Considerations for Choosing a Thermal Mass Flow Meter

4 mn read

Getting the best out of your thermal flow meter demands that you choose the right type for your application. Here is a guide on how to select a thermal mass flow meter. 

Not all meter types work the same way when measuring flow rate. Therefore, it is important to consider several factors before picking the type of meter that works well with your application. This means that you have to do extensive research and understand the operating requirements of your flow system if applicable. The right kind of thermal mass flow meter will allow you to save money as well as time. If you aim to expand your project, make equipment improvements, or upgrade the plant process. 


There are key questions that you need to ask yourself before picking any thermal mass flow meter that, if answered correctly, will allow you to make the right choice. Let’s expound on these questions to help you understand better. 

What level of accuracy are you aiming for in your application?

Checking the general accuracy statement alone is not enough to purchase a thermal mass flow meter. Installed accuracy is also critical. This type of accuracy involves calibration needs, basic accuracy capability, and flow profile disturbances. You need to factor in if the flow meter can compensate for both the installation and gas temperatures. 

Knowing or establishing the thermal flow meter requires actual calibration. In any given situation, the actual gas calibration determines accurate reading results. When performing an actual calibration of the thermal flow meter, it must be within the set process temperature and pressure conditions. If your main focus is repeatability when picking a thermal mass flow meter, then the actual gas calibration is the best option. 

Few gas flow systems won’t work well with an actual gas calibration. The process won’t be economical, practical, or achievable. In such gas flow applications, the answer lies in the equivalency calibration option. This is ideal for applications that require high safety standards or those that involve complicated gas mixtures. 

What gas type are you using in your flow system or application? 

It is crucial to clearly understand the type of gas you intend to use in your gas flow system to select the right type of thermal mass flow meter. Thermal flow meters are designed differently to cater to different types of gases. There are those designed for inert gases, hydrocarbon-based gas, or air. There are also thermal mass flow meters designed to cater to single gas substances or a mixture of gases. That is why it is very important to know the gas constitution. 

If the plant operator aims to use the thermal flow meter to measure a mixture of gases, then the proportions of each gas need to be known. Establishing whether the plant is going to deal with clean or dirty gas is also important. For dirty gas, its quality and quantity have to be established. Does the gas have any moisture, or is it corrosive? These are the questions that need to be answered to allow the manufacturer to suggest the best thermal mass flow meter to use in your flow system. 

Any kind of manufacturer can design ideal flow meters for handling clean, dry gas. If your plant deals with gas that contains moisture, then the best metering technology will be constant power. A thermal mass flow meter uses heat transfer when measuring the flow rate of gases. Readings spike or fluctuations can be due to sudden heat transfer changes brought about by moisture or condensate interacting with the heated sensor. 

The method of measurement used by these types of thermal flow meters is constant ∆T (CT). This technique of gas flow measurement reacts to moisture droplets. The slightly heated sensors with temperatures above the dew point of the gas mean that thermal gas devices using the constant power (CP) method won’t be affected by moisture. When this happens, an inaccurate or unstable flow measurement is created. 

What are the requirements of the flow range? 

One of the most compelling features of many thermal mass flow meter types is the wide turndown capability. A turndown ratio of 100:1 is observed by the majority of thermal mass flow meters suppliers. What separates suppliers and the technology used to create thermal flow meters is flow range capability. The sensor power drawbacks make CT-type technology meters have low range capabilities than CP-type devices. Pushing the measuring range capability to 1000 fps [300 mps] requires special techniques used by manufacturers. 

What makes an ideal response time? 

When using thermal mass flow meters to control flow, faster response time is not always ideal. This applies to other flow meters. Faster response time leads to excessive valve responses. That is if the thermal flow meter uses a PID control loop. When this happens, unstable flow control or valve failure will be experienced. 

It is also not good to have a very slow response time using a thermal mass flow meter. The reason being that there will be less control due to lagging behind control valve action. 

What is the appropriate pipe size and size to use with a thermal flow meter? 

In many applications that use the thermal flow meter, the size of the pipe and the type of pipe you are going to use matter. As a plant operator, you need to know if the application requires a round or rectangular pipe. The size of the pipe is measured in diameter, and if the pipe is a rectangular duct, it has to be measured in terms of dimension. For insertion flowmeter types use, the socket’s dimension needs to be known in the thermal mass flow meter selection. It should be known whether the flow meter is through a ball valve or any other type of valve. 

Conclusion 

These are the basic considerations and factors to be observed when selecting a thermal mass flow meter properly. Proper selection of these types of mass flow meters will not only save you extra expenses of installing and maintaining the flow meter, but it will also ensure that the flow meter functions as expected. 

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