Question and Answer Guide: Thermocouples
Thermocouples are a tough nut to crack, there is so much to digest! If you’ve read through our other blogs on the topic but have some lingering queries, well you’re in luck! Read on to find out some common questions, and their answers, about this temperature sensing enigma. If you haven’t already, be sure to read through our main blog “What is a Thermocouple” to give you an introduction to the instrument, then come back here to get more in-depth answers to some queries you may have.
Q: Do thermocouples tell you the temperature?
A: Yes, but they do so in a different way than other thermometers. When one end of a thermocouple is heated it produces a voltage, also known as a difference in potential. The voltage produced is relative to the temperature difference between the measurement and reference junction. By using established tables we can translate the voltage reading into a temperature reading.
Figure 1: Enercorp thermocouple with display
Q: Is a thermocouple a thermometer?
A: Yes, a thermocouple is a thermometer. Let’s define what a thermometer is: The word “thermo” means “temperature” and the word “meter” means “measure”. So a thermometer is something that literally “measures temperature”. We can look at other types of thermometers to see how they qualify as temperature meters. A mercury thermometer works by measuring the level of mercury within a tube as the metal expands and contracts with the changing temperature. A Resistance Temperature Detector, or RTD, measures the resistance of an electrical signal as it is altered by rising temperature. And our thermocouple, of course, produces a measurable voltage that correlates into a certain temperature. All of these devices take the physical temperature and translate it into a readable signal; they are all thermometers.
Q: Why are two different metals required in a thermocouple?
A: It is the inherent properties of metal that affect how electrons behave within the wires of a thermocouple. For example, electrons will flow in a different way through iron than they do through copper. The two wires of a thermocouple are exposed to the same temperature at the measurement junction, exciting electrons and initiating a current. However, the electrical flow through each wire is slightly different so if you take a voltage measurement at the reference junction one wire will have more potential compared to the other. It’s this difference in potential that we use to calculate the temperature. If our thermocouple had two wires of the same metal the electrons would flow in the same manner through both wires. We would have no differences with which to calculate a temperature because there would be no voltage produced.
Figure 2: Cross-section showing two different metal wires inside protective sheath
Q: Can thermocouples only measure hot temperatures?
A: No, thermocouples can actually measure cold temperatures as well as hot. All sorts of temperatures, ranging from -200°C to +2300°C, can be measured by this device. Two dissimilar metals make up a thermocouple. These metals can be pure elements or mixtures of elements (alloys) that have different properties. The combination of specific metals make for thermocouples that perform well in certain temperature ranges; some types of thermocouples are good for measuring really hot temperatures, some for really cold. Indeed, knowing the typical temperature range that will be measured is an important aspect that should be considered when choosing a thermocouple for an application.
Q: Do thermocouples need heat to work?
A: Thermocouples do not need heat to work. Thermocouples need a temperature difference between the measurement junction and the reference junction to work. That temperature difference can be from something heating one end of a thermocouple, or something cooling one end of a thermocouple. That doesn’t mean we have to have something hot, we just need the temperature of the process being measured to be different than the temperature of our reference junction.
Q: What is cold junction compensation?
A: The “cold junction” isn’t some new part of the thermocouple we haven’t talked about, it’s just a more traditional term for “reference junction.” Thermocouples give off a measurable voltage difference between the measurement and reference junctions; allowing for a calculation with standardized tables to figure out what the corresponding temperature would be. Sometimes the reference junction is not always at a temperature that works with a standardized table; the calculated temperature of the thermocouple would be off.
We’ll apply a “cold junction compensation.” This is simply a mathematical adjustment to the reference junction. Now, no matter the temperature of the cold junction, we can accurately calculate the temperature that the thermocouple is measuring. In the figure below, we would take the measurement of the cold junction (°C temperature along the top) and compare it to the measured thermocouple voltage (millivolts in the body) to get the temperature of the hot junction (°C temperature along the left).
Figure 3: Thermocouple reference chart with Cold Junction Compensation
Q: Do thermocouples need a thermometer to work?
A: It may sound crazy, but yes, thermocouples need a thermometer to work. The voltage measurement of a thermocouples changes depending on the temperature difference between the measurement and the reference junctions. So we have three variables: voltage, which we can use a voltmeter to measure; reference junction temperature, which we need another sort of instrument to tell us; and measurement junction temperature, which in practice is always going to be unknown since we’re trying to measure it with a thermocouple in the first place. If we know two of these variables we can calculate the third. So yes, we need to know the temperature at the reference junction and have a way to measure the voltage of a thermocouple. Only with those two pieces of information can we then determine the temperature at the measurement junction.
Q: Are you serious? Thermocouples need a thermometer to work?
A: For a thermocouple to work we need to know the reference junction temperature. That temperature can be determined by another temperature sensing instrument like a resistance temperature detector (RTD). But there are other ways to tell temperature; we don’t necessarily need another type of thermometer to measure the reference junction. How about using water to sense temperature? It may seem odd but think of the temperature that water turns to ice. How about if your reference junction was submerged in a pot of boiling water? That would be 0°C and 100°C, respectfully. These are natural thermometers that we can use; a universal way of telling what the temperature is. Also, these constants never change; we can use them to know the temperature of the reference junction without even using a thermometer, anywhere, anytime!
Q: Measuring temperature with a thermocouple seems like a lot of effort…why use a thermocouple?
A: There are many different ways to measure temperature other than a thermocouple. We go over thermistors and RTD sensors in their own blogs here. There are also semiconductor sensors and mercury thermometers. You could use your hand as a thermometer; like your parents did when they put their hand on your forehead to see if you were running a fever. Each way of sensing temperature has potential advantages and drawbacks. For example, thermocouples can measure much hotter temperatures than RTDs, however, RTDs are usually more accurate. It really comes down to choosing the best temperature sensor to meet your requirements. Whether those are: measurement range; accuracy; cost; the operational environment; or some other variable. We’ve got an entire discussion about the benefits of thermocouples and how they are employed over at “What are Thermocouples used for?”
Temperature sensing is a process that occurs in all sorts of activities going on around us. I hope you have learned a little bit more about the role of thermocouples, how they work, and why they are used. While you’re here and learning, why not check out some of our other blogs? You never know what you might find out about the sort of instruments we manufacture here at Enercorp; sensors, controllers, and all sorts of other devices that play a role in the daily activities that we take for granted.