The Temperature Measurement System

A temperature measurement system consists of the temperature sensor and the evaluation electronics. The sensor is usualy a temperature-dependent resistor or thermocouple and is normally fitted in a protection tube which is suitable for heat transfer. The two components of the measurement system is connected with cable. The sensor and evaluation electronics are often combined in a temperature probe (terminal head). The evaluation electronics can be either a transmitter (standardized output signal) or a direct-reading electronic indicating instrument (digital indication of the temperature). The temperature measurement system with thermocouple is shown on the Picture 1. On this picture, as example is shown "J" thermocouple (Iron-Constantan). This thermocouple has range from -336 to +2193°F, or from -204 to +1200°C, or from 68 to 1473 K (this values can vary). Another types of thermocouples are "K" (Chromel-Alumel), 'T' (Copper- Constantan), "E" (Chromel-Constantan), "R" (Pt-Pt-13%-Rh), "S" (Pt-Pt-10%Rh), "B" (Pt-13%-Rh-Pt-6%Rh), etc.

Picture 1: The temperature measurement system with thermocouple


A thermocouple is an element which always measures the temperature difference between the measurement point and the reference junction. The reference junction terminal, at the transition from thermocouple to copper cable, has the temperature T2. This temperature must be known in order to be able to measure the temperature T1. The connection between the thermocouple and the evaluation electronics is provided with a compensating cable. This cable has the same thermoelectric properties as the thermocouple itself, so, this way it prevents an additional thermoelectric emf arising at the junction. However, it is essential that the appropriate compensating cable is always used,and also plugs and sockets free from thermoelectric emf must be used.