Development and testing of a novel single-wire sensor for wide range flow velocity measurements

Thermal flow sensors with a wide dynamic range, e.g. 1:1000 and more, are currently not available in spite of the great demand for such sensors in practical fluid flow measurements. The present paper introduces a sensor of this kind. The new sensor is mechanically the same as the 'sending' wire of the two-wire thermal flow sensor described by Durst et al, but it is excited by discrete, widely separated, square waves of electrical current rather than a continuous sinusoidal current. The nominal 'output' of the new sensor is the increase in wire temperature so that an integral of the resistance over the pulse length can be used for measurements. This 'output' is a function of the time constant ('thermal inertia') of the heated wire and thus also of the velocity of flow. The time constant decreases as the flow velocity increases, while the heat transfer increases. At very low flow velocities the response is determined almost entirely by the time constant of the wire while at high velocities the device acts almost like a 'constant current' hot-wire anemometer. That is, the effect of thermal inertia augments the output signal of the basic hot wire, thus increasing the flow rate range/sensitivity of the device, especially at the low-velocity end, above than that of a simple hot-wire flowmeter. The sensor described here was developed for slowly changing unidirectional flows, and uses one wire of 12.5 µm diameter. It is excited at 30 Hz frequency and its usable flow velocity range is 0.01–25 m s−1.