The thermal Mass Gas Flow Meter principle is that a fluid flowing past a heated temperature sensor removes a known quantity of heat as it passes. In a thermal flowmeter, the fluid has to pass two temperature sensors. One sensor obtains a reference value by measuring the actual process temperature. The second sensor is a heating element and it receives precisely as much energy as is needed to compensate for dispersed heat and maintain a precisely defined temperature difference.
The higher the mass flow past the heated temperature sensor, the more heat is dissipated and the more power has to be input to the element to maintain the temperature difference.
Heating current, therefore, is a measure of the mass flow of the gas.
The sensor temperature is always 30℃ higher than the medium temperature (environment temperature), and the meter adopts method of constant differential temperature, therefore the meter does not need to do temperature and pressure compensation in principle.
The sensing element
includes two resistance-type temperature detector(RTD), which are protected by heat casings. When measuring gas, one RTD (heated RTD) is heated, and another one(reference RTD) measures the temperature of the gas. When the sensor is in medium with no flow, the two RTD will have a temperature difference (ΔT)
because of the action of the heater. And when the medium flows, the molecule of the medium will take away some heat from heated RTD because of the principle of heat conduction, and the temperature of reference RTD will remain the same. Power of heated RTD (ΔP), temperature difference (ΔT), and mass flow of gas (Q) have function relation, which is as follows:
Based on the above function, it can be divided into temperature detection method and power measurement detection method according to different
variables.The former, also known as the constant power method, refer to keep constant power(ΔP) of heating temperature sensors to provide heat, then measures the temperature difference value(ΔT) that changes with the flow; the latter, also known as the constant temperature difference method, refers to keep constant temperature difference(ΔT) between the heating element and measured
fluid, then controls and measures the power(ΔP) of heating temperature sensors, and the power consumption will increase with the increment of flow.