Precautions for the use of gas mass flowmeters

Precautions for the use of gas mass flowmeters According to the principle and performance of this gas mass flowmeter, combined with my experience and test conditions, the following aspects are worth noting: 1. When the working conditions of the gas mass flowmeter change (such as changing the medium , large changes in ambient temperature, etc.), the zero position of the instrument should be readjusted. We adjust through the zero adjustment hole on the side of the air inlet on the outer cover, and the outer cover can also be opened for adjustment. Adjust the position of the potentiometer as shown in the figure below. Note: the flow line cannot be ventilated (or the valve is closed) during zero adjustment; it must be warmed up for more than 15 minutes after starting the machine, and then proceed after the zero point of the flowmeter is stable. Usually, except for the zero adjustment potentiometer, other potentiometers should not be adjusted easily. 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 must not vibrate or sway, so it is not suitable for use on ships. 2. The linearity of the gas mass flowmeter is related to the size of the range. The larger the flow rate, the worse the linearity, that is, the more serious the nonlinearity. Therefore, the range is generally limited within 0 to 4 standard liters/hour (air) to ensure good performance. 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 will change with the pressure, and even some gases have relatively large fluctuations, the measurement accuracy of the instrument can still be maintained within a certain range. 4. In the selection of conduit material, in addition to considering corrosion resistance, it is better to choose materials with better thermal conductivity. Taking the measurement of nitrogen as an example, it is also tested in the range of 0~100kgf/cm2 pressure and 0~7 standard liters/hour flow rate. The measurement accuracy of nickel tube is 2~2.5%, while that of stainless steel is 3~4%. (The thermal conductivity of nickel is about 3 times that of stainless steel). 5. Since this type of gas mass flowmeter can only work normally when the specific heat of the gas is relatively stable, when the gas composition is unstable, there is fog in the gas, and the working conditions are close to the liquefaction critical area of ​​the gas, etc., due to The specific heat value is very unstable, so it is not suitable to use this kind of instrument. For example, the critical point of liquefaction of ethylene is 50kgf/cm2 and 9.9℃. When the pressure exceeds 30 kgf/cm2 during the test, the reading of the gas mass flowmeter will start to lose stability. 6. If the gas medium to be measured is changed, it is best to re-calibrate. In the instruction manual of the instrument, it is often introduced that the uncalibrated gas flow rate is converted only according to the specific heat of the two gases without recalibration. Although this is simple and convenient, it will cause large errors, especially when working under high pressure. We found that the sensitivity of the meter is not necessarily proportional to the specific heat, and it should be re-calibrated. 7. The gas mass flowmeter must be turned on and preheated before use. Before the gas mass flowmeter is fully preheated, the operation of the gas mass flowmeter is 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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