What causes drift in Thermocouples?

What causes drift in Thermocouples? Is there a solution?

It goes without saying temperature measurement is one of the key factors when working in the heat treatment industry. However, the type K and N base metal thermocouple sensors used today to conduct these measurements have seen little change or improvements in the development in their performance since their initial development (110 and 50 years ago respectively).

With the development of the new low drift high-temperature mineral insulated base metal thermocouples, I decided to take a look into the workings of type K and N base metal thermocouples and why they drift.

Let’s break it down… so, what actually is a thermocouple?

A thermocouple is a temperature-measuring device consisting of two dissimilar conductors or wires that join together at one end. The Hot Junction is the end that is joint together. The other end of these wires or dissimilar metals are referred to as the Cold Junction.

What are the causes?

Thermocouple drift is plain and simply from use. The use of a thermocouple in a harsh, high-temperature environment changes the physical state of the material.

The main causes of this are;

• – Oxidisation
• – Metallurgical changes to the thermal element
• – Vaporisation
• – Contamination from the atmosphere and protective outer sheath

These result in the original starting alloy or dissimilar metals of the element slightly change to essentially different alloys.

Now we’ve established what causes drift in type K and N base metal thermocouple sensors. Let’s look into solving the problem.

How do we solve it?

Over 10 years ago a University student, Michele Scervini, studying at the University of Cambridge started a project looking at temperature measurement using contact sensors.

In effect he conducted his research on the drift characteristics of base metal thermocouples and studied the reason why base metal thermocouples drift.

The conclusion of the study was the major issue of drift in base metal mineral insulated thermocouples, therefore caused by the outer sheath. This is something that cannot change due to the properties of the sheath. Therefore, it was essential to the high-temperature application in use.

A second inner sheath was creating a barrier between the outer sheath and the conductors. This was in order to reduce migration, improve temperature measurement accuracy, extend thermocouple life by significantly reducing the drift characteristics.

The new cable then underwent many continuous days and months of rigorous testing at the CCPI Europe UKAS accredited (no. 0600) thermal calibration laboratory and University of Cambridge laboratory. In comparison to a standard-issue type K and N base metal mineral insulated thermocouple, the drift was reduced by over 80% at 1200 ℃ (2192 ℉) and 90% at 1300 ℃ (2372 ℉).

In industries where the accuracy of the heat treatment is fundamental, the new low drift cable allows the user to know the reading is accurate and reliable!