TC technology basics

The thermocouple modules can evaluate thermocouples of the types B, C, E, J, K, L, N, R, S, T and U. The characteristic curves are linearized and the reference temperature determined directly within the module. Temperatures are output in 1/10°C, for example (device-dependent). The module is fully configurable via the bus coupler or the control system. Different output formats may be selected or own scaling activated. In addition, linearization of the characteristic curve and determination and calculation of the reference temperature (temperature at the terminal connection contacts) can be switched off.

Measuring principle of the thermocouple

Thermocouples can be classified as active transducers. They exploit the thermo-electric effect (Seebeck, Peltier, Thomson). A voltage referred to as thermovoltage occurs over the length of a cable with different temperatures at both ends. It is an unambiguous function of the temperature and the material. In a “TC element” this effect is utilized by operating two different conductor materials in parallel (s. fig.)

Example:

In the following example, the voltage U_th is given which is present at a type-K thermocouple at the temperature Tm.

U_th = (k_NiCr - k_Ni) x ΔT

with

ΔT = T_m - T_v

A type-K thermocouple consists of a junction of a nickel-chrome alloy and nickel, where k_NiCr and k_Ni represent the thermoelectric coefficients of nickel-chrome and nickel respectively. By adapting the equation according to T_m, the sought-after temperature can be calculated from the voltage measured across the thermocouple. Based on the difference to the cold junction temperature, the temperature at the measurement point can be determined to an accuracy of better than one tenth of a Kelvin with the aid of the above thermocouple equation.

Sensor circuit A modification of the sensor circuit with additional devices such as change over switches or multiplexer decreases the measure accuracy. We strongly advise against such modifications.

Internal conversion of the thermovoltage and the reference voltage

Since the coefficients are determined at a reference temperature of 0°C, it is necessary to compensate for the effect of the reference temperature. This is done by converting the reference temperature into a reference voltage that depends on the type of thermocouple, and adding this to the measured thermovoltage. The temperature is found from the resulting voltage and the corresponding characteristic curve.

U_k = U_m+ U_r

T_out = f(U_k)

Overview of suitable thermocouples

The following thermocouples are suitable for temperature measurement:

* not standardized according to EN60584-1
** according to DIN 43710