Thermal mass flow controllers are used in gas flow applications and utilize heat to measure flow. The heat is introduced into the flow and the dissipation is then measured using one or more temperature sensors. The operation of thermal flow control is attributed to L.V. King (1). King’s Law revealed how a heated wire in a fluid flow measured the mass velocity at a point in the flow.
A flow controller's accuracy is typically expressed as a percent of Full-Scale flow (%FS). This means that the flow controller is most accurate when operating at its rated full-scale flow rate. This also means that its accuracy, expressed as a percent of Setpoint, drops off significantly when operating at lower flow rates.
For example, a 1000 cc/min flow controller with a /- 1%FS accuracy, operating at a setpoint of 1000cc/min can be expected to be accurate to /- 1% of the set point. However, when operating at a setpoint of 100 cc/min, the accuracy expressed as a percent of setpoint drops to /- 10% of the setpoint.
Environics instruments use flow controllers calibrated to an accuracy of /- 1% of SETPOINT, NOT FULL-SCALE (2). This means that the accuracy of the flow controller is the same, regardless of the operating point of the flow controller.
(1) L.V. King, On the Convection of Heat from Small Cylinders in a Stream of Fluid: Determination of the Convection Constants of Small Platinum Wires with Application to Hot-Wire Anemometry, Phil Trans. Roy. Soc, A214, 373-432, 1914.