Thermal Flow Switch

A thermal flow switch uses the same principle as a thermal mass flowmeter, reduced to a switch. Two sensors sit in the process; one is heated, and the temperature difference between them depends on how fast the medium carries heat away. Flow, or the presence of liquid at the probe, changes that difference and trips the output. With no moving parts, the switch is immune to fouling, wear and vibration and works equally on liquids, gases and slurries.

Insertion lengths range from 1.2" to 18" with threaded, flanged, sanitary Tri-Clamp or retractable packing-gland mounting. Ranges cover liquids from 0.01 to 5 ft/s and gases from 0.25 to 120 Nft³/s, with additional model-dependent ranges to 20,000 cc/s for very low gas flows. Process limits are 241 bar and −100 to +454 °C. Wetted parts are 316L stainless steel, Hastelloy C, Monel 400 or titanium, with a sanitary finish down to 20 Ra for food and pharmaceutical service.

Outputs are SPDT or DPDT relays or an analogue signal; options include explosion-proof enclosures, dual-port housings, a calibration mode and fail-safe logic.

The same probe serves as a flow switch in pipes, a level or interface switch in vessels, and a temperature alarm. Users include oil and gas, chemical, power generation including nuclear, food and pharmaceutical, mining and water treatment, often for pump protection, seal-flush monitoring and dry-run prevention.