How to Use the Thermocouple for Temperature Measurement

Among the different types of sensors available to measure temperature the thermocouple or TC is by far the most common. The key reasons are that thermocouples are low cost, extremely rugged, can be run long distances, are self powered, and there are many types of thermocouples available to cover a wide range of temperatures. Low cost speaks for itself in many applications. Ruggedness means they will last in many different environments, including outdoors and with exposure to harsh factory environments. Metal-sheathed TCs are available to help protect them in harsh or corrosive environments, or they can be run inside conduit piping. Different alloys allow different range and sensitivity of measurement. Some common types of TCs include J, K, T, E, R, S, B, and N, which refers to the type of material from which they are constructed (as in table 1). The type J, K, and T are the most common and are readily available in spools or pre-made forms. The ranges for all types of thermocouples can be found in NIST (National Institute of Standards and Technology) reference tables at www.nist.gov.



One important property of thermocouples is their non-linearity; that is, thermocouple output voltage is not linear with respect to temperature. Consequently, to convert output voltage to temperature accurately requires mathematical linearization.



Thermocouples consist of two dissimilar metals joined (either welded or twisted) together at one end and open at the other. They operate on the principle of the thermoelectric effect and can be thought of as the junction of two different metals producing a voltage when a thermal difference exists between the two metals (also known as the Seebeck Effect). The voltage signal at the open or output end is a function of the temperature at the closed end. As the temperature rises, the voltage signals increases.



Here’s what really happens. The open-end signal is a function of not only the closed-end temperature (the point of measurement) but also the temperature of the open end. Only by holding T2 at a standard temperature can the measured signal be considered a direct function of the change in T1. The open-end voltage, V1, is a function of not only the closed end temperature (the temperature at the point of measurement), but also the temperature at the open end (T2). The reason the voltage is developed is because different materials produce different voltage for the same temperature difference. This is the reason for the two different metals. If they were the same metals, then the voltage would be zero.



The industry standard for T2 is 0ºC. Most tables and charts make the assumption that T2 is at 0ºC. In industrial instrumentation, the difference between the actual temperature T2 and 0ºC is usually corrected electronically within the instrument. This adjustment is known as cold junction compensation or ice-point reference.



Advantages

Thermocouples have many advantages over other types of temperature sensors. For one,

they are self powered, requiring no external power supply. They are also extremely

rugged and can withstand harsh environments. Thermocouples are also inexpensive compared to other temperature sensors such as RTDs and thermistors and come in a wide variety of types with wide temperature ranges.



Disadvantages

Thermocouples are non linear and require cold-junction compensation (CJC) for

linearization. Also, the voltage signals are low, typically in the tens to hundreds of

microvolts, requiring careful techniques to eliminate noise and drift in low-voltage

environments. Accuracies are typically in the range of 1-3% depending on wire alloy consistency and cold junction accuracies.



Common Errors

Avoiding some common mistakes when setting up and using thermocouples will yield

better measurements. One common problem is that the CJC is not configured or

compensated properly or at all. This leads to inaccurate or nonlinear temperature

measurements.



Another mistake is not to use copper wire from the thermocouple connection to the

measurement device. Normally the measurement devices (voltmeters, DMMs, etc.) have copper input terminals. Using another alloy (tin, aluminum, etc.) essentially introduces another thermocouple into the measurement. This is because any junction of dissimilar metals forms a thermocouple. On the measurement device side, the voltmeter being used may not be sensitive or accurate enough for thermocouple measurements. To avoid the problem, make sure the voltmeter is sensitive and accurate enough for the low-voltage signals (uV to mV) of the thermocouples. Some proper shielding would also prevent any external noise. Surround the sensitive circuit with a conductive shield and connect it to circuit or measurement LO for maximum effect.