How to calibrate Type S or Type T thermocouples

The main way to calibrate Type S and Type T thermocouples is the comparison method. This includes comparing the data from the thermocouple that needs to be calibrated with that of a standard thermocouple in a constant temperature environment to find out how far off its measurements are. Type S thermocouples are high-temperature types made of precious metals, whereas Type T thermocouples are low-temperature types made of base metals. They need to be handled separately since they have different temperature ranges, calibration equipment, and operational features.

I. General Calibration Procedure (for both Type S and Type T)


No matter what type of thermocouple it is, Type S or Type T, the essential procedures for calibrating it are:

Setting up Standards and Tools

Standard Thermocouple: Use a Class I or Class II Type S standard thermocouple for Type S calibration. For Type T, a standard platinum resistance thermometer (such WZPB-2) can be used as a reference.

For Type S, use a tube furnace or a dry well furnace. For Type T, you can use a constant temperature water bath, a freezing point bath, or an oil bath.

A potentiometer or handheld calibrator that is accurate to within 0.05% or less is a measuring instrument.

Linking and Setting Up

Put the measuring ends of the thermocouple you want to calibrate next to the measuring ends of the standard thermocouple. Then, bind them to the same vertical plane and put them in the constant temperature zone of the temperature control equipment.

Put the cold junction in the bath at the freezing point (0°C) to keep the reference junction temperature the same.

Setting Points for Calibration

S-type: Choose high temperature values like 600, 1000, and 1300 degrees Celsius;

T-type: Choose points with temperatures between -100 and 300 degrees Celsius.

S_measured is the difference in thermoelectric potential (mV/°C). Figuring Out Acceptance

A variance that is within the permissible tolerance range is acceptable:

S-type: ±1.5℃ or ±0.25%t (whichever is bigger)

T-type: ±0.5°C or ±0.4%t, whichever is larger

II. Important Things to Remember When Calibrating an S-type Thermocouple (High-Temperature Precision Type)

People generally utilize S-type thermocouples (platinum-rhodium 10-platinum) to measure temperatures below 1300°C with a lot of accuracy. During calibration, you should pay close attention to:

Using the Fixed-Point Method to Make Things More Accurate

In laboratory-level calibration, the International Temperature Scale (ITS-90) sets fixed values for calibration. For example, the freezing point of tin is 231.928 degrees Celsius, and the freezing point of copper is 1084.62 degrees Celsius. You can keep the inaccuracy within ±0.1°C.

Avoiding contamination and "platinum brittleness"

The operating environment should be clean and devoid of dust, and it should not come into contact with reducing gases (H₂, CO). To keep contamination from happening, it is best to employ a high-purity alumina protective tube. Suggested Equipment Combination

A tube-type calibration furnace, a Class II standard S-type thermocouple, a freezing point bath, and a 0.02-grade potentiometer.

III. Important Things to Know About Calibrating a T-type Thermocouple (Low Temperature, High Sensitivity)

For measuring low temperatures between -200℃ and 350℃, T-type thermocouples made of copper and constantan are very common. Their calibrating features are as follows:

For low-temperature ranges, the freezing point approach is the best choice.

Set the cold junction to the correct reference temperature by using a 0°C ice-water mixture.

You can do multi-point calibration from -20°C to 100°C by using a constant temperature water bath.

Be aware of how cold junction compensation affects things.

T-type thermocouples are very sensitive to variations in the temperature of the cold junction. To eliminate mistakes caused by changes in the temperature of the air, it is best to employ an automatic cold junction compensation module or freezing point compensator.

Suggested Set of Tools

A digital multimeter or calibrator, a Class II standard platinum resistance thermometer, and a constant temperature water bath or oil bath.



IV. Quick Calibration Method on Site (No Need to Take Apart)

In industrial situations where shutting down is not an option, an on-site comparison method can be used:

Use a portable high-precision thermometer (with a built-in standard platinum resistance thermometer or standard thermocouple) near the end of the thermocouple that you are testing.

Read the temperature values of both at the same time. If the divergence is greater than the acceptable level, it means the device needs to be sent back to the factory for calibration.

The good thing is that it works quickly (in a few minutes), but the bad thing is that it isn't very accurate (±1°C) and can only be used to tell if anything is pass or fail.