What is a thermocouple and how do they work?

What is a thermocouple?

Thermocouples are devices that are used to do temperature measurements. One of the major advantages of this device is that it can be used for a wide range of temperature measurements, we can use this device to measure zero degrees to thousand degrees Celsius. Thermocouples are widely used in many industrial applications, they are composed of two wires and they would be joined to form two junctions. This is the most widely used temperature measurement method with an electric output signal.

How is a thermocouple constructed?

Thermocouples are constructed by using two different materials, it doesn’t have electronics inside them. These devices are created by welding two metal wires. Mostly the metals which are used would be iron and copper. The wires would be joined to form two junctions and it would create a small electromotive force when there is a temperature difference between them. Thermocouple cables are called instrumentation cables because they are mostly used for the measurement of the process temperature.

How does a thermocouple work?

The thermocouple working principle is based on three effects and they are

Seebeck effect

In this effect, it says that if two different metals are connected together at two junctions then an electromotive force would be generated at the two junctions. This generated EMF would be different for the different metal combinations.

Peltier effect

In this effect, it states that when two dissimilar metals are joined together it forms two junctions. Then EMF would be generated due to the different temperatures of the metal devices.

Thomson effect

In this effect, it states that if two dissimilar metals are joined together which forms two junctions, then there would a potential in the entire circuit due to the temperature gradient of the entire circuit.

Thermocouple working and how its reading is measured

One of the metal joints in the thermocouple will be connected to the body of the object which is to be measured and it is called a hot junction or measuring junction. The other junction will be connected to the body of the known temperature this is called a cold junction or reference junction. So the thermocouple would do the measurement of the unknown temperature of the body with reference to the known temperature of the body. In order to determine the measuring temperature, the output from the thermocouple circuit should be calibrated against the unknown temperature. So the output signal of the thermocouple would give the value of the unknown temperature. This output signal would be mostly measured by using a potentiometer or a galvanometer.

We must consider while selecting the metals to create a thermocouple. If both the junctions of the thermocouple have the same temperature then the EMF which is created would be equal and opposite and due to this, the total current would be zero. So in a thermocouple, we should maintain different temperatures for the metals so the generated EMF won’t be equal and the current won’t be zero. The total EMF in a thermocouple is dependent on the metals which are used.

What are the different types of reference temperature systems used in thermocouples?

• Ice baths
• Electronically controlled references
• Zone boxes

In the above image, Ice baths are used for the cold junction compensation because they would provide constant temperature or known temperature. The hot junction is the measurement of the temperature which we need to determine.

What are the different combinations of the metals used in a thermocouple?

Wiring color code details in thermocouple?

ANSI standards

How to calculate the temperature in a thermocouple?

εαθ

ε = EMF produced in V

θ= Temperature difference between the junctions

What are the different types of thermocouple material color codes?

What are the different types of thermocouples?

Type K

This is the most used type of thermocouple it’s less expensive and really accurate. But it has a limited range and the metals which are used would be chromel and alumel. The maximum operating range would be 1260 Deg C.

Type J

This is also really sensitive and its range is limited. Iron and constantan are the major metals used for this and its maximum operating range would be 870 Deg C.

Platinum type

The platinum type thermocouple such as B, S, T, and R types are very less sensitive but all these would have a good measurement range.

How should we choose a thermocouple type?

• It must be selected according to the Temperature range
• Accuracy
• Process compatibility must be considered like chemical and mechanical
• Instrument compatibility

What are the advantages of thermocouples?

• It can be used to do the direct measurement of temperatures up to 2600 Degree C
• We can ground the thermocouple junction and it can be bought to direct contact with the material which is to be measured
• These devices are rugged and due to that can be used roughly
• The output of this device will be an electrical signal and due to this it can be used for indicating, recording, or micro-processor-based control system
• The output of this device is not dependent on the length of the wire
• It has good stability for a long period of time
• Self-powered it doesn’t need any current or voltage source

What are the disadvantages of thermocouples?

• Cold junction compensation is required
• Chances of error are high due to the complex measurement process
• Non-linear relationship between the process temperature and the thermocouple signal
• Low voltage output
• A reference point is required
• Thermocouple calibration is complex, it should be done while it is in use by comparing it to a nearby comparison thermocouple

What are the applications of thermocouples?

• It can be used for the temperature measurement of kilns
• Gas turbine exhaust
• Diesel engines
• HVAC systems
• Chemical plants
• Food industry
• Petroleum plants
• It is used in the gas machines to determine the pilot flame
• Heaters and boilers
• It is mostly used to do the high-temperature measurements

What is a thermowell?

Thermowells are external fittings that would be in cylindrical form and will be used to protect the temperature sensors. It is used where the process condition is really hot and pressurized. It can also be used to prevent corrosion and abrasion. Basically, it is a hollow tube that is used to protect the temperature sensor. Thermowell materials are mostly constructed by using Hastelloy, carbon steels, etc. The thermowell material will be chosen according to the process conditions.

What are the different types of tests conducted in thermocouples?

Mostly the welded protection in the thermocouples would be subjected to leakage and pressure tests

Leakage test

In this test a vacuum will be produced inside the protection tube and from outside helium will be applied to the protection fitting. In case there is any leakage then the helium would penetrate inside the protection tube and it will be detected through analysis. The time duration of this test would be 10 Secs.

Pressure test 1

In this test, a positive pressure of 1-50bar will be applied to the protective tube from the outside. In case there is any leak in the fitting flow would be established in the tube and this would be detected. The time duration of this test would be 10 Secs

Pressure test 2

In this water pressure of 50 – 300 bar would be applied to the protective tube, and the water pressure would remain constant in case there is no leak. The time duration of this test would be 10 Secs.

Some useful questions related to the thermocouple

What is an ungrounded junction (probes) in a thermocouple?

In this junction, the conductors would be totally separated from the protecting covers. This junction can be mostly used for high-pressure applications, the major advantage of using this junction is that it can decrease the stray magnetic field effect.

What is a grounded junction(probes) in a thermocouple?

In this type, there would be a connection between the metal wires and the protecting covers. This type can be used to do the temperature measurement in an acidic atmosphere.

What is exposed junction (Probes) in thermocouple?

This type of junction would come in handy when a quick response is required. This type of junction is mostly used to measure the gas temperature