principles of heat conduction, the molecules in the medium will carry away some of the heat from the RTD at the measurement end, while the temperature of the RTD at the reference end remains constant. As a result, the temperature difference between the two RTDs continues to decrease. Changes in temperature are related to the flow rate of the medium and the thermal properties of the medium itself; higher flow rates or media with greater density will accelerate the change in the temperature difference between the two RTDs. A linearized circuit is used to convert changes in temperature difference into an output signal corresponding to the flow rate. In this way, the measurement of traffic can be completed.
The basic principle of heat conduction is as explained on the second floor. There is also another approach, which involves maintaining a constant temperature difference between the two RTDs; in this case, the power consumed by the system is directly proportional to the flow rate. Flowmeters based on this principle are suitable for measuring single-phase gases; due to the relatively slow response time, the data obtained is fairly stable. Thanks to technological advancements, these flow meters now feature a completely unobstructed design, which results in very low pressure losses. They are also suitable for measuring fluids with low flow rates; they can be used for measuring liquids as well. However, due to liquid evaporation, a two-phase condition can easily occur, which has a significant impact on accuracy. Not suitable for media with extremely high temperatures.