Multi-point thermal gas mass flow test method experiment

On the basis of the single-point thermal gas mass flow meter that has been developed, in view of the current problems in the measurement of gas mass flow in large-caliber or irregular pipelines, a thermal gas mass flow test method based on multi-point measurement is proposed. The article conducted a lot of experimental research on the temperature characteristics of the sensitive element (hot film probe) and the multi-point test method with the hot film probe as the measuring point. The experimental results show that: in the multi-point test method, the logarithmic linear method; the multi-point thermal gas mass flow test method can significantly improve the large deviations in some single-point measurements, and the measurement accuracy can reach 1.5%. The uncertainty is less than 3.4%.

Thermal gas mass flowmeter is a new type of gas flow detection instrument developed on the basis of the early hot wire anemometer. It has been widely used in aviation, aerospace, energy, medicine, automobile industry, natural gas pipeline transportation and other industries. At present, the domestic research and application of thermal gas mass flow meters are still in the initial stage, and most products need to be imported, especially thermal gas flow meters for gas flow measurement in large and medium-sized pipelines. Therefore, the study of thermal gas mass flow meters will be of great significance to the development of my country's national economy. Based on the single-point thermal gas mass flow meter that has been developed, this article focuses on the low single-point measurement degree, the large pressure loss of the differential pressure meter, and the velocity-area in the current large-diameter or irregular pipeline gas mass flow measurement. The method is difficult to measure and other problems, and a thermal gas mass flow test method based on multi-point measurement is proposed.

1 Measuring principle and temperature characteristics of sensitive components

Thermal mass flow meters are divided into hot wire and hot film types according to different heating elements [1, 2]. This article uses thin-film platinum electrodes (platinum film resistors or platinum film probes) as sensitive components. As a new type of temperature sensing element, platinum film resistor has the characteristics of small size, fast response, easy to match with integrated circuits, and has the advantages of wide temperature measurement range, high accuracy, good linearity, and stable performance. It has a wide range of applications in temperature compensation, temperature and flow measurement and control.

When the platinum film resistor is used as a flow sensor, since the heat dissipation condition of the probe is greatly related to the ambient temperature, its signal output will be affected by the change of the ambient temperature. In order to quantitatively grasp the temperature characteristics of the hot film probe, we heat according to the thermal flowmeter The working temperature range of the probe is to test the temperature characteristics of the hot film probe at different ambient temperatures. Place the hot film probe in a thermostat with adjustable temperature, add a constant working current i to the probe, measure the voltage v across the probe under different working conditions, and then calculate the resistance of the hot film probe. The experimental device for the temperature characteristics of the hot film probe is shown in Figure 1.

Multi-point thermal gas mass flow test method experiment

Since the working temperature of the hot film probe is difficult to measure, in the static temperature characteristic experiment, first establish the curve of the working resistance of the hot film probe with temperature, that is, under the premise that the hot film probe has no self-heating effect (working current ≤ 1 ma) , The resistance of the hot film probe changes with the ambient temperature. According to the actual situation, this article chooses pt20 hot film probe as the experimental research object.

In the absence of self-heating effect, the relationship between the resistance of the hot film probe and the ambient temperature is shown in Figure 2.

Multi-point thermal gas mass flow test method experiment

The relationship between the resistance r t of the hot film probe and the ambient temperature t is established from the static temperature characteristic curve of the hot film probe:



In the industry, thin-film platinum resistors are used as temperature measurement elements, mainly using the r t-t characteristics of the platinum resistors under the premise of no self-heating effect. Under normal circumstances, the current through the platinum resistors is not more than 1 ma. This article uses thin-film platinum resistance as a heating element, passes a larger current, and uses its own thermal effect to make the platinum film probe reach a certain working temperature as a sensor for measuring gas flow.

Under different experimental conditions (u = 0 and u ≠ 0), the hot film probe is applied with the same current (i = 70 ma). The relationship between the resistance rt and the ambient temperature t is shown in Figure 3.

Multi-point thermal gas mass flow test method experiment

The hot film probe is under static experimental conditions (u=0), and different working currents are applied to the hot film probe. The relationship between the resistance r t and the ambient temperature t is shown in Figure 4.

Multi-point thermal gas mass flow test method experiment

Through the analysis of Figure 3 and Figure 4, it can be concluded that when the hot film probe is working in a constant current state, its resistance changes almost linearly with the change of the ambient temperature, that is, the difference between the working temperature of the hot film probe and the ambient temperature is $t basic constant. The heat dissipation caused by the forced convection of the hot film probe under different working conditions does not change with the change of the ambient temperature. For example, the heat dissipation of the selected hot film probe caused by forced convection is about 31 mw. The working temperature of the hot film probe increases with the increase of the working current. For example, when the ambient temperature is 20℃, when i = 50 ma, the working temperature of the hot film probe is about 77℃; when i = 70 ma, the working temperature of the hot film probe is about 142℃; when i = 90 ma, the working temperature of the hot film probe is about 142℃; The working temperature is about 256°C. So we can determine the working temperature of the hot film probe by selecting the working current of the hot film probe.

2 Multi-point test method analysis

Multi-point thermal gas mass flowmeter mainly arranges multiple sensing elements in the diameter direction of the pipe cross section to detect the gas flow at different points in the pipe section, as shown in Figure 5.

