The theory behind thermocouples and cold junction compensation (CJC), while not difficult to apply, seems to confuse control and process engineers frequently. In this article, I will explain what a thermocouple is, and attempt to demystify the concept of cold junction compensation.
Thermocouples are a relatively inexpensive thermoelectric device that is used frequently in industrial applications. If you’ve found yourself asking the question, what is a thermocouple, then I assure you that you’ve come to the right place.
What is a thermocouple?
A thermocouple is an electrical thermometer consisting of two dissimilar metal wires joined at one end to form the “hot junction”, and voltage sensing device such as a volt meter, or PLC at the other end to sense the voltage. The thermocouple junction or “hot junction” is the point where the two dissimilar wires are joined. This can be clearly seen in the image below.
When the hot junction is at a different temperature than the cold junction, a measurable voltage is generated across the cold junction. The “cold junction” or “reference junction”, is the end of a thermocouple used to provide a reference point.
What is a thermocouple used for?
Thermocouples are used to measure the temperature of solids, liquids and gases, in a variety of industrial applications. They are without question the most common temperature measurement device used in industrial applications today.
This is due to several reasons, which include:
- Thermocouples are simple in construction.
- They are fairly inexpensive.
- They have a wide temperature range.
- Thermocouples have reasonably good accuracy (although non-linear with voltage).
- They are self-powering, meaning, the device (i.e., PLC) receiving the thermocouple’s signal does not have to supply electric power to it.
What does a thermocouple look like?
In theory, a thermocouple is nothing more than two dissimilar metals twisted together at one end. In fact, I’ve made rugged electrical control panel temperature sensors by doing exactly that!
Simply take some of your wasted, or cut-off thermocouple wire, twist one end of them together to form the hot junction, and wire the other end into a spare channel on your PLC’s thermocouple module. It adds a great control panel temperature feature to your HMI display!
However, this is crude at best. The most common way of constructing a thermocouple consists of welding the two thermocouple wires together and then slipping ceramic beads down the open ends of the wires to provide separation and to keep them insulated from the thermowell.
Thermocouple theory of operation
Thermocouples work on a phenomena known as the Seebeck Effect. The Seebeck Effect is the conversion of temperature differences directly into electricity.
When the circuit is opened at the cold junction, an electrical potential difference (the Seebeck voltage) exists across the two dissimilar wires at that junction.
The Seebeck effect causes an electrical potential and a current to flow when two dissimilar wires are joined and the end is heated. The voltage produced depends largely on the composition of the two wires (what they are made of – more on that later) and the temperature difference between the hot junction and the cold junction.
It is important to realize that the voltage is NOT generated at the junction of the two metals but rather along the length of the two dissimilar metals that is subjected to a temperature gradient.
Since both lengths of dissimilar metals experience the same temperature gradient, the end result is a small, measurable potential difference (mV) between them.
As mentioned previously, however, this relationship is not linear, typically an 8th order polynomial that looks something like this…yikes!
Luckily, we don’t have to crunch the numbers to figure out what a temperature should read at a given voltage…enter “Thermocouple Tables”. We use tables instead that list temperature curves (T) vs Voltage curves (V) – more on this a little bit later!
What is Cold Junction Compensation?
To measure a temperature, one of the junctions – normally the cold junction – is maintained at a known reference temperature ( the ice point or 0°C) and the other junction is at the temperature to be sensed forming the hot junction.
If you examine the image below, you can see that we are trying to measure the Seebeck Voltage at the cold junction by placing what could be a digital multimeter across the Iron and Constantan leads of the thermocouple.
The problem is, when we connect the copper leads to the thermocouple leads we are creating another Seebeck Effect because our meter leads are dissimilar to the metals of the thermocouple. This introduction of a new or “intermediate” metal needs to be handled…let’s find out how!
There is a law known as, The Law of Intermediate Metals, that states, a third metal may be inserted into a thermocouple system without affecting the voltage generated, if, and only if, the junctions with the third metal are kept at the same temperature.
Therefore, to get around this problem, the cold junction is placed in the “ice bath” so this junction of the metals is at a known reference temperature, we’ll call Tref.
Of course, having a bucket of ice at every thermocouple junction is not very practical in an industrial setting is it…this is where Cold Junction Compensation comes into play.
