Thermocouples and resistance temperature detectors (RTDs) are temperature sensors that can be used in various applications. However, their design and mode of working are different. The sensor is made of two dissimilar metal wires joined at one end and leads to a thermocouple thermometer. There are various designs, but all of them follow the basic principle.
On the other hand, an RTD sensor consists of a platinum wire (PT) that wraps around a core made of ceramic material. The sheath is to protect the fragile material inside. There is a variation of RTDs called Thermistors. They differ from typical ones in that they have a polymer or ceramic resistor instead of metal.
Thermocouples also come in various types with type J, K, T and E being base metal thermocouples. There are also noble metal thermocouples that include R, S and B and are used in high-temperature measurements.
Here is a quick look at the differences of thermocouples vs RTDs to help choose the right one for each application, for a more in depth look into thermocouples, ProcessParameters.co.uk has an informative guide.
There is a huge difference between the two sensors in terms of temperature range. Many of the RTD sensors have a temperature range of between -200 and 8500C. This is a narrower range compared to a thermocouple that has a range of between -200 and 26000C.
The typical measurement range is between -50 and 3000C for a pt100 B and between 0 and 11000C for a type K thermocouple. There are thermocouples for very cold applications such as ultra-low freezers or cryogenics. These sensors include the Type T thermocouples that have a measurement range of between -2000C and 2000C.
Thermistors have a low-temperature range but higher than most RTDs. A typical PTC thermistor has a range of between -100 and 325°C. This range varies from one device to the other.
In a thermocouple, one end of the metal is the one that is used for temperature measurement and referred to as ‘hot junction.’ The other becomes a reference point, which is referred to as the ‘cold junction.’ The reference junction is usually held at 0°C.
However, it may be left in ambient room temperature and adjustments made on the external sensor to compensate for variations in temperature. A small voltage is generated when the external probe is cooled or heated. The charge is equivalent to the change in temperature.
The RTD uses the metal base as the resistive material. As said earlier, most of these sensors are standard platinum resistance thermometers. However, some probes are made of nickel and copper.
The resistance to the flow of electricity onthese Positive Temperature Coefficient (PTC) devices increases with the rise in temperature. Therefore, you can measure the increase in resistance (ω) by passing some current at the material when it is heated or cooled. The grouping of a PTC thermistor depends on the metal used, manufacturing process and the structure.
This resistance is then converted to temperature based on the characteristics of the material at hand. In essence, RTDs are resistance temperature devices. The most popular ones, the PT100 sensors use platinum as the resistive material and have a resistance of 100 ohms at 0°C.
Both TRDs and thermocouples differ from negative temperature coefficient thermistors (NTC) in that the metals used in NTC devices have lower resistance in higher temperatures. Besides, a small change in temperature results in a greater change in resistance, which makes them more accurate than the rest of the sensing devices.
RTDs have higher accuracy compared to the rest of the temperature sensors. The international society of automation recommends these detectors for applications where a temperature measurement accuracy of between ± 0.05 and ± 0.1 °C is required. On the other hand, thermocouples have a lower measurement accuracy or between ± 0.2 to ± 0.5 °C.
RTDs are passive resistance thermometers that do not produce an output on their own. There must be a current source with the resistance being measured by another device. The device gets the voltage readings as the temperature increases on the RTD wire, where the resistance of each of the metals is known.
However, thermocouples are self-powered and do not require an external source of power. This is because they generate voltage if there are temperature variations. This allows them to be put in applications where it is not possible to add some source of power.
A thermocouple has a short response time (usually less than a second) because it uses point-to-point temperature variations for its reading. Its probes are highly sensitive. However, it takes some time to reach thermal equilibrium due to the cold junction compensation discussed in the ‘mode of working’ section above.
On the other hand, the RTD has a response time of between one and seven seconds. Its sensitivity is dependent on the length of the wire, with several wire lengths in the market. On the other hand, it has a better reaction to the change in temperature because the resistance changes immediately with heat variations.
The drift in RTDs is quite small, primarily due to their design. This allows them to be used for stable readings over a longer period than thermocouples do. On the other hand, thermocouples have a higher drift over the same period due to the inhomogeneity of the probe metals due to the exposure to heat.
The high drift means that thermocouples require frequent calibration adjustments than RTD sensors. Depending on the application and number of devices, this can be an extra regular cost to the organisation.
Industrial and electrical engineers have to choose the right thermoelectric temperature sensors for specific applications. They should consider the temperature range, accuracy sensitivity, mode of action, drift and reaction time, among other factors. Depending on the individual application, they need to list all these factors in the order of priority.
Applications can range from temperature monitoring in printed electrical circuits, gas or oil flow meters, heaters, heat sinks and electrical appliances. The right choice of resistors ensures accurate reading and the ease of taking temperature changes in the particular application.