The flow sensor of the thermal gas mass flowmeter is a non electric conversion component, and its internal structure is shown in Figure 4. Four windings w56 are symmetrically wound on a thin-walled tube. Overlapping on W12, W78 overlaps on W34, forming a Wheatstone bridge that supplies a constant current to heat the tube. When there is no flow rate, the temperature distribution of the measured tube is symmetrical as shown in Figure 5. When there is flow rate, the curve deviates from symmetry, and the larger the flow rate, the greater the deviation. The four arm winding serves as both a heating element and a temperature measuring element, and the DC millivolt signal (D.Cmv) detected diagonally by the bridge is linearly proportional to the magnitude of the flow rate within a certain range.

When there is no flow, the average temperature rise of the upstream and downstream sections of the thermal gas mass flowmeter is symmetrical and can be expressed as

In the formula: Q - the heat obtained by the power supply for each half tube.

A - Heat conduction between each half pipe and the environment. When there is flow, the airflow passing through the upstream section of the pipe exhibits heat absorption, while the airflow passing through the downstream section exhibits heat release. This heat exchange is mainly conducted, and due to the slow flow velocity and small diameter of the pipe, it can be considered that the heat exchange between the pipe wall and the measured gas is sufficient. Therefore, the average temperatures of the upstream and downstream half pipes corresponding to a certain flow rate are shown in the table below

And △ T=T-To

Substituting equation (5) into equation (4), the average temperature difference between the upstream and downstream pipes is:

Obviously, for specific media and instrument structures, while maintaining a constant heating power, their specific heat C, maximum temperature rise T, and heat transfer coefficient A are all constant.

(6) The formula expresses the basic equation of the instrument, hence the name of the thermal gas mass flowmeter. According to equation (6), the measuring premise of this instrument is that the specific heat C is constant, so the thermal flowmeter is only suitable for measuring the flow rate of a single component gas or a mixture gas with a constant component ratio. The signal output by the bridge (D.Cmv) is a measure of T2mT1m, and a correction curve for mv and mass flow rate (or flow rate expressed in volume) can be given in Figure 6. From the curve, it can be seen that the linear segment is very wide throughout the entire measurement range of the instrument. After 4.51/hr, it deviates from the linearity, indicating that the flow rate has been too high and the airflow has not had enough time to fully exchange heat with the pipe wall, carrying out some heat from the pipe. As a result, the linearity is broken, which is more important for the linear range of the small 4 pipes.

According to equation (6), if the instrument is calibrated with mass flow rate, the flow rate will be basically independent of temperature and pressure, because the specific heat of gas is approximately independent of temperature and pressure over a wide range. However, there are still many difficulties in calibrating the quality of gases, so many foreign countries still use volume calibration as the main method, such as the U-70 model in France and the LF and HF series instruments in the United States. Obviously, the volume method is sensitive to operating pressure and temperature, and fluctuations in ambient temperature will also cause changes in the displayed values. The magnitude of this change is related to the symmetry of the instrument structure, especially the pairing of the measuring element's process winding.

According to equation (6), as the ambient temperature To changes, the K value will also change, and the signal value corresponding to the same flow rate will also change. The experimental measurement of the effect of the flow rate curve on the ambient temperature change is shown in Figure 7. In general, when the ambient temperature rises by 20 degrees Celsius, the flow curve shifts downward (almost in a parallel relationship) by about 7-8%. Therefore, it is necessary to design a large heat capacity aluminum isothermal internal measurement element and adopt temperature compensation measures. The setting of isothermal body is to reduce the influence of instantaneous temperature fluctuations, while the temperature compensation circuit will eliminate the influence of slow environmental temperature fluctuations.

The development of thermal gas mass flow meters focuses on two breakthroughs. Firstly, seeking an effective, simple and feasible environmental temperature compensation measure that is convenient for mass production; The second is to improve the response speed of the instrument to transient flow. Due to the fact that the heating and temperature measuring elements are wrapped around the outer wall of the measuring tube, the lag time is relatively long. The measured low-pressure type (diameter 3 * 0.1 nickel tube)

Its time constant is 33 seconds; The high-pressure type (diameter 4 * 0.2 stainless steel tube) takes 55 seconds. For the measurement of pulsating flow (such as the outlet of a reciprocating pump), the instrument can provide an indication of the average flow rate, but for rapidly changing instantaneous flow rates, the instrument cannot respond immediately. Excessive lag is not allowed in the automatic regulation system, which will seriously affect the quality of regulation. The pure hysteresis time of the experimental measurement of the thermal mass flowmeter is very short, about 0.5-1 second. The hysteresis of the flowmeter to the step flow signal is characterized by a large time constant and an integral exponential curve. This makes it possible for us to perform pure circuit reprocessing on the output signal. Obviously, this processing should be in differential form, which plays a leading role. In actual circuits, amplification and closed-loop integral negative feedback form are used. Reasonable selection of feedback components can greatly shorten the response time.

From Figure 4, it can be seen that in order to ensure sufficient mechanical strength, the measuring tube sleeve of the thermal gas mass flowmeter is welded inside the reinforced tube. The reinforced tube extends to the environment through a brass heat dissipation block with a large heat capacity for external pipeline connection. The copper heat dissipation block is considered to be at the ambient temperature To, and the aluminum isothermal body and the shell form a double-layer shielding of electricity and magnetism.