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What is a thermocouple sensor?
Thermocouple sensor is perhaps the most simple and robust electronic temperature sensing devices imaginable. They consist only of two wires made out of dissimilar metals, attached together in a junction, which is placed where the measurement is to be obtained. Thanks to the Seebeck effect—named after German physicist Thomas Seebeck—if there is a temperature differential between the junction of these two wires, and their other measured ends are some distance away, there will be a voltage differential between them.

This voltage differential can be correlated to the actual temperature at the junction, thanks to the Seebeck coefficient, which relates the voltage potential to the temperature. This is often expressed as microvolts per degrees Kelvin (μV/°K), which is equivalent to microvolts per degrees Celsius. If needed, that can be converted to Fahrenheit by dividing by 1.8. While this seems simple enough, several factors are at play here that make the correlation slightly less than straightforward.

Temperature measurement using a thermocouple
While thermocouples can be made from a wide variety of metal combinations, the Seebeck constant varies depending on the construction. This number also changes over the thermocouple usable temperature range, so even if you’re using a standard metal combination, you’ll either need a chart or a mathematical model to correlate the temperature value. You also need to know the temperature at the measurement point, since the voltage is due to the temperature difference, not the absolute temperature itself.

The good news is that standard thermocouple metallurgical combinations are well documented, so if you know the type of thermocouple and are able to measure the temperature at your meter, this can all be worked out. The even better news is that more than likely you don’t even have to do any of that. Electrical meters with thermocouple temperature sensing capabilities are able to automatically correlate this information when the proper probe is used, and can even measure the internal temperature to give an output in actual degrees—not just a temperature difference.

Type K thermocouples and more varieties
Standard thermocouple metal combinations are designated with a letter, such as “type M” or “type T,” and the most common general-purpose thermocouple is known as a “type K.” Type K thermocouples are made from alloys known as chromel and alumel, with a Seebeck coefficient generally pegged at 41μV/°C. This number, of course, varies over its useful measurement range of -200°C to +1350°C.

The type K’s measurement range isn’t unusual, and highlights this class of sensor’s versatility, temperature-wise. Since such a device is simply two wires coupled together, they can be made inexpensively and tend to be quite durable.

How does an RTD sensor work and what RTD types are there?
RTD stands for Resistance Temperature Detector, which refers to a type of temperature sensor. As the name suggests, it uses changes in resistance to detect temperature, but that in itself might not tell you how an RTD works. To know more about tenperature sensor types, you can read our article on temperature sensors

How does an RTD work?
When the temperature increases, the RTD’s resistance increases, and vice versa. The control system or transmitter constantly sends a current through the RTD sensor. When the temperature changes, the resistance on the current can increase or decrease. Here, the RTD detects this shift and thereby reports it.

Vendors build RTD sensors out of a variety of materials. Platinum, copper, and nickel show up the most often, with platinum as the fan favourite because it offers the best stability in a wide range of temps.

Also, we can choose RTDs with two, three, or four wires, but what does that mean? These wires create the compensation you need for the cable. Of course, the two-wire option doesn’t, so we should choose it when you need only an approximate value. The three-wire option is most common in many applications.