Principle of operation for thermal flow switches

Thermal flow sensing technology relies on the cooling effect of flowing liquids. A higher flow velocity results in a greater cooling effect and this can be reliably measured. The images below depict heat being conducted from the sensor tip by a flowing liquid.

The sensor tip houses two platinum RTD elements and a heat source. When power is applied, the tip of the sensor is heated. The difference in temperature between the two RTDs is held constant (Δ 4 °C) by varying the amount of power applied to the heater. The flow rate is determined as these RTDs react to the physical effect of a flowing medium conducting heat energy away from the sensor tip.

This temperature-based operating principle can reliably sense the flow of virtually any liquid or gas. The general rule of thumb is “if the medium is pumpable, the flow is detectable.”

The flow measurement is in units of velocity (ft/sec) and it is measured at a single point in the pipe. When the pipe size is known, velocity can be calculated in units of volumetric flow rate (gpm, lpm, etc.) by multiplying the velocity by the cross-sectional area of the pipe.