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Dec 19, 2018

# Application and selection of thermal gas mass flow meters

Thermal gas mass flow meters are being recognized and used by more and more users due to their unique and excellent characteristics, and are now used in many gas flow measurement applications. However, there are many problems and misunderstandings in these applications. Many manufacturers' irresponsible propaganda misleading and product quality defects have caused many obstacles and blind spots in product use, causing many users to misunderstand the thermal gas mass flowmeter. Some blindly think that the thermal flowmeter can measure the gas under all working conditions, and some reject and oppose the thermal flowmeter. In recent years, I have been committed to promoting and promoting thermal gas flowmeter products, and have contacted many domestic and foreign brands of flowmeters. I have encountered many application problems in actual promotion, and accumulated a large number of cases. Some of my own feelings, willing to come out and share with friends.

This article will answer the doubts from a professional point of view, helping the thermal workers in various industries to correctly select the thermal gas mass flow meter. We will elaborate on three aspects:

## One. Correct understanding of the principle of thermal gas mass flowmeter

The origin of thermal gas mass flow meters comes from hot wire anemometers. The temperature of the platinum wire is proportional to the change in fluid flow rate: the faster the flow rate, the more heat is removed. The following is a brief explanation of why the mass flow of gas can be obtained by detecting changes in heat.

Formula: H=m×Cp×ΔT

The measured heat H is simultaneously fixed to the temperature difference ΔT, and for the gas having a fixed specific heat capacity Cp, the mass flow rate m of the gas can be directly obtained.

Below we explain microscopically why it is a direct gas mass flow measurement.

As shown, the gas molecules contact the heated wall to complete the heat transfer and carry away the heat from the probe. Since the ability of different gas molecules to carry away heat is different, so in the case where 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 It is the mass number) to obtain the mass flow rate of the gas.

We convert the formula: H = m × Cp × ΔT, and we can get: m = H / Cp × ΔT

Where: m is the mass flow rate of the gas

H is the amount of electric power to be compensated

Cp is the constant pressure specific heat capacity, related to the type of gas

ΔT is the temperature difference between the two probes

From the above formula, two kinds of thermal principles can be derived: the temperature difference of the denominator is constant, and 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 type currently on the market. If the heat H of the molecule is made constant, the magnitude of the temperature difference is measured in real time to obtain the mass flow of the gas, which is a constant power type.

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 also has the ability to respond quickly. The constant power principle can detect high 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 the second response time is worse than the constant temperature difference.

## Two. Correct understanding of the excellent performance of thermal gas mass flow meters

A good quality thermal flow meter product must have two main features: a good probe manufacturing process and a complete real-flow calibration device. We analyze these two characteristics separately:

a. Probe manufacturing process: We talked about the principle of the thermal flowmeter. We can see from the principle that the probe is the core of the whole thermal gas mass flowmeter. Its performance can determine the measurement accuracy and repeat of the flowmeter. Saturation, service life, low flow rate characteristics. The probe consists of platinum wire and sheathed stainless steel. Platinum heating wire is conductive, and sheathed stainless steel is also conductive, which requires that the gap between them must have very good thermal conductivity, but not conductive. This leads to the core of all thermal flowmeters - the gap filler material and packaging process for the heated probe:

The thicker the interstitial layer, although the insulation is good, the thermal conductivity is poor, the temperature sensitivity is also poor, and the response lags. If the interstitial is organic, it is easy to age and cause cracks in the filler, and the flowmeter exhibits a zero drift. If there is a small amount of air in the gap, since the probe is always heated, the tiny air expands, causing zero fluctuations.

