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For high-purity hydrogen, its density under standard conditions 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, when using a thermal mass flowmeter to measure the flow of hydrogen, another difference occurs. That is, the difference in thermal properties.

As we all know, thermal mass flowmeters must be calibrated by actual flow before leaving the factory, and when they are sent to the metrology verification agency for verification, they must also be calibrated by actual flow. For thermal mass flowmeters, there are many types of gases, and it is impossible to build a flow calibration device for each gas. In order to solve this problem, the substitution method is generally used for calibration and verification.

GB/T20727-2006 "Thermal Mass Flowmeter for Fluid Flow Measurement in Closed Pipes"/ISO14511:2001 stipulates that thermal gas mass flowmeters can be calibrated using substitute gases that are and/or similar to the process gas to be measured. Then use the K factor for correction or numerical calculation to convert it into the process gas to be measured and/or the conditions under the operating conditions. The literature believes that it can be calibrated directly with air and then corrected with the K factor. Experiments have shown that the uncertainty increases by about 2%. The conversion coefficients of several gases given in the literature are listed in Table 1. The conversion coefficients of thermal mass flowmeters provided by a manufacturer are listed in Table 2. The last line of the table emphasizes: Different data sources will have different data.

Using this method to calibrate the flowmeter is actually a last resort. The thermal properties of nitrogen and oxygen are slightly different from those of air, because the main components of air are nitrogen and oxygen, so the additional error introduced by the conversion method should be small. However, hydrogen is much different from 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 specific heat capacity cp at constant pressure is 13 times different. These differences will cause large errors in the conversion. In addition, the temperature and pressure of the fluid will also cause additional errors in the zero point and range of the thermal mass flowmeter put into operation.

According to relevant industry standards, it is necessary to use the actual measured gas under the actual working temperature and pressure conditions to perform zero point adjustment. Regarding the influence of temperature and pressure on the range, GB/T20727-2006 stipulates that a standard table must be provided to provide reference measurement values ​​under working conditions before comparison and adjustment can be made. It is often difficult to find a standard table that meets the requirements. The above influences will cause large errors to the thermal mass flowmeter.

The above is the application of thermal mass flowmeter in high-purity gas flow measurement compiled by Beite editor. I hope it will be helpful to everyone. If you have other content you want to know, please contact us and we will provide you with thoughtful help.