Thermal Mass Flowmeters

Thermal Mass Flowmeters

Thermal Mass Flowmeters measure gas mass flow by quantifying the heat removed from a heated sensor as gas passes by. A common implementation uses two PT100 temperature sensors: one senses the process temperature as a reference, while the second is heated to maintain a constant temperature differential at zero flow. As flow increases, convective cooling rises; the electrical power needed to hold the temperature difference becomes a direct measure of mass flow.

Because the signal is mass-flow based, pressure and temperature compensation is generally unnecessary for many applications, and the output can include both mass flow and gas temperature. High turndown ratios (on the order of 100:1) and strong low-end sensitivity support measurement of both base load and small leakage flows. Fast thermal response also makes these meters effective where rapid changes in demand need to be tracked and controlled.

Thermal meters are frequently deployed as an alternative to DP measurement on low-pressure gas systems where permanent pressure loss is undesirable. With no moving parts, maintenance requirements are low, and long-term stability is typically strong when sensor fouling is managed. Insertion-style designs extend coverage to large ducting or pipe sizes, while inline designs are used when defined geometry and the highest repeatability are required.

Typical applications include compressed air consumption and distribution, natural gas to burners and boilers, and industrial gases such as carbon dioxide, argon, nitrogen, oxygen, and biogas. Aeration air measurement in wastewater treatment is a common use case, where turndown and low-flow sensitivity improve blower control and energy optimization. Thermal mass measurement also supports plant-wide utility auditing programs by converting flow into normalized usage metrics for cost allocation.

Selection focuses on gas type and composition stability, pressure/temperature limits, installation geometry, and exposure to contamination or condensate that can insulate the sensor. Where gas mixtures vary significantly, configuration and calibration strategy should align with expected operating envelopes. For critical custody or highly variable composition service, alternative technologies may offer lower uncertainty, but thermal meters remain an efficient choice for utility and process-gas monitoring.

Forberg Smith, an exclusive authorized representative of sales and service for Endress+Hauser.