6 key Differences Between RTD’s and Thermocouples
1. Measuring Range
The main advantage of thermocouples is their range. Most RTD sensors are limited to temperatures of up to 400-500 °C, and in some cases higher, whereas certain thermocouples
can be used to measure above the 1400 to 1800 °C range, which makes them suitable for a large range of applications.
provide the highest accuracy and may be the preferred solution when a temperature measurement accuracy is required to be around ± 0.05 to ± 0.1 °C.Thermocouples in comparison, have a lowered accuracy around ± 0.2 to ± 0.5 °C.
Although a thermocouple sensor system usually has a
faster response time due to the changing temperature at its point of contact, it generally takes longer to reach thermal equilibrium. This is largely due to the presence of the cold junction compensation, which does not respond to the change in temperature as quickly as the hot junction located at the tip of the sensor does. In comparison an RTD sensor is designed to be more durable and react faster to temperature changes (naked tip).
The RTD sensors drift is small due to their design, which makes them produce stable readings for longer durations than thermocouples can. Unlike RTD sensors, a thermocouple
has a relative high drift-over-time, which is typically caused by inhomogeneity of the conductor wires resulting from heat- and chemical exposure or mechanical damage, such as having been bent, tugged or squeezed during use. Due to this, frequent calibrations and adjustments are mandatory for thermocouples.
5. Single Point Measurements
Due to the design of a thermocouple, it is possible to narrow the measuring point down to the exact spot where the two metals are welded together. This point can then be defined very accurately when operating thermocouples
with “naked tips”. For RTD sensors however, measurements are calculated by taking the average value along the entire surface of the PT (platinum) element itself. This is mainly a disadvantage for large elements like PT100, whereas smaller elements like the PT1000 rarely have this issue, as some leading suppliers can provide PT1000
elements as small as 1x1.5 mm.
When it comes to cost, thermocouples are generally less expensive than RTD sensors, as most thermocouples cost between half to one third of an RTD. As mentioned however, thermocouples require regular adjustments and calibration, which in addition to the longer installation and setup times, adds to the long-term costs of the product.