Basic function principles
ELX3312 and ELX3314
41
Version: 1.3.0
4.3
Basics of thermoelectricity and thermocouples
The thermocouple terminals can evaluate thermocouples of the types E, J, K, N, R, S und T. The
characteristic curves are linearized and the reference temperature determined directly within the terminal.
Temperatures are output in 1/10°C, for example (device-dependent). The terminal 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 (see following fig.)
Fig. 30: Principle of the thermocouple
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
