How Does a Thermocouple Work?

Introduction


A thermocouple is an electric device that is used to measure the temperature of an objects via electric conductivity through two meta wires. These two metal wires connect together at one end and to a thermocouple meter at the other end. 

Quite simply, a thermocouple is the most widespread temperature sensor anyone can find. Therefore, it’s paramount for any professional worth his name in salt to gain mastery on the ins-and-outs of the device. Frankly, not doing so could be disastrous. Already, countless industries, scientific research, and engineering applications are putting their faith in thermocouples, reaping the rewards in the process.

It’s not hard to understand why thermocouples are mankind’s favorite temperature sensors. Right off the bat, you have a sensor that can be deployed at the most extreme of circumstances delivering temperature data where and when it matters most. We’re talking about the hottest and the coldest temperatures known to man. Plus, they’re some of the cheapest robust tools you can find being self-powered without the need for an external form of excitation. However, one caveat. If you’re looking for a precision of less than one degree Celsius (°C), you may have to look elsewhere.

Here we bring light into the mechanics of how a thermocouple operates, its best versions and how you can make the most of these versions. Indeed, not all thermocouples are created equally.

How Does a Thermocouple Measure Temperature?

In its essence, a thermocouple is a temperature-measuring device made up of two distinct metals. It sure sounds simple enough.

You have two wires of different metal compositions joined together to form a closed circuit creating two electrical junctions in the process. One junction, dubbed the “hot junction”, is where the unknown temperature is to be measured. The other junction, dubbed as the “cold junction” or reference point, is directly connected to a known temperature.

But to understand how a thermocouple gives us the temperature data, we need to step back. And learn about the Seebeck effect which governs the operation of the device.

A Baltic German medical doctor who chose to study physics, Thomas Johann Seebeck (1770 - 1831) observed that when exposed to a temperature source, two dissimilar metals joined together cause a deflection on the magnetic needle of a compass. Seeing how thermal difference produced varying degrees of electromotive force, he called the phenomenon the thermo-electric effect.

Today, we refer to it as the Seebeck effect.

A simpler way to understand this principle in action is to do a little cooking experiment. Specifically, heat a frying pan using a burner. As you hold the pan over the fire, heat travels from the fire up to your hand. What you don’t realize is that electricity also travels the same path.

Looking closer, certain parameters were observed to cause the Seebeck effect. First, there must be a difference in the two junctions, namely the cold and hot junctions mentioned above. If by any chance, the temperatures of the two junctions are the same, there will no electric current produced as the electromotive forces generated from each junction will ultimately cancel one another.

Secondly, it is this temperature differential that produces the electromotive force which is then measured by a voltmeter. Care must be observed in reading the current as the voltage produced is in the micro-volt level. And to obtain a more reliable reading, the temperature at the cold juncture must be known right from the onset.

Thirdly, the amount of voltage also depends on the material of the wire used. Different metals used to create a thermocouple will have different results. Also, know that a thermocouple since it is capable of producing electricity, albeit in minuscule amounts, is self-powered.

Moreover, thermocouples when used together can act as a thermopile, the heart of the thermal camera, or infrared thermal imaging. A thermopile converts thermal energy into electrical energy.

Types of Thermocouple 

Thermocouple Types

Over the years, many types of metals were coupled to form a thermocouple, producing varying results in the process. Eventually, letters were used to designate each metal pair allowing different industries to make the most of each depending on the application.

By the process of elimination, four types became the most widely adopted. These were types J, K, N, and T. It must be distinguished that although they exhibit the same Seebeck phenomenon, each particular type of thermocouple has distinct characteristics. We’re talking about temperature sensitivity and temperature ranges. It is therefore incumbent on a particular industry to choose the best type to suit its needs.

K-type Thermocouples

To boot, K-type thermocouples are the most widely used thermocouples. The secret of this type’s popularity lies mainly in its composition. Made by combining two nickel-based metals (e.g., chromel/alumel), K-types are sturdy even when operating at a wide range of temperatures ( -200 to 1260°C) with a precision of ±0.75%.

To note, nickel-based metals are strong, exhibiting an above-average resistance to oxidation and corrosion. Usually, the positive leg of the thermocouple consists of a metal made of 90% nickel and 10% chromium. On the other hand, the negative leg consists of a metal made of 95% nickel with 2% manganese, 2% aluminum and 1% silicon.

J-type Thermocouples

Though not as popular as K-types, this thermocouple type is also widely used. And that’s basically because it demonstrates a narrower temperature range (-40 to 750°C). Added to this, its lifespan is also shorter compared to the sturdier K-type thermocouple.

The one benefit this type lords over is the price. J-type thermocouples offer the least expensive price tag compared to other thermocouple types. Plus, it has been shown to demonstrate greater efficiency even in vacuums and non-oxidizing atmospheres. However, take note that J-type does not work well in moisture-laden environments as its metals are prone to oxidation.

The positive leg of this J-type usually consists of an iron wire while the negative leg is made up of copper-nickel combination alloy (e.g., constantan).

N-type Thermocouples

You can call an N-type thermocouple a K-type on steroids. Indeed, the N-type is designed to overcome certain shortcomings of the K-type in order to better perform in specific environments. We’re talking about greater stability that’s perfect for nuclear situations. Plus, N-type has greater success in resisting oxidation in the hottest environments.

You could say N-type performs a lot better than K-type. It can operate in a wider temperature range of -270 to +1300 ° C. It’s also more expensive.

Legs usually are made up of nicrosil-nickel alloys. This category includes silicon, nickel and chromium.

