General Selection of Thermal Gas Mass Flow Meters
2021-06-04
Thermal gas mass flowmeter based on the principle of thermal diffusion, using constant temperature difference method designed, is a professional gas metering instrument. It has the characteristics of high precision, wide range ratio, small pressure loss, direct measurement of gas standard volume flow without temperature and pressure compensation, and many other flowmeters do not have, and has been widely used in all walks of life.
The following points need to be noted when selecting and installing:
Thermal gas mass flowmeter constant temperature difference measurement center point is to ensure that the temperature between the two probes is constant, the heat of the medium (flow rate) and the heat supply current required for heating is proportional to the medium temperature and medium specific heat as the key parameter, the flow meter converter is stored in more than 60 kinds of conventional gas specific heat conversion coefficient, the coefficient is set as a single matrix clean gas. However, in the actual measurement of the field, the purity of the medium is not so high, although the design accuracy of the flowmeter has not declined, but the measured value and the actual value may have a certain amount of deviation, which is why the thermal gas mass flowmeter is suitable for measuring dry gas; The temperature of the medium is a major difficulty of the hot gas mass flowmeter, and it is necessary to heat the heating probe to 50 degrees higher than the medium temperature when working. If the temperature change frequency of the medium itself is too high during the measurement, that is, the power supply current required for heating is large and small, resulting in large flow fluctuation of the meter head display, its measurement accuracy will fall straight down, which is not conducive to detection, if the medium itself is high temperature, the probe needs a higher heating temperature, the flowmeter design power is less than 18W, when the heating temperature is too high, The power supply current required for heating is too large (the power supply current is not enough) will produce a variety of abnormal conditions, such as the head flow from small to large cyclic changes, so the thermal gas mass flowmeter is mostly used to measure the medium temperature change frequency is not high and the temperature does not exceed 400 degrees of gas.
Thermal gas mass flowmeter is a high-end gas measuring instrument, with many advantages, wide range ratio, plug-in connection mode can be selected when large diameter, convenient for subsequent disassembly and maintenance, various power supply modes, 24V or 220V can be used, ground pipe local display, high altitude pipe separate display, etc. However, thermal gas mass flowmeters are not suitable for all occasions.
Medium temperature. The thermal gas mass flowmeter is designed based on the principle of thermal diffusion. The probe of the flowmeter determines the heating temperature of the probe according to the temperature of the measured medium. Generally, the heating temperature of the probe is 50 degrees higher than that of the medium. It can be seen that the temperature of the measured medium is relatively constant when the flowmeter is normally measured, and the heating current is proportional to the flow rate of the medium at this time.
Medium composition. Hot gas mass flowmeter can measure almost all single gas medium (except acetylene), but in the actual measurement there are a lot of mixed gases need to be measured, at this time the use of hot gas mass flow timing, it is very necessary for the mixed gas. All gas flow meters use air as the measured medium during calibration, and the standard conversion coefficient can be found when a single medium is used. However, the conversion coefficient of the mixed gas needs to be converted according to the percentage of the measured medium composition in the field time, but when the medium composition changes during measurement, the measurement error of the flowmeter will be increased.
Stress. Thermal gas mass flowmeters are not recommended for use under high pressure conditions. The important point of thermal measurement is the heat carried by the medium, when the field pressure is too high, the heat carried by the medium per unit volume is very large, under the double influence of the compression system and the reaction rate, the measured value of the flowmeter will be smaller than the actual use, so it is recommended that the thermal gas mass flowmeter be used within 4.0Mpa.
It's high temperature. Measuring the high temperature medium means that the temperature resistance of the sensor is higher, and the power consumption of the whole machine circuit increases when measuring the high temperature medium. At this stage, the hot gas mass flowmeter can withstand high temperature to 400 degrees, which can meet the vast majority of on-site requirements.
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