How to measure temperature?
Industrial temperature measurement is usually conducted in one of the two ways: with either a thermocouple or an RTD (resistance temperature detector). Both methods use the changing electrical properties of the sensing material with temperature to measure it.
How a thermocouple works?
A thermocouple is a pair of wires of different material that are joined at one end. At the other end there can be a small voltage measured between depending on the difference in temperature between the joint (hot junction) and the measurement point (cold junction). The relation between the voltage and temperature is non-linear and depends on the type of wire material. Different wire pairs form different type of thermocouples (K, T, J, E, R, S, T, C, B) and differ from each other in sensitivity and usable temperature range.
Thermocouples can measure temperature from almost absolute zero to more than 2000 degC, depending on the type.
What equipment is needed to measure temperature with thermocouples?
- Internal humidity sensor
- Unlimited logging on any platform
- Digital I/O
K-type thermocouple generates voltage of approximately 40 µV/degC, which means that 1 mV of measurement error would translate into 25 degC of temperature error. The measurement system therefore needs to have very good accuracy at low voltages. It also needs to have good protection from external disturbances such as 50 Hz noise from mains voltage or higher frequency electro-magnetic noise because even low voltages induced in measurement cables can translate in large temperature error. Measured voltage is linearized in software to compute the temperature, also taking into account the cold junction temperature.
A very important part of the measurement system is also the cold junction measurement. Cold junction is a point in the system where the thermocouple wires are joined with copper or gold (on the measurement device PCB or in the connector). The temperature at that point must be measured precisely since it is the reference for determining the absolute temperature at the hot junction.
How an RTD works?
RTD utilizes a change in resistance with temperature. The relation between temperature and resistance is non-linear, but not to the same extent as with thermocouples. Nominal resistance of an RTD at 0 degC determines its type: usually between 100 ohm (PT100) and 1000 ohm (PT1000). RTD’s are more accurate than thermocouples but also more expensive and can measure temperatures from -200 to 850 degC.
What equipment is needed to measure temperature with an RTD?
To measure resistance a precise current excitation is needed that generates a voltage drop accross the RTD. Low currents in the range of mA are needed in order not to overheat the RTD and cause measurement error due to self-heating. Voltage is then measured to determine the resistance and linearization is applied in software to calculate the temperature. No reference temperature is needed contrary to the thermocouples.