Take the Stress Out of Thermocouples

by Ian 0 Comments
Take the Stress Out of Thermocouples

Take the stress out of thermocouples by watching out for damaged or broken wires from rough installation. This is an essential troubleshooting hint.

According to https://www.processparameters.co.uk/thermocouples-sensor/what-is-a-thermocouple/ – typically, thermocouples are preferred over other temperature sensors due to their simplicity in operation and ability to withstand physical stress. You may have already encountered them in your workplace or home. However, do you know all the types of thermocouples? Or why is a particular thermocouple preferred to another in different temperatures? This article gives you a comprehensive insight into all these questions.

A thermocouple is a sensor that measures temperature. It has two wires of different metals (electrical conductors) joined together at one end to form an electrical junction. Temperature is measured from this Hot End Junction. When a temperature change is experienced, a voltage is created, interpreted, and calculated using a thermocouples reference table. The creation of the voltage is known as the Thermoelectric Effect. The Thermoelectric Effect is a combination of three separate effects; Seebeck Effect, Peltier Effect, and Thomson Effect.

How does a Thermocouple work?

Do you have a gas water heater or any appliance with a gas burner that cycles on and off? That is a thermocouple performing. It works with the standing pilot when the machine is in use. The thermocouple ensures the pilot remains lit by transmitting an electric current to a sensor in the gas valve, thus signaling it to stay open and keeping your appliance running.

What is the Response Time of a Thermocouple?

In a thermocouple Physics experiment, it was found that if you take a thermocouple and put it in a preheated furnace at 750 degrees Celsius, it will equilibrate to that temperature. Then, quickly take the thermocouple and place it in still air of 20 degrees Celsius and let it record the readings as it cools. The time taken to cool from a high temperature to a critically low temperature is called the Response Time.

Types of Thermocouples

Thermocouples are either:

  • Base Metal Thermocouples – Nickel-alloy – Are the most common and cheaper, i.e. K, J, T, E, and N.
  • Noble Metal Thermocouples – Platinum-alloy – Used in high-temperature applications and relatively more expensive i.e. S, R, and B.

1. Thermocouple Type K

It is the most common thermocouple since it provides the widest temperature range (-200 to 1,350C). It works in most applications since it is nickel-based and has good corrosion resistance.

However, Green rot, leading to erroneous readings, will occur in temperatures exceeding 900 degrees Celsius and low oxygen concentrations. This can be prevented by substituting it with a Type N Thermocouple or addition of oxygen in the thermowell.

2. Thermocouple Type J

Although common, it has a smaller temperature range (-210 to 760C) and a shorter lifespan in high temperatures. Like Type K, it is cost-friendly and reliable.

3. Thermocouple Type T

With a temperature range of -270 to 370C, Type T is very stable. It is mostly used in low-temperature applications e.g. cryogenics.

4. Thermocouple Type E

It has a temperature range of -270 to 870C. Of all the thermocouples, Type E has the highest EMF output per degree.

5. Thermocouple Type N

It has a temperature range of -270- to 1,300C and slightly lower sensitivity than Type K. Developed to outdo Type K, aging is considerably less and is more stable in nuclear environments.

6. Thermocouple Type S

It has a temperature range of -50 to 1,480C and is easily contaminated. It is highly accurate, stable, and is used in high temperatures since reducing temperatures are damaging.

7. Thermocouple Type R

Although it has a temperature range similar to Type S (-50 to 1,480C), it has a higher EMF output and a higher Rhodium percentage, making it more expensive. Like Type S, it is easily contaminated and reducing temperatures are damaging.

8. Thermocouple Type B

With a temperature range of 0 to 1,704C, it has the highest temperature limit in the Noble Metal Thermocouples. Similar to Types S and R, it easily contaminates and reducing temperatures are damaging.

Choosing a Thermocouple

It is possible to find yourself in a dilemma on which sensor to select from the wide range. Below are factors to consider when choosing a thermocouple:

  • Where will the thermocouple sensor be used?
  • Is there any chemical resistance required for the thermocouple and sheath material?
  • To what probe ranges will the sensor be exposed?
  • What installation requirements will be involved?
  • Is there a need for abrasion and vibration resistance?

Benefits of Thermocouples

In addition to being fast responding and highly efficient, thermocouples are famous for the following reasons:

  1. Cost friendly.
    Thermocouples are inexpensive; they are three times cheaper than RTDs.
  2. Wide range measurement.
    They directly measure temperatures of up to 2,600 degrees Celsius.
  3. Self-sufficient.
    There is no need for an external power source since the output EMF increases with the temperature changes.
  4. Simple yet tough.
    These temperature sensors are designed with high strength metals that make them fit for industrial applications.

Some of the industrial applications include:

  • Controlling composite temperatures during cure
  • Measuring temperature when melting aluminum
  • Sterilization and validating of equipment in food and pharmaceuticals
  • Temperature control when curing of bricks and tiles
  • Testing break-engine cooling systems in transportation, among others.

Replacing and Purchasing a Thermocouple

If your thermocouple needs replaceing, turn off all line valves on the gas supply. Use a wrench to unscrew the connector that holds the sensor to the appliance. Unhook the probe from its clips and pull out the thermocouple.

The genesis of thermocouples was established in 1821 by Thomas Johann Seebeck. To date, the simplicity and accuracy of thermocouples have significantly been adopted in industrial gas-powered applications. The Seebeck Effect has effectively left a positive ripple effect in the Science world.

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