Thermal Mass Flowmeters

  • Thermal mass flowmeters use heat to measure mass flow. These meters are bypass or shunt flow meters. The meter uses a probe to introduce heat to the fluid, at midpoint of the sensor tube. RTDs are located upstream and downstream of the meter to measure the fluid temperature. The temperature variation (or ΔT) between the two measurements is proportional to mass flow. As the fluid velocity increases, the ΔT goes to zero. These types of meters are most accurate in low flow settings, and available as pressure or flow controllers.

  • The typical accuracy of a thermal mass flowmeter is:
    - Liquid: ±0.5% of full scale
    - Gases: ±1.0% of full scale
    These types of meters have very good repeatability, and provide a compact flow metering option that can control very low flows over a wide range (50:1). A key advantage of this type of meter is the ability to receive a mass flow measurement without the need for pressure or temperature correction. This assumes a constant fluid specific heat and thermal conductivity.
  • Common Liquid Applications
    Common Gas Applications
    Pilot plant low-flow control
    Semiconductor doping flows
    Dosing such as odorants, dyes, enzymes, flavorings, etc.
    Laboratory gas blending

    Adhesives
    Purge flows
    Drug/I.V. Flows
    Combustion control leak detection
  • The limitations or disadvantages of this type of meter are:
    - Has lower accuracies at higher flow rates
    - Can show significant errors due to ambient temperature changes
    - Can experience measurement drift caused by the coating of the “hot wire” (due to this possibility, upstream filters are sometimes required with this meter installation)
    - Response rate is typically in seconds for gasses
    - Typically requires factory calibration annually