Perhaps the greatest disadvantage of thermal mass flowmeters is their sensitivity to changes in the specific heat of the process fluid.
This makes the calibration of any thermal mass flowmeter specific for one composition of fluid only. In some applications such as automotive engine intake air flow, where the fluid composition is constant, this limitation is not a factor.
In many industrial applications, however, this limitation is severe enough to prohibit the use of thermal mass flowmeters.
Industrial applications for thermal mass flowmeters include natural gas flow measurement (non-custody transfer), and the measurement of purified gas flows (oxygen, hydrogen, nitrogen) where the composition is known to be very stable.
Another (potential) limitation of thermal mass flowmeters is the sensitivity of some designs to changes in flow regime. Since the measurement principle is based on heat transfer by fluid convection, any factor influencing the convective heat-transfer efficiency will translate into a perceived difference in mass flow rate.
It is a well-known fact in fluid mechanics that turbulent flows are more efficient at convecting heat than laminar flows, because the “stratified” nature of a laminar flowstream impedes heat transfer across the fluid width.
In some thermal flowmeter designs, the walls of a heated metal tube serve as the “hot” element cooled by the fluid, and the difference between the rate of heat transferred by a laminar flowstream from the walls of a heated tube versus a turbulent flowstream can be great.
Therefore, a change in flow regime (from turbulent to laminar, and vice-versa) will cause a calibration shift for this design of thermal mass flowmeter.
Thermal Mass Flow Meter Advantages
Measure gas mass flow rate directly
Suitable for applications where temperature and pressures fluctuate
Highly accurate and repeatable measurements with a typical accuracy of ± 1% FS
Able to measure accurately low gas flow rates or low gas velocities
Excellent turn down ratio, typically 50:1
No moving parts
Thermal Mass Flow Meter Limitations
Gas mass meter use is limited to clean, non abrasive fluids
Presence of moisture or droplets can lead to measurement inaccuracy
Thermal properties must be known: variation from calibrated values can cause inaccuracies
Relatively high initial cost
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Thermal Flow Meter Applications
Thermal mass flow meters are suitable for a variety of processes requiring mass flow measurement and are frequently used for the regulation of low gas flows.
Some common gas flow applications for thermal mass flow meters include:
Compressed air flow and distribution
Natural gas consumption eg for burner and boiler feed control
Monitoring and control of stack or flue gas (where composition known)
Landfill gas recovery
Flare gas measurement
Gas flow mixing & blending
Gas leak testing and detection