Precautions for the use of gas flow meters1

According to the principle and performance of this instrument, combined with our use experience and test conditions, the following aspects are worth noting: 1. When the working conditions of the instrument change (such as changing the medium, the ambient temperature changes greatly, etc.), the zero bits should be readjusted. At the same time, the conduit of the instrument must be installed horizontally and calibrated with a level. Otherwise, the influence of the change of working conditions on the zero drift will be increased. The frame should not vibrate or sway, so it is not suitable for use on ships. 2. pair is equivalent to 0—A large number of test data in the range of 100kg/cm2 pressure and 0-7 standard liters/hour flow (air) are correlated, and the linear regression equation is calculated by the principle of the least square method. The correlation coefficientλThe values ​​are all in the range of 0.999 to 0.9999, which proves that the instrument has good linearity. However, linearity and range size have a relationship. The larger the flow rate, the more serious the nonlinearity. Therefore, the range is generally limited to 0 to 4 standard liters/hour (air) to ensure good linearity. In order to measure a large flow rate while ensuring linearity, the shunt principle can be used to expand the measuring range of the meter. If the bypass tube, Venturi tube, orifice plate are used together, the measuring range can be expanded to tens, hundreds, thousands of standard liters per hour, up to tens of thousands of standard cubic meters. 3. Although the specific heat of the real gas varies with the pressure and even the variation range of some gases is relatively large (see Table 1), the measurement accuracy of the instrument can still be maintained within a certain range (see Table 2). 4. In the selection of conduit material, in addition to considering corrosion resistance, it is better to choose materials with better thermal conductivity. Take the mesh nitrogen as an example, also at 0—Tested within the range of 100kg/cm2 pressure and 0-7 standard liters/hour flow rate, the measurement accuracy of nickel tube is 2-2.5% and that of stainless steel is 3-4% (the thermal conductivity of nickel is about three times that of stainless steel. ). 5. Since the gas flowmeter can only work normally when the specific heat of the gas is relatively stable, the specific heat value is very unstable when the gas composition is unstable, there is fog in the center of the gas, and the working condition is close to the liquefaction critical area of ​​the gas. , should not use this instrument. For example, the critical point of liquefaction of ethylene is 50 kg/cm2 and 9.9°C. When it is found that the pressure exceeds 30 kg/cm2 during the test, the instrument reading will start to lose stability. 6. If a gas medium is changed, it is necessary to re-calibrate. In the instruction manual of the instrument, it is often introduced that there is no need to re-calibrate, and it is simple and convenient to convert the uncalibrated gas flow rate only according to the specific heat of the two gases, but it will cause a large error, especially when working under high pressure. It is found that the sensitivity of the meter is not necessarily proportional to the specific heat, and it is more appropriate to re-calibrate. 7. The instrument must be turned on and preheated before use. Before it is fully preheated, the instrument will be unstable. For better models, the boot warm-up time is within two hours. 8. During use, when the gas flow rate changes suddenly, the temperature in the tube must be redistributed through heat transfer, so it takes a certain amount of time for the output signal to re-stabilize. In order to reduce this hysteresis, the manufacturer often adds a differential network to the electrical circuit of the instrument to make the output signal respond quickly. This is especially necessary when cooperating with other instruments for automatic flow control.

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