Application of Thermal Gas Flow Meter in High Purity Gas Flow Measurement

Application of thermal gas flowmeters in high-purity gas flow measurement Thermal gas flowmeters are divided into thermal distributed and submerged according to the structure of the detection components. The latter is used when the pipe diameter is larger. Due to the working principle and structural characteristics of the thermal gas flow meter, its accuracy is usually expressed by a reference error or a combination of indication error and reference error. For high-purity hydrogen, its density in the standard state is stable and known, so using a differential pressure flowmeter to measure its flow can obtain the same accuracy as measuring nitrogen and oxygen. However, there is another difference when measuring hydrogen flow with a thermal mass flow meter. That is, the difference in thermal characteristics. Mass flowmeters should be calibrated one by one before leaving the factory, and actual flowmeters should also be calibrated one by one before leaving the factory. For thermal gas flow meters, there are so many different types of gases that it is not possible to build a flow calibration setup for each gas. To address this issue, calibration and verification are often performed by substitution. GB/t20727-2006 'Thermal gas flowmeters for fluid flow measurement in closed pipelines'/iso14511:2001 stipulates that thermal gas mass flowmeters can be calibrated using and/or alternative gases similar to the process gas to be measured. Then, the K coefficient is used for correction or numerical calculation, and it is converted into the conditions and/or usage conditions of the process gas to be measured. According to the literature, it can be calibrated directly with air and then corrected with the K factor. Experiments show that the uncertainty is improved by about 2%. Table 1 lists the conversion factors for several gases given in the literature. Table 2 lists the conversion factors for thermal gas flow meters provided by the manufacturer. The last row of the table emphasizes that the data will be different for different data sources. This method is actually a means of calibrating the flowmeter. The thermal properties of nitrogen and oxygen are not much different from those of air. Since the main components of air are nitrogen and oxygen, the additional error introduced by the conversion method should be small. However, there is a big difference between hydrogen and air. The thermal conductivity of hydrogen is 7 times that of air, the density of hydrogen is only 7.1% of that of air, and the molar constant pressure specific heat capacity CP is 13 times that of air. These differences can introduce large errors in the conversion. In addition, the temperature and pressure of the fluid will also bring additional errors to the zero point and span of the thermal gas flow meter in operation. According to the relevant national standards, it is necessary to adjust the zero point with the actually measured gas under the actual working temperature and pressure. Regarding the influence of temperature and pressure on the measurement range, GB/t20727-2006 stipulates that there must be a standard table to provide reference measurement values ​​under the working conditions before comparison and adjustment. It is often difficult to find a standard form that meets the requirements. The above effects will bring larger errors to the thermal gas flow meter.
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