Thermocouple Explained | Working Principles
In this article, we’re going to walk you through one of the most commonly used temperature sensing devices… the thermocouple.
We’ll discuss the basics of how they work, what you need to consider when choosing a thermocouple, and then the challenges associated with putting thermocouples into an industrial application.
If you’re deciding what instruments are going to be placed on a machine, or into a process, the more information you have, the better.
Having said all that, let’s start our walk down thermocouple lane…
Thermocouples – the basics
A thermocouple is an extremely simple device used to measure temperature.
Thermocouples tend to be inexpensive, durable, and can be fabricated into a variety of shapes and sizes.
Thermocouple hot junction
A thermocouple is made up of two dissimilar metal wires. Dissimilar is just a fancy way of saying Different, but for some unknown reason, that word tends to be used most of the time when discussing thermocouples.
In any case, the metal wires are connected together in only one place, typically the tip of the thermocouple.
Lots of manufacturers call that junction by different names. Hot Junction, Measurement Junction, Sensing Point, or Sensing Junction. Those terms all refer to the same thing… the place where the dissimilar metals are joined that will measure the temperature.
Thermocouple output connection
The wires at the opposite end from the sensing junction are then left available to connect to some kind of measuring instrument like a temperature transmitter, a simple electronic display unit, or even directly to a PLC thermocouple input card.
Thermocouple cold junction
The wiring terminals on the measuring instrument are most often called the Cold Junction.
While the Hot Junction refers to the tip of the thermocouple that will be exposed to the heat source of interest, the cold junction refers to the thermocouple wire connections that happen right at the measuring instrument, which typically is not exposed to the same thermal energy.
All thermocouples work the same way. They generate a small voltage when they are exposed to heat.
If you’re interested in the detailed physics of the way this works, you can research topics like The Thermoelectric Effect, or The Seebeck Effect, but to put things in simpler terms, when you heat up a piece of metal, the electrons in the metal want to move around more and will tend to move through the metal away from the heat.
Because electrons are negatively charged, the colder end of the piece of metal will have a negative charge compared to the hotter end.
A thermocouple works based on the movement of the electrons in its metal wires due to the heat difference between the hot and cold junctions.
If the two wires of the thermocouple were made up of the same type of metal, electrons in both wires would move away from the heat at roughly the same rate, so you couldn’t really measure the difference in the charge of the two wires.
But if you recall, thermocouples are made up of two different types of metal wire… and those wires are connected together only at the hot junction… the sensing end of the thermocouple.
The different metals in those wires, or more accurately the electrons in those different metal wires, react differently to heat.
Thermocouple wire leads
When exposed to heat, the electrons from one of the wires will want to move around at a certain rate. The electrons from the other wire will want to move around at a different rate.
– The wire that has the electrons that move more ends up being more negatively charged at the cold junction… and will therefore be called the negative wire lead.
– The wire with the slower electrons won’t build up as much of a charge, so it’s called the positive wire lead.
That difference in charge between the positive and negative wire leads can be measured and used to calculate the heat at the hot junction.
This is the basic principle of how a thermocouple works, so let’s look at this more closely at a specific thermocouple to make sure we’ve got it.
Let’s look closely at a Type-K thermocouple. A Type-K thermocouple is probably the most commonly used thermocouple in industrial applications because it responds predictably across a very wide range of temperatures (say around -330 °F to around +2460 °F).
The exact temperature ranges will change a bit based on how the manufacturer builds the body of the probe and the insulating materials used, but with that wide range, you can see why it can handle almost any application…
Type-K thermocouples are made from the metal alloys Chromel and Alumel.
As we heat the wires, you will notice that the electrons in the Chromel wire don’t move around as much as the electrons in the Alumel wire.
Over a very short period of time, you can see the Alumel wire has more electrons collecting at the cold junction… the cooler end… which means the Alumel wire will have a negative charge compared to the Chromel wire.
This difference in charge, also called a voltage, can be measured.
The more heat you apply to the metal wires, the more the electrons want to move around, and the more they move away from the heat.