A conventional thermal mass flowmeter heated speed sensor is packaged at the end of a stainless steel tube probe and a 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, typically epoxy resin, ceramic cement, heat resistant grease or alumina powder, magnesium oxide powder, and the like. These "wet" sensors using the above fillers have some drawbacks: for example, the surface thermal resistance increases with the use time, causing the output curve to decrease, resulting in a decrease in sensor sensitivity and ultimately affecting measurement accuracy. Because the filler of the "wet" sensor is different from the thermal expansion coefficient of the speed sensor, aging, cracking, etc. may occur due to the lengthening of the use time, and eventually the measurement accuracy of the sensor is excessively poor, and it is difficult to maintain long-term accuracy. Such a package and a filler can not be distinguished in a short time, but after half a year, problems such as poor repeatability and zero drift can be found. On the right is the SIERRA thermal flowmeter probe planing diagram. Syrac's speed sensor is the only true “dry” sensor in the world. Its unique packaging process makes the speed sensor perfectly filled with the inner wall of the stainless steel, and does not use organic matter as a filler, but a platinum-ruthenium inorganic substance. The sensitivity and repeatability of the Syracuse flowmeter products are optimized. At any time, there will be no cracks in the filler in the speed sensor, and there will be no drift due to cracking, which will greatly improve the situation. Measurement accuracy, excellent long-term accuracy. SIERRA is filled with inorganic materials with unique nano-scale insulation properties on the hot core components, and then formed by high-pressure molding to make the nano-scale fillers denser, ensuring zero drift in a decade. Two different packaging technologies have also produced different quality products.

b. Real-flow calibration technology: First, let's talk about the importance of real-time calibration of thermal gas mass flowmeters. We have already mentioned the principle of thermal measurement. It is known by the principle that the measurement of thermal gas mass flowmeter has a great relationship with the thermal conductivity of the gas medium to be tested. The specific heat capacity of each gas medium is different in temperature and pressure. are different. If you rely solely on atmospheric or negative pressure air calibration and then modify the way to get a high performance flow meter, this is still unscientific. Moreover, it is proved that there is no flow meter for measuring the non-air medium, and the data deviation is very large. At present, most of the thermal gas mass flow meters on the market use open-circuit negative pressure air calibration, and then corrected to high pressure conditions, or modified to other gases, such as argon, carbon dioxide, oxygen, hydrogen, ammonia and so on. Correction calibration will inevitably lead to measurement accuracy not guaranteed. The table after low pressure calibration is applied to high pressure working conditions. The meter after air medium calibration is applied to other gas media, and the measurement accuracy cannot be guaranteed.

## Three. Correct selection of thermal gas mass flowmeter

It is proved by the data that in the probability of failure of flow metering, the improper selection of the flowmeter accounts for about 70%, and the quality of the flowmeter product or other reasons accounts for 30%. It can be seen how important the selection is to the flowmeter. Users of thermal gas mass flow meters should pay special attention to the fact that they should not select a certain type of flowmeter unless you choose to replace the original flowmeter. It is best to give this work to the seller because they are professional. You only need to provide as much working condition data as possible and some technical requirements. For example, the flow range (recommended flow unit uses the standard), the measured medium, the diameter of the pipe, the material of the pipe, the working pressure, the working temperature, whether the explosion is required, the working temperature of the flowmeter, the signal power requirement, the accuracy level, and what is applied In terms of process (process control or trade measurement), measure the dryness and cleanliness of the medium. To measure the flow rate of the mixed gas, it is necessary to provide a percentage value of each medium.

Finally, I would like to remind you that if the site has conditions, the flowmeter should be integrated. If it is a split, since the transmission signal cable is used as a part of the measurement bridge, it has a resistance value. No matter how excellent it is, because the resistance value of the measurement probe is small, the resistance of this cable is bound to affect. The operation and processing of the electronic circuit, coupled with the influence of the field environment, will cause drift in the resistance. If you have to choose a split, it is recommended that the split distance be chosen as short as possible. If the measurement gas is other non-air gas medium, it is recommended to select a thermal flow meter product with actual flow calibration in foreign countries. This must be remembered. Of course, unless you have low requirements for the measured data, don't care about the deviation, just look at the flow trend, you can choose those low-end thermal flowmeter products. For products that are dumped at a low price, pay more attention to yourself. Calculate the basic cost to see if it is a flow meter product that can guarantee the quality of the product. There is a basic cost that cannot be avoided. The calibration cost of a gas flow meter below DN50 is about 1000 yuan. Different, slightly different). Below DN100 is 1,500 RMB. Above DN100, the calibration cost of each pipe diameter specification is about 1000 RMB. The most important thing to choose the right type of thermal gas mass flowmeter is to learn more, master the measurement principle and the performance of the product, and do more in-depth communication with the product manufacturers to clearly distinguish the characteristics of each brand. In order to be self-sufficient, choose the right type of thermal mass flow meter products that meet your requirements.