Thermocouples – The Guide

Thermocouples, the most widespread and inexpensive temperature sensors, are used in technical, experimental, or even household instruments for its compact size, response rate, and performance efficiency. It is an integral part of power production industries, pharmaceuticals, bioTech instruments, or your in-home water heater or gas furnace, and you would obviously want to get the best out of the device.

Are you in a hassle thinking about if the thermocouple of the water heater, toaster, or boiler is working accurately or not? Well, be it for industrial or household purposes, for ensuring flawless operation, you should have precise knowledge about its working process and characteristics.

To help you get in-depth knowledge about this temperature sensing equipment and make sure it is working in its best condition, I am summarising here the essentials you have to know about thermocouples.

Definition of Thermocouple

A thermocouple sensor is simply an electric instrument that comprises two distinct electric conductors and senses temperature, producing temperature variant voltage through thermoelectric effect. The electric junction the two conductors form acts as the measurement terminal, and the generated electromotive force (emf) works as the interpretation of the measured temperature.

Thermocouple Applications

As I already mentioned, thermocouples come in various uses. Let’s get insight into the many applications and benefits the thermocouple sensor offers.

Uses of a Thermocouple

From different miniature thermocouples to thermocouples with terminal heads, it comes in enormous uses and applications. The main applications include:

  • Examining and regulating the temperature of many instruments in chemical or petroleum refinery plants
  • Checking the temperature of metals in metallic industries like iron, steel, or aluminum
  • Low temperature and cryogenic treatments in the food production industries
  • Sensing and measuring pilot flame in furnaces
  • Executing the thermoelectric cooling process operating as a heat pump
  • Estimating the incident infrared radiation
  • Measuring and observing the temperatures of the exhaust of diesel engines, gas turbines, etc.
  • Sensing the temperature of household instruments like thermostats, water heater, etc.
  • Acting as a fire sensor in safety instruments for the appliances run by gas

Advantages of Using Thermocouples

Some of the surpassing benefits that come with this temperature sensor:

  •  Cost-effective way
  • Can sense temperature ranging from 270-2700°C
  • Rigid and compact in construction
  • Doesn’t involve any bridge or complex circuitry
  •  Exhibits high thermal response rate
  • Guarantees satisfactory precision
  • Dependable performance
  • Faster response time

Different Thermocouples Types

Thermocouples are of:

  • T-Type: Copper as positive conductor and Constantan as negative conductor
  • E-Type: Chromel as the positive conductor and Constantan as the negative conductor
  • J-Type: Iron as the positive conductor and Constantan as the negative conductor
  • K-Type: Chromel as the positive conductor and Alumel as the negative conductor
  • S-Type: 10% Rhodium is used as the positive conductor, and Platinum works as the negative conductor

K-Type Thermocouple

Type K thermocouple is well-known over other types because:

  • It exhibits a broad range of working temperature and ensures higher accuracy over others
  • Can maintain precision in rough environmental and different atmospheric condition
  • It is durable
  • Compact in size and shows stable outputs
  • Resistive to corrosion up to 1600°F

Thermocouple Thermometer

A thermocouple thermometer is an industrial grade and precise temperature measuring instrument that features built-in thermocouples or utilises external ones for inputs.

How Does Thermocouples Work?

Let’s get an overview of how this economical yet efficient temperature sensor works.

Operating Principle: Seebeck Effect

A thermocouple works depending on the Seebeck effect, a thermoelectric effect established by the renowned German scientist Johann Seebeck in 1821.

The Seebeck effect denotes the phenomenon of producing an electromotive force between two conductors when there occurs a temperature variance between the two joints they form. The Seebeck voltage is usually measured in mV per Kelvin.

How Does a Thermocouple Measure Temperature?

The working process of the thermocouple is as follows:

  • A thermocouple circuit is formed by welding two metal conductors building terminals.
  • The temperature variance between two terminals generates a Seebeck voltage
  • If the temperatures of both ends are the same, there induces equal and opposite charge, and thus passes no current
  • The intended temperature sensing part gets attached to the hot junction
  • The other terminal known as the reference junction gets connected to the measuring equipment and referenced at 0°C
  • The temperature difference, as per Seebeck theory, induces an emf in the circuit
  • PPMC connected to the circuit can measure the induced emf
  • The equation of the measuring voltage is E = aΔθ; where ‘a’ is the Seebeck coefficient, and ‘Δθ’ indicates the temperature difference between the junctions
  • This way, the thermocouple can turn the temperature (non-electric parameter) into voltage (electric parameter)

Signs of a Bad Thermocouple

Be it a water heater, gas wall heater, or a gas furnace, thermocouples are an integral part of any industrial or household devices. But sometimes, it can get damaged and cannot perform efficiently.

You can know whether your temperature sensor is malfunctioning if:

  • The thermocouple is filthy
  • There is any indication of decay on the thermocouple tube, like pinholes, damage, or stain
  • Corroded wires
  • Damaged connectors
  • For a water heater, if the pilot lights but doesn’t stay lit

Thermocouples are essential to ensure your appliance is working following the parameters you have set. Hence, buying from the most reliable source is crucial. Process Parameters provide quality thermocouples, offering you a variety of choices according to your requirement, backed by skilled and efficient technical support.

Thermocouple Diagram, Chart, and Colour Code

Diagram

Circuit diagram of a thermocouple.
Image Source

In a thermocouple circuit, two conductors get combined to form the measuring junction. The other ends are not connected, form the reference terminal, and get linked to the system through its Copper traces.

A voltmeter is used to measure the emf induced at the reference junction related to the temperature difference between these two points.

Type-K Thermocouple Chart

The temperature and induced voltage in a thermocouple don’t exhibit a linear relationship. Thus to get the temperature correlated to the measured voltage, we need to use the thermocouple chart, which is distinct for each type.

The temperature conversion chart for the widely-used Type-K thermocouple is:

Colour Code

The IEC colour codes applicable only within the Europe and UK from the process parameters:

Colour codeing for thermocouples.

Conclusion

The simple circuit configuration of thermocouple has made it encompass almost all sectors of industries and scientific applications. It is rugged, accurate, and ensures reliability in operations if purchased from a dependable source and operated with precision.

The above information I have gathered would be a milestone in the process of gaining knowledge about this effective electrical instrument.