Measurement of the velocity of products of combustion in gas-fired heaters

A relatively simple scheme is presented for measuring the velocity in an environment such as the flue way of gas-fired furnaces and heaters. The scheme employs a measure of the correlation between two thermocouples displaced in the axial direction. Analysis of the time-delayed correlation between these two signals shows a maximum value of the correlation function at a delayed time which corresponds to the transit time between both sensors. Knowledge of the separation between these sensors permits a measure of the velocity. While the accuracy of the device may be on the order of 10%, the procedure is straightforward and simple, requires no calibration, and is not dependent on the local properties of the fluid.