Gas flow meter use matters needing attention
1. When the working conditions of the instrument are changed (such as changing the medium, the temperature of the environment greatly changes, etc.), the zero position of the instrument should be re-adjusted. At the same time, the instrument's catheter must be installed horizontally and calibrated with a spirit level. Otherwise, the effect of changes in operating conditions on zero drift will increase. The rack should not be shaken or swayed, so it should not be used on ships.
2. Correlate the large amount of test data equivalent to 0-100kg/cm2 pressure and 0-7 liter/hour flow rate (air), and use the principle of least squares to find the linear regression equation, the correlation coefficient λ value is The range of 0.999 to 0.9999 shows that the instrument has good linearity. However, the larger the linearity and the range size, the greater the flow rate and the more severe the nonlinearity, so the range is generally limited to 0 to 4 liters/hour (in air) to ensure good linearity. In order to measure large flow rates and ensure linearity, the shunt principle can be used to extend the range of the meter. Such as the use of bypass pipe, Venturi tube, orifice plate and other use, the range can be expanded to tens, hundreds, thousands of hours per hour, up to tens of thousands of standard cubic.
3. Although the specific heat of the real gas varies with the pressure and even the range of change of some gases is still relatively large, the measurement accuracy of the instrument can still be kept within a certain range.
4. The choice of conduit material, in addition to considering the corrosion resistance, it is better to use a material with better thermal conductivity. Taking the measurement of nitrogen as an example, it is also tested in the range of 0-100 kg/cm2 pressure and 0-7 liter/hour flow rate. The measurement accuracy of the nickel tube is 2 to 2.5% and that of the stainless steel is 3 to 4%. (The thermal conductivity of nickel is about three times that of stainless steel).
5. Because the gas flowmeter must work under the condition that the specific heat of gas is relatively stable, all gas components are unstable, the center of the gas is entrained with mist, and the working conditions are close to the liquefaction critical area of ​​the gas. Because the specific heat value is unstable This type of instrument should not be used. For example, the critical point for ethylene liquefaction is 50 kg/cm2, 9.9°C. When the pressure is found to exceed 30 kg/cm2 during the test, the meter reading will begin to destabilize.
6. If a gas medium is changed, perform the calibration again. In the manual of the instrument, it is often introduced that the calibration is not based on recalibration. The conversion of the uncalibrated gas flow based on the specific heat of the two gases is simple and convenient, but it will cause large errors, especially when working under high pressure. It is found that the sensitivity of the meter is not completely proportional to the specific heat, and it is better to recalibrate it.
7. The instrument must be warmed up before use. Before it is fully warmed up, the instrument is unstable. For better models, the warm-up time is less than two hours.
8. During use, when the gas flow suddenly changes, the temperature in the pipe must be redistributed by the heat transfer, so that the output signal re-stabilization requires a certain amount of time. In order to reduce this hysteresis, manufacturers often add a differential network to the instrument's electrical circuit to allow the output signal to respond quickly. This is especially necessary when working with other instruments to cooperate with the flow control.
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