Basic principle
A thermal mass flowmeter is a flow meter that uses the temperature field change generated when the fluid flows through a pipe heated by an external heat source to measure the mass flow of the fluid, or uses the relationship between the energy required to heat the fluid to a certain temperature and the mass of the fluid to measure. It is mainly based on the principle of thermal diffusion, through the constant temperature difference method to accurately measure the gas.
Sensor structure
The sensor part of a thermal mass flowmeter usually consists of two reference platinum resistance temperature sensors:
- One sensor: continuously measures the medium temperature T1.
- Another sensor: self-heating to a temperature higher than the medium T2, used to sense the flow rate of the flow body, called the speed sensor. The temperature difference ΔT=T2-T1 (T2>T1). When there is a fluid flowing through, the gas molecules collide with the sensor and take away the heat of T2, causing the temperature of T2 to drop. To keep ΔT constant, the supply current at T2 needs to be increased. The faster the gas flow speed, the more heat is taken away, and there is a fixed functional relationship between the gas flow rate and the increased heat, that is, the principle of constant temperature difference.
Working principle
The working principle of the thermal mass flowmeter can be divided into two types: thermal distribution and immersion:
- Heat distribution: The outer wall of the measuring tube is wrapped around a Wheatstone bridge composed of a heater and a detection element, and heat is supplied by a constant heat source and transferred to the fluid in the tube by thermal conduction. As the fluid flows, it changes the temperature distribution on the measuring tube so that the mass flow rate can be calculated by measuring the temperature difference.
- Immersion type: Based on King’s law, the relationship between the parameters is expressed by using the heat loss rate of the hot wire. Two temperature sensors are respectively placed in two metal tubes in the air flow. One measures the air flow temperature T, and the other is heated by electric heating with constant power. The temperature Tv is higher than the air flow temperature. As the mass flow rate increases, the air flow carries away more heat and the temperature drops, and the mass flow rate is calculated by measuring the temperature difference ΔT=Tv-T.
Data analysis
The temperature difference measured by the sensor or the power consumed can reflect the mass flow of the fluid. In general, thermal mass flowmeters can directly display gas mass flow and standard volume flow without temperature and pressure compensation. Moreover, using RS-485 or HART communication, factory automation and integration can be realized.
In summary, the thermal mass flowmeter has a wide range of applications in steel, boiler and other industrial fields with its characteristics of high precision, high repeatability and high reliability.