Key points
- Thermal dispersion meters measure heat carried away by flowing gas.
- Liquids have far higher heat-transfer coefficients, so the heated sensor saturates at low velocity.
- Specialised low-flow thermal sensors for liquids exist in lab and microfluidic devices, but not for industrial pipe sizes.
- Liquid mass flow: Coriolis. Conductive liquid volume: electromagnetic. Clean non-conductive liquid: turbine or ultrasonic.
How a thermal mass flow meter works
A thermal dispersion meter has two temperature sensors: a reference RTD that reads gas temperature and a heated RTD held at a fixed temperature difference above it. As gas flows past, it carries heat away from the heated sensor. The power required to hold ΔT constant rises with the number of gas molecules passing per second. That is mass flow, which is why the meter needs no temperature or pressure compensation (background).
Why liquids break the measurement
- Saturation. Water's convective heat transfer is orders of magnitude higher than air's. At even modest velocity the heater cannot hold ΔT, and the output flattens.
- Property drift. Viscosity and thermal conductivity of liquids change strongly with temperature, so the calibration does not transfer between conditions.
- Bubbles and deposits. Entrained gas and fouling on the sensor change heat transfer unpredictably.
- Calibration. Supmea's thermal meters (SUP-MF, SUP-FTM210) are calibrated on gases such as air, N₂, O₂, Ar and CO₂, and their documentation excludes liquids, steam and two-phase flow.
What to use instead
| Liquid | Recommended meter | Why |
|---|---|---|
| Any liquid where you need mass (batching, blending) | Coriolis FCC-300 | Direct mass, density and temperature; up to ±0.1% |
| Water, wastewater, acids, slurries (≥50 µS/cm) | Electromagnetic LDG-SUP-A100 | No pressure loss, no moving parts |
| Clean oils, fuel, glycol, DI water | Turbine SUP-LWGY | Independent of conductivity, compact |
| Low-viscosity liquids and steam in one meter type | Vortex SUP-LUGB | No moving parts, wide temperature range |
Where thermal mass meters are the right choice
For gases they are often the best value: compressed air audits, aeration air to diffuser grids, nitrogen blanketing and natural gas sub-metering. A single SUP-MF insertion probe covers DN65 to DN1000 at 0.1–100 Nm/s.
Related questions
Can a thermal mass flow meter measure steam?
No. Steam condenses on the sensor and behaves as a two-phase fluid. Use a compensated vortex meter.
What about thermal flow switches in liquid lines?
Thermal flow switches give a flow/no-flow signal in liquids, which is a different task from measuring flow rate.
Is a Coriolis meter more expensive than a thermal meter?
Usually yes, per measuring point, but it is the instrument that actually measures liquid mass flow.
Sources and further reading
- Thermal mass flow meter (Wikipedia)
- Supmea thermal mass flowmeter range
- Mass flow meter / Coriolis (Wikipedia)
Product specifications quoted here are summarised from Supmea datasheets. Confirm the figures for your exact model before final design.