Applications and selection of thermal gas mass flow meters

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Thermal mass flow meters are being recognized and used by an increasing number of users due to their unique and excellent properties, and they are now employed in many applications for measuring gas flow rates. However, there are also many problems and misconceptions in these applications. irresponsible marketing by various manufacturers, along with quality defects in the products, create numerous obstacles and issues in their use. This has led to misunderstandings among many users regarding thermal gas mass flow meters; some believe blindly that such meters can measure gases under all operating conditions, while others reject and oppose them. Over the past few years, I have been dedicated to promoting and publicizing thermal gas flowmeter products. I have come into contact with flowmeters from various domestic and international brands, and encountered many application-related issues during this promotional work; as a result, I have accumulated a large number of cases. By analyzing these cases, I have gained some insights of my own, which I would like to share with all of you. This article will provide professional answers to various questions, helping thermal engineers in different industries choose the right thermal mass flow meter. We will explain it in three aspects: First, a correct understanding of the principle of thermal gas mass flow meters. Thermal gas mass flow meters originated from hot-wire anemometers. There is a proportional relationship between the temperature of the platinum heating wire and changes in fluid flow rate: the faster the flow rate, the more heat is carried away. Below is a brief explanation of why the mass flow rate of a gas can be determined by detecting changes in heat. Formula: H = m × Cp × ΔT. The heat quantity H measured, with the temperature difference ΔT kept constant, allows for the direct calculation of the mass flow rate m of the gas, given a constant specific heat capacity Cp. Next, we will explain in detail from a microscopic perspective why direct measurement of the gas mass flow rate is necessary. As shown in the figure, gas molecules come into contact with the heated wall to facilitate heat conduction, thereby removing heat from the probe. Since different gas molecules have varying capacities to carry away heat, knowing the thermal conductivity of the gas molecules (i.e., the value of Cp), it is possible to determine the number of gas molecules flowing through by measuring the power dissipated; this number of molecules corresponds to the mass flow rate of the gas. If we rearrange the formula H=m×Cp×ΔT, we obtain: m=H/Cp×ΔT. Here, m represents the mass flow rate of the gas, H is the amount of electrical power supplied for compensation, Cp is the specific heat capacity at constant pressure, which varies depending on the type of gas, and ΔT is the temperature difference between the two probes. From this formula, two different thermal principles can be derived: one involves keeping the temperature difference in the denominator constant, and by measuring in real time the amount of heat removed by the gas (H), the mass flow rate of the gas can be determined; this is the principle behind the constant-temperature-difference thermal mass flow meters that are commonly used in the market today. If the heat H of the molecule is kept constant, and the magnitude of the temperature difference is measured in real time to determine the mass flow rate of the gas, this is known as constant-power thermal method. The constant temperature difference principle exhibits excellent low-flow characteristics, maintaining good linearity even at mass flow rates as low as 0.1 Nm/s. It also has the ability to respond quickly. The constant power principle can detect relatively large changes in flow rate, performing very well in applications with high flow rates; however, its performance at low flow rates is not as good as that of the constant temperature difference method, and its response time is also worse compared to the constant temperature difference approach. II. Correct understanding of the excellent performance of thermal gas mass flow meters A high-quality thermal flow meter must possess two main characteristics: excellent probe manufacturing technology and a complete real-flow calibration system. Let’s analyze these two key features separately: a. Probe manufacturing process: Earlier we discussed the principle of thermal mass flow meters. From this principle, it is clear that the probe is the core of an entire thermal gas mass flow meter; its performance determines the meter’s accuracy, repeatability, service life, and performance at low flow rates. The probe is composed of a platinum resistance wire and a stainless steel sheath. The platinum heating wire conducts electricity, and the stainless steel insulation also conducts electricity; therefore, the filler material between them must have excellent thermal conductivity but no electrical conductivity. This leads us to the core element of all thermal flowmeters – the filler material for the heating probe and the encapsulation process: the thicker the filler layer, the better its insulating properties, but the worse its thermal conductivity, as well as its sensitivity to temperature; moreover, there is a delay in response. If the filler is organic, it tends to age, causing cracks in the filler, which results in zero drift in the flow meter. If there is tiny air in the filler, the probe remaining in a heated state causes this tiny air to expand, resulting in zero-point fluctuations. In traditional thermal mass flow meters, the speed sensor that is heated is enclosed at the end of a stainless steel tube probe, and a mixture is filled between the sensor and the inner wall of the stainless steel tube. This mixture must be electrically insulating while also having a low thermal resistance; common materials for this purpose include epoxy resins, ceramic cements, heat-resistant pastes, or aluminum oxide powder, magnesium oxide powder, etc. These “wet” sensors that use the aforementioned fillers have certain drawbacks: for example, their surface thermal resistance increases over time as they are used, which causes the output curve to show a downward trend, leading to a reduction in the sensor’s sensitivity and ultimately