With the two different types of metal wire, the difference in the voltage will increase and decrease with changes in heat at the sensing point.
The thermocouple Voltages are very small. The actual change in voltage per degree Celsius is minuscule. For example, for a Type K, the change is about 41 µV/°C. Also, interestingly, all T/C voltages are 0 mv at 0 °C.
Because the thermocouple manufacturers carefully select the metal alloys when they build the thermocouples, anyone can convert those voltages into temperatures using standard calculations.
In fact, most manufacturers will provide some voltage to temperature charts so that you can get a good idea of the temperature difference between the hot and cold junctions.
How to choose a thermocouple
There are many different types of thermocouples. Most manufacturers have selection guides to help you decide what to purchase.
In addition, most reputable manufacturers will also have technical support specialists available that will walk you through a series of questions to help select just the right type of thermocouple for your application.
I) Range and accuracy
You’ll need to consider things like the range of temperatures you are trying to measure, and the accuracy you would like to have.
This will help narrow down the two different metal wires that you need to use. The type of thermocouple is based on the type of metals used in the sensing wires.
1) Type-K thermocouple
As we mentioned before, a Type-K thermocouple is made from Chromel and Alumel alloy wires. It can be used to measure temperature from around -330 °F to over +2460 °F. That temperature range can cover a wide range of applications.
You need to keep in mind that the accuracy of a Type-K thermocouple may only be about plus or minus 5 °F across the entire range. Sometimes that’s fine, but sometimes more accuracy is needed.
2) Type-T thermocouple
If you were looking at a cryogenic application that needed more accuracy, but you didn’t need to worry about high temperatures, you could consider a Type-T thermocouple.
A Type-T thermocouple is made from a copper wire and a Copper-Nickel wire. Type-T thermocouples are usually accurate to within a degree or two… so that makes them about twice as accurate as Type-K thermocouples.
Type-T thermocouples can typically measure even lower than -330 °F but the upper end of the range is usually just over 600 °F. They are more accurate but have a more limited measurement range.
These are common trade-offs for thermocouples. The metal pairs that can do more are often less accurate… a sort of jack of all trades but master of none scenario. If you want greater accuracy, you either need more expensive metals, or you need to narrow the temperature range.
II) Styles of thermocouples
In addition to the different TYPES of thermocouples, there are different STYLES of thermocouples.
Many applications for temperature measurement can get away with just using thermocouple wire with the ends twisted together.
In that scenario, care must be taken to protect the wire from vibration or physical damage, but the ability to use just a pair of flexible wires can solve a lot of mechanical challenges with getting the sensing point position in the area you need to measure.
In cases where a little more protection is desired, the wires are encased in a probe, which is just some kind of protective sheath and some insulating material to help protect the wires.
Thermocouple probes come in Ungrounded, Grounded, and Exposed Junction style. The style of thermocouple probe you select will also be based on your application.
1) Exposed junction thermocouples are when the sensing wires are joined together out beyond the end of the probe sheath.
Exposed junction thermocouples have the fastest response time to temperature changes, but because the sensing junction is exposed, it is more vulnerable to breakage. These probes are usually used to measure gases.
For corrosives and liquids, Grounded or Ungrounded thermocouples work best.
2) A Grounded probe is when the sensing junction is in direct contact with the end of the sheath.
This makes the heat transfer at the end of the sheath faster, which improves the response time of the thermocouple, but it also makes the sensing wires more vulnerable to electrical noise like ground loops. This can take away from the accuracy of the grounded probe.
3) An ungrounded probe is when the sensing junction is joined just inside the end of the probe. This means there is a small layer of insulating material between the sensing junction and the very tip of the probe.
It insulates the sensing wire from electrical noise, but the heat transfer is slower because of the insulating material. So an ungrounded probe tends to be more accurate, but slower to respond.
As discussed, thermocouple accuracy is very sensitive to the types of metal used in the wiring. Unfortunately, this makes installation in an industrial application, challenging.
A) Cold junction compensation
One problem that you need to deal with is something called Cold Junction Compensation.
Earlier in this article, we stated that the thermocouple signal is based on the difference in temperature between the hot and cold junctions. But we need to make a small signal correction to convert that temperature difference into an absolute temperature.
