Applications and selection of thermal gas mass flow meters

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This post was last edited by *aojiaoya0546 on 2016-6-28 at 19:31. Due to its unique and excellent properties, thermal gas mass flow meters are being recognized and used by an increasing number of users; 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 challenges in their use. This has led to misunderstandings among many users regarding thermal gas mass flow meters; some assume 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 flow meter products. I have come into contact with flow meters from various domestic and international brands, and encountered many application-related issues during this promotion process; 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. I. The historical background of thermal gas mass flow meters: In the 1960s, there were two renowned scholars in the United States named Dr. John Olin and Dr. Jerry Koz. Both graduated from Stanford University and joined TSI in Minnesota. From 1968 to the early 1970s, they worked in TSI’s development department, primarily researching anemometers in the HVAC field. Later, they left TSI and began working together on their own thermal gas mass flow meters, realizing that hot-wire anemometers were not suitable for use in industrial settings. The early products were air anemometers and wind speed sensors, as well as air sampling devices; these formed the foundation of Sierra Instruments’ initial product line. Sierra Instruments was founded in Minnesota in 1973. In 1975, Sierra Instruments moved from Minnesota to California. After working together for a few years, the two parted ways in 1977: Dr. John Olin retained Sierra Instruments and its air sampling products, while Dr. Jerry Kurz established Kurz Instrument, taking over the company’s anemometers and wind speed sensors. Sierra has developed thermal flowmeter measurements for air and gas, including applications for multi-component gases. Sierra thermal meters are also widely used in industrial fields, including the pharmaceutical industry, biotechnology, petrochemicals, and steel manufacturing industries ； It is also used for measuring combustion gas flows, metering natural gas refueling, and monitoring flue gases. Sierra is the only company that covers the entire range of gas flow measurement, from 0.1 milliliters per minute all the way up to mass flow rates of several hundred meters per second. They are only released from the factory after individual, real-time calibration. The most prominent application of Kurz instruments is in power plants. Kurz has developed a series of plug-in single- and multi-point thermal flowmeters for measuring and monitoring waste gas and combustion gas. Kurz meters are also used in refining, cement, papermaking, and other process industries. Around the same time as Sierra and Kurz emerged, there was another famous company in the United States: FCI. FCI Company was founded in 1964; initially it manufactured thermal switches in California to determine whether the oil transported in oil fields was flowing or stationary, not to measure flow rates. It was not until 1981 that true flow meters were introduced ； That year, FCI began installing a large number of electronic components on switchboards, enabling it to introduce the first thermal gas flow meter. Later, some employees who left the three companies—Sierra, Kurz, and FCI—each founded their own companies specializing in the production and research of thermal mass flow meters. These companies gradually had an impact and spread to other regions around the world. II. Principle and Structure of Thermal Gas Mass Flow Meters Thermal gas mass flow meters operate on the principle of thermal diffusion. Thermal diffusion technology offers excellent performance and high reliability under harsh conditions; its typical sensing elements include two thermistors (platinum resistance wires), one of which serves as a velocity sensor while the other is used to measure the temperature of the pipeline. When these two sensors are placed in a gas, the temperature sensor measures the temperature of the gas inside the pipe, while the velocity sensor contains an electric heating wire that is heated to a temperature higher than that measured by the temperature sensor (which corresponds to the temperature of the gas in the pipe), and this temperature is maintained at a constant level. The airflow carries away the heat from the speed sensor, reducing the temperature difference. To maintain a constant temperature difference, additional electrical power is required to compensate for the heating, and the amount of this compensating electrical power is proportional to the mass flow rate of the gas. Thermal flow sensors used in industrial pipelines have speed probes and temperature probes both made of platinum resistors; they feature a robust protective casing. Since the platinum wires do not come into direct contact with the fluid, these sensors are widely used. The protective casing is generally made of 316L material, while for corrosive gas environments, Hastelloy or tantalum materials are used. Next, we will explain in detail the measurement principle of thermal flowmeters using formulas, and clarify why thermal flowmeters are mass flowmeters. Formula: H = M × Cp × ΔT. By measuring the heat amount H and keeping the temperature difference ΔT constant, the mass flow rate M of the gas can be determined directly for a gas with a constant specific heat capacity Cp. Let’s take a look at why gas mass flow measurement is used from a microscopic perspective: gas molecules come into contact with the heated wall, facilitating heat conduction and thereby removing the heat from the probes on the sensor. 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. The number of molecules corresponds to the mass, and thus the mass flow rate of the gas can be determined. A stainless steel sleeve is used to encapsulate the platinum heating wire, thereby addressing issues related to its lifespan and corrosion resistance. This has **expanded the application areas of thermal heating systems; however, this process is quite challenging.