Top 10 questions about Thermocouples

From our last article, what is a thermocouple, we have been asked a few questions about thermocouples so we thoight it best to put our top 10 thermocouple questions together for you.

What is a thermocouple and how does it work?

A thermocouple is a sensor that consists of two dissimilar metal wires joined at one end. This is called the hot junction. When the junction of the two metals is heated or cooled, a voltage, known as thermoelectric EMF, is produced that can be correlated back to temperature. This voltage is generated due to the thermoelectric effect. You can read a more indepth answer on our page What is a Thermocouple.

What are the different types of thermocouples?

The most common types are K, J, T, E, R, S and N type thermocouples. They are named after the metal alloys used. K type (chromel and alumel) is the most common general purpose thermocouple. Thermocouple temperature sensors can also be identified by the colour of plug or cable used. As an example you can identify a type K thermocouple in Europe by the Green outsheath and Green Positive leg. In the USA you would identify type K by the yellow outersheath of the cable and yellow posirve leg and red negative leg.

What is the temperature range of thermocouples?

Different thermocouples have different temperature ranges but in general thermocouples can measure from as low as -200°C to as high as +2500°C. K type has a range of -200 to +1300°C. Certian applications require certain thermocouples dependant on temperature and the atmosphere the thermocouple is in.

What is the thermocouple reference junction?

The reference junction is where the two thermocouple wires connect to the measuring instrument (voltmeter). To measure temperature accurately, the reference junction needs to be at a known temperature, typically achieved using an ice bath or electronic cold junction. Depending on your requirements, yo ucan have an insulated, grounded or exposed junction. The most poular is insulated. The exposed junction will offer a quicker response than insualted or grounded but it does come with its limitations.

How accurate are thermocouples?

Accuracy depends on the thermocouple type but generally thermocouples are accurate to within +/- 2°C to 0.75% of the temperature being measured. Calibration can improve accuracy. Stability and repeatability are around 1°C for most common types. Type T thermocouples offer the most accurate but only up to temperatures of 400 deg c. Accuracy can also change depending on the design of sensor, instrumentation used and length of thermocouple cable.

What is the thermoelectric effect?

The thermoelectric effect refers to the conversion of temperature differences directly into electricity. It is the principle behind how thermocouples generate voltage/EMF that corresponds to temperature.

What are the advantages and disadvantages of thermocouples?

Advantages include wide temperature range, low cost, ruggedness, small size. Disadvantages include low voltage output, nonlinearity, drift with use, and the need for cold junction compensation.

How are thermocouple signals amplified and converted to temperature readings?

Thermocouple amplifiers boost the small mV signals. Analog-to-digital converters change the voltage into digital values. Cold junction compensation is applied. Conversion equations or lookup tables then convert the voltages into temperature values.

How can thermocouples be used for temperature measurement and control?

Thermocouples can provide temperature measurement and feedback for control systems. With appropriate electronics and software, thermocouple data can be used to monitor temperature, activate controls, alarms, valves, etc. to regulate processes. There are thousands of industrial applications where thermocouplea are still the prefered choice over wireless temperature measurement.

What standards exist for thermocouples?

Key standards include ASTM E230 and E584 that specify thermocouple types, tolerances, error limits, calibration, and usage. IEC 60584 also standardizes thermocouple specifications and tolerances.