How to determine if an S-type or T-type thermocouple has failed I

S-type and T-type thermocouples are made of different materials and work in various conditions, thus they need to be diagnosed in different ways when they fail. The most common signs of S-type thermocouple failure are a consistently low thermoelectric potential, quick drift after calibration, or wire breakage. These problems are generally caused by high-temperature aging, contamination, or "platinum embrittlement.""T-type thermocouple failure, on the other hand, generally shows itself as an abrupt change in the signal, strange resistance, or symptoms of corrosion. This is often due to oxidation, humidity, or wiring difficulties. Here are some particular ways to diagnose:

I. General Diagnostic Methods: These work for both S-type and T-type thermocouples.


Check for Problems by Looking

Check the protective tube for cracks, bends, rust, or holes, especially if the thermocouples are utilized in hot or humid places.

Check to see if the junction box is sealed correctly, if the terminals are loose, rusted, or burned, and if the insulation on the cable is broken.

Continuity Test using a Multimeter

Set the multimeter to the ohm range, take the thermocouple out of the circuit, and measure the resistance of the two thermocouple wires:

The thermocouple wire is broken if the resistance is infinite;

There can be a short circuit if the resistance is near to 0Ω.

The resistance of assembled thermocouples is usually less than 2Ω. If it is more than 1kΩ, it is probably broken.

Comparison of Millivolt Signal Measurements

Use a very accurate millivoltmeter to measure the thermoelectric potential at the cold junction. Compare your results to the theoretical values on the calibration table. For example, type S should be about 0.142mV at 25°C, and type T should be about 1.277mV.

If the measured value is outside of the acceptable error range (for type S, ±1.5℃ or ±0.25%t; for type T, ±0.5℃), the thermocouple wire may be getting old or breaking down.

Actual Temperature Measurement Comparison and Verification

To compare the readings, put the thermocouple and a conventional thermometer (such a platinum resistance thermometer) in a place where the temperature stays the same, like a boiling water bath or a constant temperature oil bath. If the temperature difference is outside the range of the accuracy class and wiring faults are not possible, the thermocouple can be considered broken.



II. Distinct Failure Attributes and Assessment of Type S Thermocouples

Thermocouples of type S (platinum-rhodium 10-platinum) are made of precious metals. High-temperature working conditions are directly related to common failure modes:Aging" causes the thermoelectric potential to slowly drop over time.

After being used for a long time at temperatures above 1300°C, the platinum-rhodium wire starts to diffuse elements, which makes the measured value 50–100°C lower than the actual temperature. After calibration, the temperature drifts again quickly and can't be brought back to accuracy, which usually shows up as a long-term, slowly lower measured temperature.

Contamination that creates "platinum embrittlement" or performance drop

When the thermocouple wire comes into contact with reducing gasses (H₂, CO) or metal vapors, it becomes brittle and breaks. When the protective tube is broken, the thermocouple wire turns black or gray, which means it has been oxidized or poisoned.

When cold junction compensation fails, it causes systematic mistakes.

The thermometer is either not set to the right graduation number or the cold junction temperature sensor is broken, which causes a continuous error over the whole measurement range. The value shown changes with the temperature of the environment.

Quick way to check: The S-type thermocouple's theoretical thermoelectric potential is about 9.587mV at 1000℃. If the measured value is less than 8.6mV (with a variation of more than 10%), it is usually seen as a failure.

III. Particular Failure Traits and Diagnosis of T-type Thermocouples

T-type (copper-constantan) thermocouples are made of base metals and are mainly utilized in places with low temperatures. Corrosion and oxidation are two things that typically cause them to fail:

When the copper positive electrode gets too hot, it oxidizes, which breaks the wire.

The copper wire turns black and breaks after being used for a long time at temperatures exceeding 350°C. It will eventually melt. You can see clear oxidation traces after peeling back the protective tube.

Corrosion of the constantan negative electrode caused by moisture or sulfide production

The negative electrode is more likely to corrode electrochemically in places with high humidity or sulfur. This can cause the resistance to go up or the signal to change. Corrosion is often shown by green copper rust on the terminals.

Cold junction temperature sensitivity makes measurements unstable.

Changes in the temperature of the cold junction have a big effect on T-type meters. If the compensation isn't good enough, changes in room temperature can cause the detected temperature value to change a lot. You may check if the instrument works by shorting the input terminal and seeing if the room temperature is shown.

At 0℃, the T-type output should be 0mV. This is a quick way to check. It means that the measured variation is more than ±0.1mV.