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2. 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.
3. The choice of conduit material, in addition to considering corrosion resistance, is better to use materials 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%. .
4. Correlation calculation is performed on a large amount of test data equivalent to 0-100 kg/cm2 pressure and 0-7 liter/hour flow rate. The linear regression equation is obtained by the principle of least-squares method. The correlation coefficient λ values ​​are in the range of 0.999-0.9999. , Prove that the instrument has a 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 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, respectively, up to tens of thousands of standard cubic.
5. Before the instrument is used, it must be warmed up. Before it is fully warmed up, the instrument is unstable. For better models, the warm-up time is less than two hours.
6. In the course of use, when the gas flow suddenly changes, the temperature in the tube must be redistributed through 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.
7. Since such instruments must work under conditions where the gas specific heat is relatively stable, all cases where the gas composition is unstable, the center of the gas is entrained with mist, and the working conditions are close to the liquefaction critical region of the gas, the specific heat value is very unstable. This type of instrument should not be used. For example, the critical point of ethylene liquefaction is 50kg/cm2, 9.9°C. When the pressure is found to exceed 30kg/cm2 during the test, the meter reading will begin to destabilize.
Gas flow meter in the use of attention
1. When the working condition of the instrument is changed, the zero position of the instrument should be adjusted again. 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.