Question

A disadvantage of thermocouples is that the relationship between the process temperature and the thermocouple signal (mV) is non-linear.
Please explain what this statement means.

Answer 1

A thermocouple is a device made up of two metals, where the voltage difference across their interface is known as a function of temperature, allowing one to measure the temperature by measuring the voltage. The statement above is saying that the relationship between the temperature you want to know and the voltage that you read out is non-linear. Non-linear means not a straight line, as in it cannot be expressed as y= mx+b --- it has some sort of curve to it. It is probably a polynomial, such as
\[y = a x^2 + bx + c\]
though in reality, it is probably more complicated than that example. This is a disadvantage because it means that converting from the measured voltage to the temperature is a more complicated process.

Answer 2

So we cannot read the voltmeter reading and establish a direct relation between it and the temperature difference?