Thermocouples are used as temperature sensors in different fields of Science and technology. Two important issues in theconditioning of thermocouple signals are:
* Cold junction compensation (CJC). Problem unique to this class of transducers.
* Linearization of the transfer relation. Universal problem.
Linearizationis inherent in logarithmic operation, and this is why varieties of log- circuits are already in use for linearization of different transducer characteristics.
The issue of linearization as applied tothermocouples in general, is further complicated due to the fact that the refractory metal thermocouples meant for measurement of very high temperatures have transfer characteristics that are notonly extremely nonlinear, but are also non monotonic.
Although it is generally true that computational algorithms for linearization have better performance compared to hardware methods, there are alsoexceptions.
An example of the output of the temperature compensated in a log amplifier could be:
In order to linearize analog transducer signals, knowing that E(T) is the analog output voltagesignal from the thermocouple (after cold junction compensation) and E is the constant DC voltage used to ensure that Vo(0) = 0. Er is a DC voltage and K is a constant, this equation can be modified as:And the percentage deviation from linearity is given by:
Computational as well as PSPICE simulation studies have been carried out using standard data for:
* Copper-Constantan (T-type).Temperature ranges covered: 0ºC to 300ºC and 0ºC to 400ºC.
* Iron-Constantan (J-type). Temperature ranges covered: 0ºC to 300ºC and 0ºC to 760ºC.
* Tungsten-Tungsten 26% Rhenium (G-type). Temperatureranges covered: 400°C to 2300°C and 1000°C to 2300°C.
The optimum values of linearizing K, Er and A for the different types of thermocouples and for different temperature ranges have been...