Knowledge||Application and selection of thermal gas mass flowmeter

Due to its excellent characteristics, thermal gas mass flowmeters are being recognized and used by more and more users, and have been used in many gas flow measurement occasions. However, there are many problems and misunderstandings in these applications. Many manufacturers' irresponsible misleading propaganda and product quality defects have caused many obstacles and blind spots in the use of products, causing many users to misunderstand thermal gas mass flowmeters. Some blindly think that thermal flowmeters can measure gases under all working conditions, while others reject and oppose thermal flowmeters. I have devoted myself to the promotion and promotion of thermal gas flowmeter products in the past few years. I have come into contact with many flowmeters of domestic and foreign brands. I have also encountered many application problems in the actual promotion and accumulated a large number of cases. After summarizing these cases, I get I have learned some of my own insights, and I am willing to share them with my friends. This article will answer your doubts from a professional point of view, and help thermal workers in various industries to choose thermal gas mass flowmeters correctly. We will elaborate from three aspects: 1. Correctly understand the principle of thermal gas mass flowmeter The origin of thermal gas mass flowmeter comes from the hot wire anemometer. The temperature of the platinum heating wire has a proportional relationship with the change of the fluid flow rate: the faster the flow rate, the more heat is taken away. The following briefly explains why the mass flow rate of gas can be obtained by detecting the change of heat. Formula: H=m×Cp×ΔT Measured heat H, while fixing the temperature difference ΔT, for a gas with a fixed specific heat capacity Cp, the mass flow rate m of the gas can be directly obtained. Next, we will explain from the microcosm why it is the direct mass flow measurement of gas. Picture As shown in the figure, the gas molecules contact the heating wall to complete heat conduction and take away the heat on the probe. Since the ability of different gas molecules to take away heat is different, if the thermal conductivity of gas molecules is known (the Cp value is known), the number of gas molecules flowing through can be directly obtained by measuring the dissipated electric power (the number of gas molecules is also is the mass number), so as to obtain the mass flow rate of the gas. We use the formula: H=m×Cp×ΔT transformation, you can get: m=H/Cp×ΔT Among them: m is the gas mass flow rate, H is the compensated electric power, Cp is the specific heat capacity at constant pressure, which is related to the gas type, and ΔT is the temperature difference between the two probes. From the above formula, two thermal principles can be derived: keep the denominator temperature difference constant The mass flow rate of the gas is obtained by measuring the amount of heat H taken away by the gas in real time. This is the mainstream constant temperature differential heat type in the market. If the heat H of the molecule is kept constant, and the temperature difference is measured in real time to obtain the gas mass flow rate, it is the constant power thermal formula. The principle of constant temperature difference has very good low flow characteristics, even if the mass flow rate is as low as 0.1Nm/s, it has good linearity. In addition, it has the ability to respond quickly. The principle of constant power can detect higher flow rate changes, and has very good performance in high flow rate applications, but the low flow rate characteristics are not as good as the constant temperature difference, and secondly, the response time is relatively poor compared to the constant temperature difference. 2. Correctly understand the excellent performance of thermal gas mass flowmeter 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 flow meters——The gap filling material and packaging process of the heating probe: The thicker the gap filling layer, although the insulation is better, but the thermal conductivity is poor, and the temperature sensitivity is also poor, and the response is lagging. 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

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