Thermocouples are incredibly useful and simple tools for measuring temperature. There are many varieties of thermocouples and they can consist of different metals. Despite this variety, all thermocouples ultimately do the same thing: generate a voltage corresponding to the temperature difference between two junctions. (An instrument, like a multimeter, then measures this temperature difference.) However, when it comes to taking precise temperature measurements, this measurement and conversion process doesn’t always cut it. This is where cold junction compensation comes in.
In this article, we’ll cover what exactly cold junction compensation is, how it works in thermocouples, and how to calculate it.
What Is a Thermocouple?
Before we can dive into the details of cold junction compensation, we have to start with thermocouples. A thermocouple is a sensor for measuring temperature that is built out of two different types of metal; these different metals join at two junctions, a measuring or hot junction, and a reference or cold junction. Thermocouples operate based on the principles of the Seebeck effect: When a temperature difference exists between two different conductive or semiconductive metals in a closed circuit, that temperature difference creates electrical voltage. This voltage is nonlinear in relationship to the temperature difference between the hot and cold junctions, and it’s defined and calculated by using standard reference tables or polynomial equations.
However, the cold junction can create measurement issues in thermocouples. As we mentioned earlier, the cold junction acts as the reference point for measuring temperature, so it’s supposed to remain at a standard, consistent temperature; traditionally that temperature was 0° C (32° F). Holding the cold junction at 0° C, though, requires using an ice bath — a setup that’s simply not practical for many applications that call for using a thermocouple. To solve this issue, experts must use a technique called cold junction compensation.
What Is Cold Junction Compensation?
Cold junction compensation, or reference junction compensation, is the process of adjusting the calculation of a thermocouple’s voltage measurement to properly account for what the voltage would be if the cold junction was at the ice point, 0 °C. This compensation allows for accurate measurement without the cold junction needing to be constantly at exactly 0 °C. In other words, cold junction compensation negates the need of using an ice bath for the cold junction.
To help complete this compensation, thermocouple cold junctions are often attached to a temperature detecting device, usually a thermistor or resistance temperature detector (RTD). (While many people confuse RTDs and thermocouples, they aren’t the same.) That temperature detecting device independently measures what temperature the cold junction is at. It’s then able to properly compensate for the actual temperature at that junction point. This compensation ensures that the voltage accurately represents the temperature at the hot junction.
Cold junction compensation is an important step in taking temperature measurements with a thermocouple, as it not only guarantees far more accurate readings, but, by compensating for ambient temperature changes at the cold junction, it also helps standardize those readings and account for any potential environmental variance.
How Is Cold Junction Compensation Calculated?
The process of calculating thermocouple voltage is complicated and dependent on a number of factors, such as what type of metals a thermocouple is made of, whether or not the Seebeck coefficient of those metals changes with temperature, and more. However, the formula for calculating cold junction compensation is relatively simple and is as follows:
Eemf = −S∆T = (Sm1 - Sm2)(THOT − TCOLD)
- Eemf is the voltage output of the thermocouple.
- Sm1 is the Seebeck coefficient of the first metal type in the thermocouple.
- Sm2 is the Seebeck coefficient of the second metal type in the thermocouple.
- TCOLD is the temperature of the cold junction.
- THOT is the temperature of the hot junction.
Of course, many of these different elements of the formula require doing calculations of their own. The first step is to measure the actual thermocouple output voltage. Next, measure the temperature of the cold junction (TCOLD) and use the appropriate thermocouple reference table for your thermocouple type to find the voltage compensation that corresponds to the temperature. Depending on the degree of the difference in cold junction temperature from the ice point, the change in voltage could be anywhere from fairly small to one or two whole millivolts (mV).
After obtaining the correct number from the lookup table, sum the cold junction equivalent voltage with the measured thermocouple voltage, then use the reference table (or polynomial) to determine the temperature of the hot junction. This is now the correct temperature of the hot junction with proper cold junction compensation in place.
Alternatively, there are online calculators that you can use to greatly simply the process, such as this thermocouple table voltage calculator. (This thermocouple voltage to temperature calculator can also be useful.)
Cold Junction Compensation Still Needs Proper Calibration
Cold junction compensation may correct for ambient reference temperature errors, but it can’t help account for inaccurate temperature readings from a thermocouple that has degraded or has experienced measurement drift over time. Thermocouples, just like all measurement equipment, must be regularly calibrated to ensure accurate and reliable readings. A reliable thermocouple allows for a seamless compensation process and leads to better, more precise measurement wherever and whenever you need it.