Correct understanding of the excellent performance of thermal gas mass flowmeter1

A good-quality thermal flowmeter product must have two main features: a good probe manufacturing process and a complete real-flow calibration device. Let's analyze these two characteristics separately: a. Probe manufacturing process: We talked about the principle of thermal flowmeter earlier, we can see from the principle that the probe is the core of the entire thermal gas mass flowmeter, and its performance is good or bad It can determine the measurement accuracy, repeatability, service life and low flow rate characteristics of the flowmeter. The probe is composed of platinum resistance wire and sheathed stainless steel. The platinum heating wire conducts electricity, and the stainless steel sheath also conducts electricity, which requires that the spacer between them must have very good thermal conductivity, but it cannot conduct electricity. This leads to the core of all thermal flowmeters - the gap filling material and packaging process of the heating probe: the thicker the gap filling layer, although the insulation is better, the thermal conductivity is poor, and the temperature sensitivity is also poor. , while responding with a lag. If the filler is organic, it is easy to age and cause cracks in the filler, and the flowmeter will show zero drift. If there is tiny air in the spacer, since the probe is always in a heated state, the tiny air expands, causing the zero point to fluctuate. The heated velocity sensor of traditional thermal mass flow meters is encapsulated at the end of the stainless steel tube probe, and the mixture is poured between the sensor and the inner wall of the stainless steel tube. The mixture must be electrically insulated while ensuring a small thermal resistance, generally epoxy resin, ceramic cement, heat-resistant paste or alumina powder, magnesium oxide powder, etc. These 'wet' sensors using the above fillers have some drawbacks: for example, their surface thermal resistance will increase with the use of time, causing the output curve to show a downward trend, resulting in a decrease in sensor sensitivity and ultimately affecting measurement accuracy. The filling of the 'wet' sensor has a different thermal expansion coefficient from that of the speed sensor, so aging and cracking will occur with the prolongation of use time, which will eventually lead to poor measurement accuracy of the sensor, and it is difficult to maintain long-term accuracy. There is no difference between such packaging and filling in a short period of time, but problems such as poor repeatability and zero drift can be found after half a year and a year. On the right is the anatomy diagram of the thermal flowmeter probe of SIERRA in the United States. The speed sensor of Sierra is currently the only real 'dry' sensor in the world. Its unique packaging process makes the perfect filling between the speed sensor and the stainless steel inner wall, and does not use organic matter as filler, but platinum iridium inorganic matter. The sensitivity and repeatability of Sierra thermal flowmeter products are optimized. At any time, there will be no cracks in the filling of the speed sensor, and there will be no drift due to cracks, which ultimately greatly improves Measurement accuracy, excellent long-term accuracy maintained. SIERRA is filled with unique nano-scale insulating inorganic materials on the most core components of the thermal type, and then through high-pressure molding to make the nano-scale fillers denser, ensuring that there will be no zero drift for ten years. The two different packaging technologies also result in products of different quality. b. Real-flow calibration technology: First, let's talk about the importance of real-flow calibration of thermal gas mass flowmeters. We have mentioned the thermal measurement principle before. Through the principle, we know that the measurement of the thermal gas mass flowmeter has a great relationship with the thermal conductivity of the measured gas medium. The specific heat capacity of each gas medium is at different temperatures and pressures. are different. It is unscientific to simply rely on normal pressure or negative pressure air calibration and then correct it to obtain a high-performance flowmeter. Moreover, it has been proved that the flowmeter without actual flow calibration measures the non-air medium, and the data deviation is very large. At present, most thermal gas mass flowmeters on the market are calibrated with open-circuit negative pressure air, and then corrected to high pressure conditions, or corrected to other gases, such as argon, carbon dioxide, oxygen, hydrogen, ammonia, etc. Calibration correction will inevitably lead to measurement accuracy can not be guaranteed, the meter after low pressure calibration is used in high pressure conditions, the meter after air medium calibration is used in other gas media, the measurement accuracy cannot be guaranteed.

Beijing Sincerity Automatic Equipment Co., Ltd is considered as one of the leading supplier of mass flow meter products in China.

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Equipping mass flow meter with innovative technology and updated processes will simplify daily compliance duties so that they can focus on attracting, retaining, and developing the most engaged workforce possible.