A thermal mass flow meter is a meter designed to accurately measure the flow of gas through a pipe using the principle of conductive and convective heat transfer. This has numerous industrial applications and has specific advantages compared to other types of flow meters (such as positive displacement flow meters which measure volume).

In this article, you'll learn the fundamentals of how these devices work. We'll also take a look at different type of thermal mass flow meters and the advantages of using them. What follows is not meant to be an exhaustive treatment of the topic. Rather, consider it an overview.

How Do Thermal Mass Flow Meters Work?

When measuring flow, there are various aspects that can be monitored. For example, a velocity flow meter works by measuring the speed at which a given liquid or gas is traveling. Other devices measure force, which is often correlated with pressure.

Whereas volume describes the space taken up, mass describes the size and density, and takes weight into account. As gas can easily spread out to fill any volume, measuring it by mass makes more sense. Using heat is just one way to measure mass.

Thermal mass flow meters apply heat to the target gas using a heating element. The devices then measure the extent to which the heat dissipates as it passes through temperature sensors.

There will normally be a sensor just before the heating element as well as a sensor afterwards. The sensors can thus measure the change once the heat is applied. The thermal properties of the fluid or gas will have an impact on this change. But once this is known, it's possible to calculate the mass. Further, the variation between fluids and gasses is relatively small to begin with.

Types Of Thermal Mass Flow Meters

There are two main concepts used in thermal mass flow meters. Constant temperature differential flow meters use the two sensor configuration mentioned above – with a heated sensor and a sensor measuring the temperature of the gas. Here, the mass flow rate is computed based on how much power is required to maintain a constant difference between the two sensors.

The other method is constant current. Here, there is a heated sensor and a sensor measuring the temperature of the flow stream itself. The heated sensor receives a constant amount of power. Mass flow is a function of the difference between the flow stream and heated sensor. Both methods take into account that higher velocity of flow means greater cooling.

The Benefits Of Thermal Mass Flow Meters

Thermal mass flow meters provide high accuracy and repeatability compared with other methods of measurement. They are a very reliable means for measuring the flow of mass. They are also compact. Further, when combined with a PID controller and integral valve, they can be used to accurately control the flow of gases at varying flow rates.

Thermal mass flow meters have no moving parts and don't obstruct the flow path of the gas or liquid. In addition, they require no temperature or pressure corrections.

There are many different applications for thermal mass flow devices. The devices are commonly found in hydraulic systems tests, ventilation and aircraft manufacturing. Other markets that use them include biotech, electronics and the automotive industry.

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