Temperature measurement by thermocouple probe: influences of variable fluid velocity on the dynamic characteristics of thermocouple

Abstract

The problem of dynamic thermocouple compensation is considered. Taking into account the nonlinear dependency of the thermocouple time constant on fluid velocity, a thermocouple model and a time constant model are proposed successively for estimation of nonlinear thermocouple time constant. This technique is most suitable for measuring dynamic temperature because of the precise description of time constant. The improve scheme offers better compensation of the thermocouple response. The parameter estimation is performed using GTLS to cope with the noise present on the regressors and the output. Instruction In many industrial applications, thermocouples are used due to their high credibility and good linear dependence with temperature. However there are drawbacks of thermocouples, the most important of which is inaccuracy of measuring high frequency temperature fluctuations (1). So, by establishment of the dynamic model, thermocouple input can be reconstructed. In general, thermocouple was model as a first order nonlinear model. Time constant is the undetermined parameter of the model. As the present research,a fixed (mean) time constant (2) is almost always used, although it is variable in fluctuating velocity and thermal field. The simplification of the procedure of modeling and data processing is based on the cost of measurement accuracy. In this paper, we propose a method which identifies the system of thermocouple denoted as Gthermo(τ) whose input and output signal is fluid temperature and thermocouple response, respectively, and that of time constant denote as Gv-τ(u) whose input and output signal is fluid velocity and time constant of thermocouple, respectively, can further improve the accuracy of dynamic compensation of thermocouple. As shown in Fig. 1, Gthermo(τ) and Gv-τ(u) are both excited by step signal, meanwhile their outputs are corrupted by noise η1 and η2 respectively, and τS is sampling interval. We can obtain the relationship between time constant and fluid velocity when the model of Gthermo(τ) and Gv-τ(u) is established.