Multi-point thermal gas mass flow test method experiment

The multi-point thermal gas mass flow test method is based on the principle of velocity measurement with an even velocity tube flowmeter [3, 4]. That is, the section of the pipeline is divided into several parts with equal area, and the mass flow rate at the characteristic point of each part is measured, and the mass flow rate at the characteristic point is used to represent the average mass flow rate of this part. Multiply the mass flow rate by the area of this part to get the mass flow rate through the small area. Then add up the mass flow of each small area, which is the mass flow through the entire pipeline. The key to the multi-point testing method is how to determine the location and distribution number of feature points. The multi-point detection sensor is designed by using isotoroidal method, Chebyshev method, and logarithmic linear method. Select the experimental pipe radius r = 30 mm, the design divides the circular pipe section into two halves, takes the pipe center as the origin, and designs the characteristic point distribution diagram of the multi-point detection rod as shown in Figure 6.

Multi-point thermal gas mass flow test method experiment

According to the values of the characteristic points of the three methods given in references [5~6], the distribution positions of the characteristic points of the multi-point detection sensor under different test methods are calculated in turn, as shown in Table 1:

Multi-point thermal gas mass flow test method experiment

3 Temperature correction

The working principle of 　　 thermal gas mass flowmeter is based on the forced convection heat exchange between the hot film probe and the fluid being measured. The size of the output signal is not only related to the mass flow rate, but also related to the temperature of the measured medium [1]. Therefore, when the fluid temperature in the measurement environment is different from the fluid temperature at the time of calibration, it will directly affect the accuracy of the measurement result. Therefore, the temperature correction of the thermal gas mass flowmeter must be performed.

The commonly used temperature correction methods for thermal gas mass flow meters can be divided into two categories: analytical correction method and automatic correction method.

Analysis and correction method, that is, an independent temperature sensor is required to detect the ambient temperature t a, and then t a is inserted into the selected heat transfer relationship. In this method, the hot film probe works in a constant resistance state. With the application and development of computer and microelectronics technology, most of the current temperature corrections of thermal mass flow meters use analytical correction methods [7,8]. The key to analysis and correction is to determine the heat transfer relationship of the thermal film anemometer, that is, the function relationship between the output signal and the wind speed and temperature. The current research on temperature correction of anemometers is basically to determine the heat transfer formula [7-9].

Automatic correction method, that is, adding a temperature sensor to the Wheatdens bridge to automatically compensate for changes in ambient temperature. At this time, the hot film probe works in a non-constant resistance state. The automatic calibration method adopted is to change the arm resistance of the Wheatdens bridge opposite to the hot film probe into a series-parallel circuit containing compensation resistors, as shown in Figure 7. The rc is a platinum resistance with a positive temperature coefficient, which is placed in the same flow field as the hot film probe rw.

Multi-point thermal gas mass flow test method experiment

The basis of the compensation circuit design is the temperature characteristic of the hot film probe and the resistance temperature parameter of the compensation platinum resistance. In order to make the sensor output not change with the ambient temperature, theoretically, it should meet the requirements of any ambient temperature:

Multi-point thermal gas mass flow test method experiment

Table 2 shows the experimental data of the multi-point thermal gas mass flowmeter without temperature correction and after analysis correction and automatic correction respectively. Among them, the analysis correction output corresponds to the same set of temperature values, and the automatic correction output corresponds to another set of temperature values.

Multi-point thermal gas mass flow test method experiment

The temperature drift of thermal mass flow sensor can be divided into zero temperature drift and sensitivity temperature drift [10]. Zero temperature drift is the drift of the sensor's output caused by temperature changes in the static state, expressed by tcr, as shown in equation (3); Sensitivity temperature drift is the drift of the sensor's output at a certain flow state caused by temperature changes, expressed by t cs, See formula (4).

Multi-point thermal gas mass flow test method experiment

Table 3 gives the analysis of the results before and after the temperature calibration of the multi-point thermal mass flowmeter.

Multi-point thermal gas mass flow test method experiment

4 Output signal calibration

At present, there are three main methods for fitting the characteristic curve of thermal gas mass flow meters, namely power law fitting, extended power law fitting and polynomial fitting. This article uses fourth-order polynomial fitting for analysis. The fourth-order polynomial fitting formula of the multi-point thermal gas mass flowmeter can be expressed as

Multi-point thermal gas mass flow test method experiment

Multi-point thermal gas mass flow test method experiment

Multi-point thermal gas mass flow test method experiment

Multi-point thermal gas mass flow test method experiment

5 Uncertainty analysis

The measurement error of the multi-point thermal gas mass flow meter is composed of random error and systematic error. Random errors such as the noise of the signal conditioning circuit; system errors such as calibration error, linearization error, signal conditioning circuit error, measurement error caused by the measured fluid temperature, etc. Among them, the noise of the signal conditioning circuit and the error of the signal conditioning circuit are one order of magnitude smaller than other errors, which can be ignored as minor errors [12-13]. The uncertainty analysis of the multi-point thermal gas mass flowmeter is shown in Table 5 of this article.

Multi-point thermal gas mass flow test method experiment

6 Concluding remarks

(1) When platinum film resistor is used as a flow sensor (larger heating working current), its resistance temperature characteristic still has good linearity under different working conditions; when working at constant current, its working temperature is less than the ambient temperature. The difference is basically constant.

(2) When there is no temperature correction, the output signal of the thermal mass flowmeter has a large temperature drift. After the temperature correction is adopted, the temperature drift of the system is obviously improved and good results have been achieved. At the same time, for the temperature correction method used in this article, the temperature correction effect of the automatic correction method is better than that of the analytical correction method.

(3) The multi-point thermal gas mass flow test method can improve the large deviations in some single-point measurements, and at the same time achieve better measurement accuracy.