Tutorial: Thermal mass flowmeters (insertion type) for gas applications

10/18/2007


In our last issue, we began considering the problems of gas flow measurement, given the problems associated with its compressibility. Mass flow measurement approaches for gas are immune to this problem, and there are two main options: thermal and Coriolis. Thermal mass flowmeters are based on the understanding that a given mass of fluid will remove a known amount of heat from a given body. While some designs handle liquids as well as gasses, liquid applications are far less common for this sensor type.

Within the thermal mass flowmeter technology, there are two approaches: the in-line type and insertion type. We will consider the in-line type in a future issue. Insertion type sensors use a probe that is inserted through the pipe wall into the process gas line. The probe includes a heating element and multiple temperature sensors, typically RTDs. One sensor measures the temperature of the ambient gas stream. Another measures the temperature of the heating element.

A sensor uses one of two approaches. One feeds a specific amount of current into the heating element and calculates flow by measuring how much lower the actual temperature is than it should be for the amount of current. The other heats the element to a specific temperature, and calculates flow by measuring how much current it takes to maintain that temperature. The current levels and temperature differences provide the data to calculate mass flow. Most designs are relatively precise and offer accuracy in the

This technology has some practical application considerations. (These are generalities, so discuss specifics with your supplier.)

--Since the design uses a heating element, it operates continuously (or at least with long on periods) at high power levels, so battery powering isn’t a practical option.

--Given the probe size, insertion designs are best with relatively large pipes and ducts.

--The nature of the measurement is for a small section of the gas stream. The normal practice is to place the sensing point in the center of the pipe and the final flow calculation is based on normal flow profiles for that size pipe. This means that the probe length has to be adjustable or fixed for a specific pipe size. It also means that turbulence has to be minimal, which calls for flow stabilizers or long sections of straight pipe up and downstream.

--Some probe designs depend on careful axial, or yaw, positioning. If the probe is twisted relative to the gas stream, the reading can be affected.

--For large ducts or where turbulence is unavoidable, some suppliers offer probes with multiple sensors. These take readings at a range of points across the flow profile and correct for poor gas distribution.

--Dirty and corrosive gas streams can leave deposits on critical surfaces and interfere with measurements or damage fragile sensing points. Some designs are more tolerant of these than others, so discuss these potential problems with your suppliers.

Thermal mass flowmeters are available from:

And others. Consult the Control Engineering Supplier Search for more options.

—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Weekly News





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