** The platinum heating wire conducts electricity, and the stainless steel coating also conducts electricity; this means that the material used to fill the gap between them must have excellent thermal conductivity but no electrical conductivity. This leads to the importance of the filler material for the heating probe and of the sealing process in all thermal flow meters. Looking at all the manufacturers of thermal flowmeters around the world, few brands truly solve this problem. Domestic manufacturers use organic materials such as epoxy resin, ceramic cement, heat-resistant pastes, aluminum oxide powder, and magnesium oxide powder. Sensors made from these materials have certain drawbacks; for example, their surface thermal resistance increases over time as they are used, which causes the output curve to decline and reduces the sensitivity of the sensor, ultimately affecting its accuracy. Using these materials has the drawback of reducing the sensor’s lifespan and increasing its failure rate. Many domestic manufacturers have also analyzed a large number of products from foreign brands and carried out extensive research and development efforts. However, due to insufficient scientific research capabilities, as well as limited expertise among R&D personnel and inadequate funding, effective solutions have not been achieved yet. As a result, their products fall far short of the advanced standards of foreign counterparts, which limits their practical application. Dr. John O’Leary of the American company SIERRA, along with his team, through decades of meticulous effort and perseverance, finally succeeded in 1999 in developing a patented, revolutionary technology for measuring gas mass flow rate – the drift-free “Dry Sensor”. The patented Dry sensor technology utilizes unique inorganic fillers with nanoscale insulation properties; high-pressure molding is then applied to make these nanoscale fillers even more compact, ensuring that the sensor experiences no drift over a period of ten years. For thermal mass flow meters, although they feature highly advanced measurement principles and manufacturers employ scientific and standardized quality control methods, achieving excellent technical performance still requires a scientific and rigorous real-flow calibration technique. We know the principle of the thermal type: the mass of the fluid is proportional to the electrical power used for heating, and it is also closely related to the specific heat capacity of the fluid medium being measured. The specific heat capacity of each gas medium varies at different temperatures, and the differences are even greater for some gas media. It is not scientific to rely solely on normal-pressure air calibration followed by corrections in order to obtain high-performance flowmeters. Moreover, it has been proven that flow meters without actual calibration yield significant data deviations when measuring non-air media. In this regard, no domestic company possesses real-flow calibration technology; all of them use open-loop low-pressure air calibration. Therefore, when applying to non-air media, if high requirements must be met, please try to avoid choosing products calibrated for air. III. Applications and Selection Criteria Characteristics of thermal gas mass flow meters: a. Extremely wide range: Thermal gas mass flow meters possess an advantage that no other type of flow meter can match——an extremely wide measurement range, enabling them to detect fluid flow speeds as low as 0.05 Nm/s and as high as 100 Nm/s (some brands can even handle flow speeds above 200 Nm/s). b. True mass flow meters: Thermal gas mass flow meters differ from other gas flow meters in that they do not require pressure and temperature corrections; they measure the mass flow of gas directly, making them true direct-type mass flow meters. c. No voltage stabilization compensation device: When measuring gas flow, thermal mass flow meters do not require pressure and temperature sensing devices, as is the case with other derived flow meters. **It simplifies the configuration of on-site equipment, reducing the frequency of failures and repairs. d. Low pressure loss: Thanks to its special sensor design, the thermal mass flow meter **reduces the pressure loss caused by the fluid flow sensor, and can be used in pipes of any shape with a known cross-sectional area. e. Easy installation: The thermal mass flow meter requires very little installation work. Thanks to its insert-type design, it can be installed and maintained online, **thereby reducing construction and maintenance costs.** f. Diverse measurement methods: Thermal mass flow meters can offer solutions such as single-point measurement and multi-point measurement, depending on the actual operating conditions and measurement requirements in the user’s facility. Single-point measurement involves using a single flow meter to directly measure the actual gas flow rate in the pipeline. Multi-point measurement is primarily used for flow measurement in large pipelines; the signals from multiple flow meters distributed across the pipeline cross-section are fed into a flow controller for processing, thereby obtaining an accurate flow value. This has great applicability for large-diameter fluid measurement, where corporate technological upgrades and process limitations lead to flow field instability. Numerous experiments over the years have shown that thermal gas flow meters exhibit good output characteristics; they have high sensitivity, especially at low flow rates, with a flow measurement range of 100:1 or even higher. There are specific application scenarios in the field of gas flow measurement, but there are also many unresolved problems. The issue of low power consumption: since thermal probes require a certain amount of heating current, it is difficult to achieve low-power operation using battery power. Regarding the impact of humidity and correction issues, since it is difficult to regulate and detect humidity levels, there are currently no effective methods for experimental data on the impact of humidity on thermal flowmeters or for corresponding correction techniques. The issue of dust interference: if a thermal probe operates for an extended period in an airflow environment containing dust (mainly wet dust, as dry dust hardly adheres to the probe), its surface will become contaminated, and its heat dissipation properties will change, which in turn affects the output characteristics of the flow meter. However, the exact extent of this impact cannot yet be described with numerical data. Another limitation is that it cannot be applied in situations involving changes in multiple media. For operating conditions with a mixture of multiple media, if the proportions of each medium remain constant, the requirements can still be met through the use of correction factors. Additionally, if there are significant variations in the operating temperature on site—for instance, changes exceeding 100 degrees—using a thermal gas flow meter may result in a loss of accuracy. Therefore, careful consideration is required when making this choice. How to select a thermal gas mass flow meter? Data shows that improper selection of flow meters accounts for approximately 70% of the failure rates associated with flow meter operation. It can be seen how important selection is for flow measurement. Users of thermal mass flow meters should take special care not to select a particular model of flow meter on their own. Unless you intend to replace an existing flow meter, it is advisable to leave this task to the seller, as they are 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 class, the application in a particular process (whether it is for process control or trade measurement), the degree of dryness and cleanliness of the medium being measured; when measuring the flow rate of mixed gases, it is necessary to provide the percentage composition 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. The above are some of my experiences over the past few years; I hope they can be helpful to everyone. If there are any issues you’d like to discuss, you can do so privately.