Dynamic Study of Temperature Transducers by Use of an Optical Method

Abstract

The dynamic study of thermal processes is often restricted by the limitations of the temperature transducers. The thermocouple is one of the most employed transducers because of its small size, but it presents several problems such as: (a) The delivered output is very small and for precision measurements problems appear concerning signal to noise ratios, and (b) the transient response is often not well known and is also sometimes insufficient. For this reason thermocouples have been studied by varying the different parameters. To be able to get a reference, permitting accurate measurements of a temperature variation as a function of time, an optical method has been constructed called the “Schlieren” method which allows visualization of a temperature variation and thereby renders feasible the recording of these variations without any appreciable time delay. The thermocouple itself is installed in the water-channel in which the temperature variations are recorded by the foregoing principles. The comparison of the two recordings defines the time constant of the thermocouple. For these measurements a special device has been constructed permitting the temperature to vary without any influence upon the other parameters (flow, pressure, and so on). The experiments have been made for different flow rates. Using the experimental values, an electric model for the thermocouple’s behavior has been devised by which the equivalent thermal resistors and capacitors could be determined. Thus the time constants of the thermocouples corresponding to other parameter values can be calculated. Similar tests to determine time constants have been made for thermal resistors, hot wires, and hot films. All tests described have been made by measuring water temperatures, but the results could easily be applied to other media.