About RTDs

 

   RTDs are suitable when extremely stable and precise measurements are required, or when accuracy over a prolonged time is the most important factor (the accuracy and precision of an RTD often exceeds that of both a thermistor and thermocouple). RTDs follow Deutsche Industrie Normen (DIN) and/or Joint Information Systems Committee (JISC) national standards and with good tolerance specifications, off-the-shelf RTDs are very consistent regardless of their batch number.

RTDs are very delicate, and while the melting temperature of an RTD element is sufficiently high enough to survive many high-temperature manufacturing operations, they do not tend to survive aggressive mechanical operations (such as compaction), which results in them being difficult to embed into custom mechanical devices. This limitation can be reduced through the use of metal-sheathed assemblies that remove the fragility, but this is at the cost of response time. Additionally, their larger size typically results in slower response times than comparable thermocouples.

For a typical 100ohm RTD, wire and termination resistance associated with long lead lengths and multiple connections can become a significant source of error. To achieve the highest accuracy, three or four-wire RTDs are often used. The electronics can be constructed to dynamically remove error associated with lead resistance, but there is also a trade-off in terms of cost and the number of wires required to perform the measurement.

Noise from external sources can create additional measurement problems, but can be mitigated in much the same way as thermocouples – by using differential, ungrounded, and shielded elements. These effects can also be limited through optional electronics that perform 10% duty cycle measurements to limit self-heating power without reducing signal strength. However, the trade-off for utilizing low-level signals (power) to drive an RTD is that even further measures may then be required to minimize the effect of external noise.