Thermal flowmeters

Thermal flowmeters are used for measuring gases. The area of application is extremely broad. There are two different types which have different fields of application: bypass and immersion flowmeters. Bypass thermal flowmeters are only suitable for measuring small gas flows. Immersion flowmeters are more often used in industrial environments in pipe diameters up to about 8 inches.

Immersion Thermal Flowmeters

The immersion gas flowmeters are often the ideal solution for more "industrial" applications. Say in 1 inch lines and larger. For the 1-8 inch pipes it is best to choose for an in-line model. In pipelines from 4 inches, a plug-in flowmeter can be considered. Incidentally, one can also place plug-in flow meters in rectangular pipes ("ducts"). And in certain cases, one can choose to place more measuring points in one pipe and then determine the average in a central transmitter. These are the so-called multi-point instruments.

The accuracy specification of the immersion thermal flowmeter is good, especially for the inline versions with built-in flow conditioning. The robust immersion flowmeters are more suitable for bulk gas measurements.

The measurement principle of a thermal immersion flow meter

An immersion flowmeter has two thermal sensors. These sensors are in the tube, where the gas passes. One of the sensors is heated and the other measures the temperature of the heated sensor. The flowing gas cools the heated sensor. The higher the flow, the stronger the cooling. Industrial thermal flowmeters come in two types:

Constant current

In this type, a constant amount of energy is supplied to the sensor, which heats the sensor. At 0-flow or low gas velocities, the sensor becomes relatively warm. The advantage is that this technique is relatively simple.

Constant temperature anemometry

In this variant the temperature difference between the reference and the heated sensor is kept at a constant level. When the cooling of the flowing medium is higher, more energy is added. This technique is more complicated but has higher accuracy and shorter response time. In the following article you can learn more about the principle of this type of flowmeter.

Applications with liquids

Thermal flowmeters have also been developed for liquids, which are then almost always used for very low flow rates. The main reasons why thermal flow meters are generally less suitable for liquids are: the higher thermal capacity of liquids, but also factors such as viscosity, Re-numbers and flow profiles. The thermal concept is widely used in flow switches for liquids (and gases). Precisely because this measurement method is so sensitive to flow changes in the lower range, thermal flow switches are applicable to low-flow detection for purposes such as pump and dry-running protection.