eFunda: Introduction to Thermal Flowmeters
engineering fundamentals Thermal Flowmeter
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Thermal flowmeters measure the heat carried away from the sensor by the passing flow to determine the mass flow rate.

Further Information
The primary sensor of a thermal flowmeter is a heated wire or film, typically a platinum or tungsten Resistance Temperature Detector (RTD), that is exposed in the flow. A second sensor, possibly another RTD, is used to measure the flow temperature.

As the flow passes over the hot wire, it carries away heat. The heat loss depends on the mass flow rate, the heat capacity of the fluid, and the temperature difference between the wire and the fluid.

Since the heat capacity of the fluid is known and the temperatures are monitored in real-time, the mass flow rate can be determined from the heat loss (related to the electric resistance of the wire via the Ohm's law) and the thermal expansion coefficient of the wire, as discussed in the hot wire theory section.

Common Specifications

Common specifications for commercially available thermal flowmeters are listed below:

  Fluid Phase:

Score Phase Condition
 Gas  Clean 
 Gas  Dirty 
: Recommended
: Limited applicability

  Line Size: Inline models: 6 ~ 100 mm (0.25 ~ 4 inch)
Insertion models: 64 ~ 1500 mm (2.5 ~ 60 inch)
  Turndown Ratio: 100 ~ 1000 : 1

Pros and Cons

  • Pros:
  - Medium initial set up cost
  - Low pressure drop
  • Cons:
  - Fragile, high repair cost
  - For (clean) gas only

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