Experimental Study of the Influence of Plunge Depth on the PRT Time Constant Measurement

The response time of a platinum resistance thermometer (PRT) is a key performance parameter, which can be tested by plunge method. The plunge test is adopted in many international standards. The test apparatus and process should meet the requirements in standards to achieve credibly measurement results. The influences of plunge depth on the measured time constant τ of an industrial PRT and a nuclear PRT were studied. A thermal model of time constant based on “thermal circuit” was deduced, showing less effect of plunge depth. The sensors elements in the tested PRTs were measured and analyzed by its X-ray images. The experimental results at different water velocities 0.4 m/s and 1 m/s, show that to diminish the influence of the plunge depth on the measured τ, the Pt sensor element should be immersed under water. The plunge depth which is larger five times than the outer diameter of the PRT is suggested. This conclusion is useful for the plunge test process and is useful to determine the apparatus measurement range.