Thermal gas mass flow meter is designed on the basis of thermal dispersion, and adopts method of constant differential temperature to measuring gas flow. It has advantages of small size, easy installation, high reliability and high accuracy, etc.
The meter contains two platinum resistance temperature sensors. The thermal principle operates by monitoring the cooling effect of a gas stream as it passes over a heated sensor. Gas flowing through the sensing section passes over two sensors one of which is used conventionally as a temperature sensor, whilst the other is used as a heater. The temperature sensor monitors the actual process values whilst the heater is maintained at a constant differential temperature above this by varying the power consumed by the sensor. The greater the gas velocity, the greater the cooling effect and power required to maintain the differential temperature. The measured heater power is therefore a measure of the gas mass flow rate.
The format of gas velocity and power is shown as below:
Where: is specific gravity of medium
V is velocity
K is balance coefficient
Q is heater power
Δ T is differential temperature
The medium temperature range of meter is -40℃～220℃.
In the format (1), the specific gravity of medium is related to the density:
Where: is the medium density in working condition（kg/m3）
ρn is the medium density in standard condition, 101.325kPa and 20℃ （kg/m3）
P is the pressure in working condition（kPa）
T is the temperature in working condition （℃）
In the formats (1) and (2), there is a certain functional relationship between the velocity and pressure in working condition, medium density, the temperature in working condition.
Due to 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 do not need to do temperature and pressure compensation in principle.