Thermal mass flow meters measure heat transfer in fluids and can be divided into thermal dispersion meters and thermal diffusion meters based on the principle of thermodynamics. Thermal dispersion is used as a non-contact instrument, suitable for measurement of small diameter and small flow. Thermal diffusion type is a plug-in instrument with a sensor placed in the middle of the pipe, which is suitable for large-diameter flow measurement.

1. Thermal dispersion mass flow meter

The working principle is based on the principle of the temperature distribution of the pipe wall before and after the fluid flow transfer heat changes. The sensor part consists of a heater and two temperature measuring elements. The heating element is placed in the middle of the pipe to perform heating with a certain power. The two temperature measuring elements are symmetrically placed upstream and downstream of the heating element to measure the pipe wall temperature.

When the flow rate is zero, the temperature distribution is the same at both ends of the heating component, in this case, the pipe wall temperature distribution. If the flow velocity is non-zero, the fluid flow will transfer heat from the upstream pipe wall to the downstream pipe wall, thereby changing the temperature distribution of the pipe wall. In this case, the red line of the pipe wall temperature distribution. Within a certain flow range, a linear relationship is formed between the temperature difference T between the temperature measuring elements and the measured flow Q. Therefore, by measuring the temperature difference between the two temperature measuring elements, the corresponding flow rate can be calculated directly.

Thermal dispersion mass flow meter measures very small flow velocity with high resolution. The sensor is installed on the pipeline and does not come into contact with the fluid to prevent pressure loss of the fluid. Later, based on the thermal dispersion mass flow meter, by improving the development of the capillary thermal mass flow meter, a bypass pipe with a much smaller diameter was added near the main pipe where the sensor was installed to the bypass pipe. Some test media flows through the bypass, changing the temperature distribution of the bypass.

2. Thermal diffusion mass flow meter

Thermal diffusion mass flow meter is based on King's law. Its sensor is surrounded by a stainless steel sleeve and inserted into the middle of the pipe. One sensor measures the temperature of the fluid, one sensor is called a temperature probe, and the other sensor is used for heating. This is called a speed measurement probe. When the fluid flow rate is stable, the heat taken away by the fluid flow is equal to the heating heat provided by the circuit, thereby reaching thermal balance. The mass flow rate can be calculated based on the relationship between the heating power or the temperature difference between the probes and the mass flow rate.

Thermal diffusion mass flow meter sensor has no moving parts, the measured fluid pressure loss is small, the measurable flow rate range is large, and it can achieve a distance ratio of 100:1 or 1000:1. It has been widely used in chemical industry, power generation, ventilation measurement and other fields.

According to different measurement methods, thermal diffusion flow meters can be divided into two types: certain power and constant temperature difference. The two have great differences in sensor design, signal conditioning circuit, signal processing method, etc.