Cold Junction Compensation
Cold Junction Compensation (CJC) is the process of using automatic compensation to calculate temperatures when the reference or cold junction is not at the ice point (or 0°C). Instead, we incorporate an artificial cold junction using a thermally sensitive device such as a thermistor, RTD, or diode to measure the temperature of the input connections at the instrument or PLC.
Most PLC thermocouple modules have Cold Junction Compensation built-in, but care needs to be taken to ensure you’re not creating cold junctions in your field wiring. This is why it is important to always use Isothermal (Constant Temperature) Blocks whenever you are field wiring thermocouples in junction boxes or control panels.
It should also be clear that care should be taken to minimize any temperature gradient between these terminals. However, if the voltage or temperature from a known cold junction can be measured or simulated, then the appropriate corrections can be applied. This is known as Cold Junction Compensation.
What are the different types of thermocouples?
Thermocouples come in various types and are identified by there ANSI codes J, K, T, E, N, R, S and B. The thermocouple letters indicate the type of alloy used in the construction of the thermocouple.
For example, from the table below, a type J thermocouple is comprised of the two dissimilar metals, Iron (Fe) and Copper-Nickel (Cu-Ni). Where the positive lead is Iron and the Negative lead is Copper-Nickel.
Also, looking at the table below you can see the temperature range of a J-type thermocouple is between -346°F and 2193°F when using thermocouple grade wire and 32°F and 392°F when using extension grade wire. This large operating range makes thermocouples very versatile for a variety of applications.
It should be noted, that type J and type K thermocouples are among the most widely used in industrial applications today. This is due to their high operating ranges as well as their greater sensitivity to temperature change (meaning a significant mV change per degree) when compared to a type B thermocouple, for instance.
The table below lists the different thermocouple types, their metal (alloy) combinations, their color coding standards, and their maximum operating ranges.
How to use thermocouple tables
Typically Thermocouple Tables will list millivolt (mV) readings from 2 to 3 decimal places. They usually come in temperature steps of 1, 5, or 10 degree, where 1 degree steps result in a 1 degree resolution.
The following is an example data sheet for a type J thermocouple.
To use a thermocouple table, such as the one above, follow these steps:
- Locate the correct thermocouple table of the type of thermocouple you are using.
- Find where the reference junction of the circuit is and, using an accurate thermometer, measure and record its temperature.
- Measure and record the voltage produced by the thermocouple. Watch the polarity (red lead is NEGATIVE) and record the proper sign with your reading. This is the measuring junction voltage, based on the reference junction of the circuit.
- Look up the reference junction voltage from the thermocouple table. Include the sign.
- Algebraically (i.e. include the signs) add the voltage produced by the thermocouple and the reference junction voltage found in the tables. This will “correct” the measured voltage to the reference junction temperature in the thermocouple tables. Where Vmeasured + Vreference = Vtrue
- Go to the tables and find this new (total) voltage. Note the temperature this is associated with. That is the temperature of the process at the measuring (or hot) junction.
I know that probably sounds confusing. Let’s do a quick example.
Example Thermocouple Calculation
Question: Imagine a type J thermocouple reads +15.935mV on a digital meter at its cold junction and has a reference junction at 25°C. Calculate the true process temperature at the hot junction.
Answer: Recall that, Vtrue = Vmeasured + Vreference
So what we need to do is lookup what the reference voltage is at 25°C from the table above. We find that a temperature of 25°C corresponds to a voltage of 1.277mV (make sure you find it yourself!).
Now we simply add the measured voltage and reference voltage we just found together to calculate what the true voltage reading is at the hot junction.
Therefore, Vtrue = 15.935mV + 1.277mV = 17.212mV
Now we need to refer back to the table to lookup the corrected 17.212mV that we just calculated. Using the correct row and column we see that the temperature corresponding to a voltage of 17.212mV is 316°C.
Because our table is in 1°C increments that is our maximum resolution.
Therefore, the temperature being read at the hot junction is 316°C.
Now you’re probably asking, well what if the true value for the voltage falls between two of the numbers in the table? Should you just round to the closest number?
Well, technically NO, however, depending on your application this might be fine. The more correct way to do it would be to perform a linear interpolation, however, we’ll save that discussion for another time.
Well I hope I’ve given you some things to think about and you now have a better understanding of not only how thermocouples works, but also the concept of Cold Junction Compensation.
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