Test & Measurement
How to Use the Thermocouple for Temperature Measurement
The most widely measured physical parameter is temperature. Whether in process industry applications or in laboratory settings, accurate temperature measurements are a critical part of success. Accurate temperature measurements are needed in medical applications, materials research in labs, electrical/electronic component studies, biology research, geological studies, and electrical product device thermal characterization.
AmonOne 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.