T-type Thermocouples

T-types don’t have a freakishly wide range with its -200 to +350 ° C temperature range in its sleeve. But you can’t fault it when it comes to stability. It’s no accident T-types are the go-to device for environments with extremely low temperature such as deep-freeze laboratory experiments.  

Its legs consist mainly of copper-constantan alloys.

 

Thermocouple Types and Performance Table

Type

Legs Composition

Temperature Range (° F)

Peculiarities

B

Platinum 30% Rhodium/

Platinum 6% Rhodium

2500 up to 3100

Low electrical output; not suitable for lower temperatures

E

Nickel-chromium/

Constantan

32 up to 1600

Most stable than K-type with higher degree of accuracy

J

Iron/Constantan

32 up to 1400

Widely used; cheapest

K

Nickel-chromium/

Nickel-aluminum

-328 up to 2300

Most widely used

N

Nicrosil/ Nisil

-454 up to 2372

Widely used; Better performance than K type

R

Platinum 13% Rhodium / Platinum

1600 up to 2640

Similar to type-S but better stability

S

Platinum 10% Rhodium / Platinum

1600 up to 2640

Can be used in very high temperatures but need protection from moisture

T

Copper/Constantan

-75 up to 700

Widely used; Very stable best for very low temperature

 

Thermocouple Applications

The many uses of Thermocouple

Thermocouples offer two main benefits for the industry. The first is its ability to measure the widest range of temperatures even in extreme situations. Its ruggedness is tops and today it’s safe to say no temperature sensor comes close. Secondly, and most importantly for Finance Managers, thermocouples are cheap. They can beat just about any temperature sensor, RTDs, and thermistors including, on the market today.

With that said, it’s no accident thermocouples are the most widely used sensors on the planet. Below are some of their most common applications:

  • Thermostats

Don’t look now but thermocouples are in your daily lives, starting with the thermostat that controls your HVAC. Indeed, thermostats have thermocouples as one of their key components. As thermocouples sense heat, a thermostat can act on the data to turn your AC on or off.

  • Medical Thermometers

When you talk about a hospital-grade thermocouple, it’s highly likely you’re seeing a thermocouple at work. Even other medical equipment that requires detection, diagnosis, and even treatment often uses the temperature sensor. Usually, these are miniaturized to produce highly-reliable temperature readings in heat-critical set-ups.

  • Vehicle Diagnostics

There’s another industry that needs to monitor heat and that’s the aerospace and automotive industry. Usually, this has to do with engine performance. Indeed, machines for transport reach abnormally-high levels of temperature making it timely for a thermocouple to operate.

We’re talking about spark plug function, battery health, and exhaust gas readings for instance. As these need to be monitored in real-time, things can get especially heated.

  • Boilers and Ovens

Here’s another way a thermocouple could be lurking in your abode without you knowing. In your kitchen. Hot water systems and ovens need a high-temperature gauge. And thermocouples fit the job to a T.

 

How to Choose Which Thermocouple Type?

How to Choose Which Thermocouple Type is Best

As often in technology, there’s a stage of hit and miss when first-time users dealt with thermocouples. Over time as each type’s characteristics became clear, certain industries have favored a particular type of thermocouple as most suitable for their use.

There are several factors in choosing which type is best for your needs. Of course, product knowledge is paramount. Knowing the ins and outs of a particular thermocouple type is the way to go. Below are key factors in choosing which thermocouple works for your particular application.

  • Cost

We’ve placed cost first so you are aware that each thermocouple type has a different pricing range. Some thermocouples are cheaper than the rest. Though in general, these sensors do not command a hefty price tag giving you tons of advantages.

  • Temperature Range

Each thermocouple type has a particular range. You should therefore determine at what temperature your industrial application will be. And make a good choice which is fit thereafter.

  • Abrasion/Vibration Resistance

The environment to be measured must be taken into account. Will there be direct contact with another material during the measurement process? Will the thermocouple “rub skin” with other entities? Secondly, is the environment free from vibration? Or will there be lots of shaking and instability? In essence, rugged environments would require more stable thermocouple types.

  • Chemical Resistance

Again this will have to do with the environment to be measured. Will there be moisture present? Is it a vacuum? If so, you will have to find a thermocouple that works best in this setting. For one, J-type thermocouples have gained a lot of attention for vacuum environments in industrial applications. Additionally, the presence of other chemicals which can be corrosive should also be factored in. If so, sheath materials such as stainless steel and Inconel should be considered.

  • Life Expectancy

Take note that the accuracy of a thermocouple could take a hit as time goes by. So knowing how sturdy a particular type is can help get you better results. It’s good to know that there are key considerations when talking about how long the device sensor will last. These factors include:

-  operating temperature

-  wire size of thermocouple

-  sheath protection of thermocouple

-  operating environment

-  accuracy required

You will know how much a thermocouple is losing its accuracy by measuring its drifting millivolts. The greater this drift is, the less accurate your reading becomes. And the more time has given your device a bad hit.

A good preventive measure to maintain its strength is to recalibrate the thermocouple every now and then. The time span of 3 to 6 months is usually the recommended interval to give your device a timely check.

Installation Requirements

Every industry has different uses for a thermocouple. Thus, you need to review your installation requirements so you can choose which type of thermocouple fits best. A thorough look at holes where the device is to be used should tell you what particular probe diameter fits the job.

It’s always best to do your due diligence when dealing with these devices. When you do, you’ll find no temperature sensing job could be too big with a thermocouple in place.


Older Post Newer Post

English