Thermocouple failure reasons

The thermocouple has become one of the most used instruments to measure temperature. Although the devices are inexpensive, convenient, and easy to use, there can be significant errors associated with temperature measurements.  If these errors are acknowledged and sensors are designed and used judiciously, the temperature measurement can be estimated with much greater accuracy.  This paper briefly explains some of the errors associated with their use.

What causes a thermocouple to fail?

The common reasons for thermocouple failures are the following:

Polarity

Polarity determines the direction of current flow in an electrical circuit. The output from a thermocouple has polarity. If this is not observed, an error will result.

It is necessary to determine thermocouple polarity in the field.

Electrical interference

Within thermocouple temperature measurements, Seebeck coefficients were reported to be strongly dependent on the magnetic field. Due to strong magnets in cryogenic experiments, thermo-magnetic effects become significant in studies of superconductivity. There is some recent evidence that magnetic effects caused by dc magnetic fields of low amplitudes are the source of measuring errors with thermocouples.

Therefore, thermocouple wires should be a twisted pair or, ideally, shielded.

Metal fatigue

Like everything, thermocouples can’t stay new forever. Changes in temperature can cause regular expansion and contraction in metal, which will cause thermocouples to weaken over time. The wire used in some thermocouples tends to be relatively brittle, and cannot withstand much flexing.

The only solution is to periodically replace the thermocouple.

Oxidation

Metals and their alloys undergo oxidation at high temperatures when exposed to oxygen and other oxidizing agents forming the oxide film on their surface. Being brittle and less strength than the metal itself reduces the mechanical strength of a corresponding thermoelement. In addition, with changing, as a result of oxidation, the alloy structure and the composition of its thermoelectric properties also change.

The only solution is to make periodical maintenance.

Using the wrong type

Different types of thermocouples have totally different characteristics. The electronics to decode the signal from a thermocouple must be matched to the type of thermocouple being used. The plugs on the ends of the thermocouple wires are often retained with screws. A detached plug should be replaced immediately, to avoid the error of attaching it to the wrong type of thermocouple.

The technical parameters of thermocouples should be checked from the manufacturer’s catalogs.

Heat damage from creating a thermocouple

If a thermocouple is made from two wires on a DIY basis by welding the tips of the wires together, minimal heat must be used to avoid degrading the alloys in the wires.

A thermocouple connector needs to be selected according to the measuring thermocouple to interface thermocouple wire to extension cable without introducing additional EMF generators. The temperature of the connector may affect the measurement if a proper extension wire is not selected.

Make sure no fans or other sources of cooling or heating are located near the reference junction.

Impurities

If the thermocouple gets any impurities inside during its manufacture or installation, the deterioration of the thermocouple will occur more rapidly.

Try to use high-quality products

Poor welding

When welders create the metal for the thermocouple, a poor weld can cause an open connection. An open thermocouple check – which is the breaking of the hot junction of the thermocouple — can detect this open connection. Since this form of failure is easy to detect, it is unfortunate that this mode of failure is uncommon uniformity.

Try to use high-quality products and check the welding before the installation.

Grounding more than one location

A thermocouple should be grounded at only one location. If it is grounded at more than one location, a “ground loop” can be created with current flowing through the thermocouple from one ground to the other. This is likely to generate electromagnetic fields, which can lead to radio-frequency-interference-related problems that can impact measurement accuracy.

Ground either the transmitter (connection head) or the controller/recorder, but not both.