Understanding Immersion Thermal Flowmeters

Operating principle thermal flowmeters

Insertion thermal flowmeters are also called "full bore" thermal flowmeters or industrial thermal flowmeters. These flow meters have 2 thermal sensors that insert into the pipeline in the flowing gas. One sensor measures the temperature of the gas, the other is heated by a built-in resistance wire. Because the heated sensor has a higher temperature than the environment it can cool down. The amount of cooling depends on the amount of molecules that dissipate kinetic energy. So the higher the speed of the gas, the stronger the cooling. But also the higher the density (due to higher pressure) the stronger the cooling. And the lower the temperature (also higher density) the stronger the cooling.

We supply Fox Thermal flowmeters which use technology that keeps the temperature difference between the two sensors constant. The electrical power required to keep the temperature difference constant is then directly proportional to the mass flow of the process gas. The instrument electronics measure the required power and thus the cooling effect of the gas flow. The microprocessor then linearizes this data to provide a 4-20mA output signal.

Photo: overview of several thermal flowmeters from Fox Thermal.

Large pipes and point measurements

The disadvantage of insertion flowmeters is that they are point meters. They measure the gas mass speed at a specific point in the pipeline. That may be in the middle of the pipe. This makes the flow meter highly sensitive to flow profiles. This need not be a problem if the inlet and outlet lengths are long. But if this is not the case, there are greater uncertainties. Flow conditioning is actually recommended in all cases. Fox Thermal supplies specifically for its thermal flowmeters a flow conditioner which gives a negligible pressure drop.

Another way to solve the problem of a point measurement is a multipoint flowmeter. In very large pipes - such as in the air supply for burner stoves in power plants - a "network" of flow meters is placed, the so-called multipoint flow meter. By averaging the various point speeds, the uncertainty in the flow measurement is reduced. And in this example also an advantage is that there is less pressure drop - for which again must be compensated - compared to venturi differential pressure flowmeters.

Measuring gas mass flow

By definition, the thermal flow meter measures mass flow directly. The great advantage of this is that a temperature and pressure sensor are not required and no pressure and temperature compensation is needed. The measured value can be used directly to control processes. Flow meters measure volumetric flow, expressed in a volume unit per time, for example m3/h. This does not tell you how much gas has flowed through the pipe, because gas expands and shrinks with temperature and also with pressure. These values must therefore be measured in order to know the actual number of gas molecules. And that is important when it comes to chemical reactions such as controlling combustion.

Photo: The inline type thermal immersion flowmeter. The flow conditioner can be seen on the left.

Photo: The insertion type thermal immersion flowmeter.

The advantages of immersion thermal mass flowmeters

• Measures mass flow directly, so temperature and pressure do not have to be measured. This makes installation easier, the flow meter cheaper and the measured value directly usable for controlling processes.
• Especially in large pipe diameters, the advantage is that this flow meter does not work with a meter body, which makes it cheaper. This makes it cheaper.
• Cost-effective: approximately 6 times less expensive than coriolis flowmeters.
• Have almost no disturbing effect on the flow profile.
• No pressure loss that has to be compensated with heavier compressors or fans.
• Less sensitive to contamination than the alternative pitot tube.
• No moving parts like the alternative plug-in turbine.

The limitations of immersion thermal mass flowmeters

• Accuracy is good, but not good enough for calibration measurements.
• Is sensitive to the composition of the gas, which is less certain if it fluctuates.
• Point measurement makes the flowmeter sensitive to flow profile. If it fluctuates, then the measurement is less certain.
• Has no AGA certification and therefore not suitable for natural gas measurements.