For example, if the temperature in a reactor is 700 °F, but the outside ambient temperature at the location of the measurement instrument cold junction is 70 °F, the thermocouple millivolt will only show a temperature of 664 °F.
Even worse, as the ambient temperature goes up and down during the day and night, the measured reading can change, even if the reactor temperature remains constant.
Fortunately, most measurement instruments can perform cold junction compensation… either with standard built-in equipment or with an optional cold junction compensation add-on.
Cold junction compensation components measure the temperature of the metal at the cold junction wiring connections and then make a signal calculation correction.
That way, the reactor will now read the true 700 °F, and the reading won’t change… day or night… as long as the reactor temperature doesn’t change. So for the most part, cold junction compensation is pretty easy to deal with.
B) Remote temperature monitoring
A bigger challenge for thermocouples is if you need to make a temperature measurement in a remote location.
1) Extension wire & terminal block
If you need to extend the wiring of the thermocouple, you must use something called a thermocouple extension wire in order to reduce the amount of error.
Trying to splice standard copper signal wiring or even using standard terminal blocks to extend the thermocouple signal can end up creating additional cold junctions in the circuit and introduce more signal error.
Thermocouple extension wires are sold by Type… just like thermocouples.
A thermocouple extension wire is constructed of the same types of metal as the thermocouple, so when you need to extend a circuit, a thermocouple extension wire of the same type as the thermocouple must be used.
Similarly, if you need to use terminal blocks to connect the wiring, the terminal blocks must be constructed of the same types of metal as the individual wires.
If you have a Type-K thermocouple, you need to use Type-K extension wire and special terminal blocks made of Chromel and Alumel.
Trying to extend a thermocouple using standard copper wire will introduce errors into the measurement because the electron movements will be disrupted by the different metals.
2) Temperature transmitter
It is not recommended to run thermocouple extension wire over a long distance due to the sensitivity of the signal to electrical noise.
If you are trying to go more than 50 to 100 ft, you need to consider using a temperature transmitter that converts the millivolt signal over to another signal type such as 4-20 mA.
3) Remote I/O
If you have multiple temperature measurements to make in a remote location, a remote I/O rack from your PLC might end up being a good option.
Thermocouple connection examples
Let’s look at two different examples. First, we have a refrigerated warehouse at one end of an industrial facility. The control room or a more centralized control system main rack is located a few hundred feet away.
A simple refrigerated warehouse may not require a lot of automation, but you may need to make sure the temperature inside the warehouse remains cold.
You could try using thermocouple extension wire, which is cheap, but you now know the signal level of a thermocouple can’t run a long distance. So now what do you do?
You could install a remote I/O rack from a PLC system, and then put a thermocouple I/O card in the remote rack, but the rack and the I/O card can begin to increase the overall cost of the solution.
Instead, you can purchase an inexpensive temperature transmitter that converts the millivolt signal from the thermocouple into a standard 4-20mA signal that can more easily travel across a larger distance with greater resistance to electrical noise.
What if you needed to take several measurements at that same warehouse. What if there were multiple HVAC zones in the building that needed to be optimized to keep energy costs down.
Now, with several measurements to take, having to buy multiple transmitters for signal conversion may cost more than a remote I/O rack…
Thermocouples are a good solution for temperature measurement, as long as you do some homework upfront about your application, and pay attention to some of the challenges that might come up.
So hopefully you’ve enjoyed your walk through the world of thermocouples. While the science of how electrons move through metal can seem complex to the average person, taking advantage of simply connecting two different metal wires together to make a temperature measurement is pretty straightforward and inexpensive.
There are other types of temperature sensors that can be used to solve installation challenges, such as an RTD… but that’s for another article…
If you haven’t already read the following related article, you might want to give that a look:
While this article can stand by itself and get you up to speed on everything you need to know about thermocouples, the other article will give you some good general information on several different types of temperature sensors.
If you have any questions about the thermocouples or about temperature sensors in general, add them in the comments below and we will get back to you in less than 24 hours.
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