affecting the accuracy of measurements. “The filler of the “wetness” sensor, due to its different coefficient of thermal expansion from that of the speed sensor, undergoes aging and cracking over time as a result of prolonged use; this ultimately leads to inaccurate measurements by the sensor, and it becomes difficult to maintain accurate performance over the long term. With such encapsulation and fillers, no difference can be seen in the short term, but after half a year or a year, issues such as reduced repeatability and zero drift can become apparent. On the right is an anatomical diagram of the thermal flow meter probe from SIERRA in the United States; Sialle’s speed sensor is currently the only true \"dry\" sensor available in the world. Its unique encapsulation process ensures perfect filling between the speed sensor and the stainless steel inner wall, using inorganic materials such as platinum iridium instead of organic fillers. The sensitivity and repeatability of Sialer thermal flowmeter products are optimized; there are no cracks in the filler inside the velocity sensor at any time, nor is there any issue of drift due to cracking. This results in a significant improvement in measurement accuracy, as well as excellent long-term precision. SIERRA uses a unique inorganic filler with nanoscale insulating properties in its core components of the thermal type; high-pressure molding is then applied to make these nanoscale fillers even more compact, ensuring no drift over a period of ten years. Two different packaging technologies also result in products of varying quality. b. Actual flow calibration technique: First, let’s discuss the importance of actual flow calibration for thermal mass flow meters. As we mentioned earlier, the principle of thermal measurement indicates that the operation of a thermal gas mass flow meter is closely related to the thermal conductivity of the gas being measured. The specific heat capacity of each gas varies at different temperatures and pressures. It is not scientific to rely solely on normal or negative pressure air calibration followed by corrections in order to obtain high-performance flowmeters. Moreover, practice has shown that flowmeters without real-flow calibration yield significant data deviations when measuring non-air media. Most thermal gas mass flowmeters available on the market currently use open-circuit negative pressure air for calibration, and then the readings are adjusted to high-pressure conditions or to other gases such as argon, carbon dioxide, oxygen, hydrogen, ammonia, and so on. Adjusting the calibration inevitably results in an unguaranteed measurement accuracy; gauges calibrated for low pressure should not be used in high-pressure conditions, and those calibrated for air should not be used with other gases, as the measurement accuracy cannot be guaranteed in such cases. III. Proper selection of thermal gas mass flow meters: Data shows that improper selection of flow meters accounts for approximately 70% of the failures that occur in flow measurement systems, while issues related to the quality of the flow meters or other factors account for 30%. This highlights just how important proper selection is for accurate flow measurement. Users of thermal gas mass flow meters should be particularly careful not to select a particular model of flow meter on their own, unless they are replacing an existing one; it is best to leave this task to the seller, as they are the professionals in this field. You only need to provide as complete operational data as possible along with some technical requirements. For example, the flow rate range (it is recommended to use cubic meters as the unit for flow rate), the medium being measured, the diameter of the pipeline, the material of the pipeline, operating pressure, operating temperature, whether explosion protection is required, the operating temperature environment for the flow meter, requirements regarding signal power supply, accuracy grade, the application in a particular process (whether it is for process control or trade measurement), and the level of dryness and cleanliness of the medium being measured. When measuring the flow rate of a mixed gas, it is necessary to provide the percentage values of each component. One final reminder: if conditions permit on-site, choose an integrated flow meter. In the case of a split design, since the signal transmission cable is part of the measurement bridge and has its own resistance value, no matter how well it is designed, the low resistance value of the measurement probe means that the resistance of this cable will inevitably affect the operations and processing carried out by the electronic circuitry. Additionally, due to the effects of the operating environment, this resistance value can drift. If a split type has to be chosen, it is recommended that the distance between the two parts be as short as possible. If the gas being measured is a medium other than air, it is recommended to choose thermal mass flow meters that have been calibrated using actual gas flows abroad; this point must be kept in mind. Of course, unless you have low requirements for the measured data and don’t care much about the degree of deviation, merely wanting to observe the flow trend, you can opt for those lower-end thermal mass flow meter products. Be more careful with products sold at low prices through dumping. Calculate the basic costs to determine whether a flow meter can ensure product quality. There is one fundamental cost that cannot be ignored; the calibration cost for a gas flow meter with a diameter of DN50 or less is around 1,000 yuan (fees vary slightly depending on the local metrology institute). For DN100 and below, it is 1500 yuan. For DN100 and above, the calibration cost increases by about 1,000 yuan for each additional pipe diameter size. The most crucial thing when choosing a thermal gas mass flow meter is to learn as much as possible, understand the measurement principles and the performance of the products, engage in in-depth discussions with the sales staff from the manufacturers, and identify the characteristics of different brands. Only then can one establish a solid foundation for themselves and make the right choice of thermal mass flow meter product that meets their requirements. (Transferred from Haichuan Chemical Forum; copyright belongs to the original author)