|Accuracy, Stability, and Repeatability.|
These three terms are often confused, but it is important to understand the difference.
Class A: Δt = ±(0.15 + 0.002 • | t | )
Class B: Δt = ±(0.30 + 0.005 • | t | )
| t | = absolute value of temperature in °C
Class A applies to temperatures from –200°C to 650°C, and only for RTDs with three- or four-wire configurations. Class B covers the entire range from –200°C to 850°C.Stability.
This is the sensor's ability to maintain a consistent output when a constant input is applied.
Physical or chemical changes can cause calibration drift. The material that the platinum is adhered to,
whether wound on a mandrel or on a substrate, can expand and contract, straining the wire.
Drift rates conservatively specified by manufacturers are typically 0.05°C/yr.
Repeatability is the sensor's ability to give the same output or reading under repeated identical conditions.
Absolute accuracy is not necessary in most applications. The focus should be on the stability and repeatability of the sensor. If an RTD in a 100.00°C bath consistently reads 100.06°C, the electronics can easily compensate for this error. The stability of RTDs is exceptional, with most experiencing drift rates of 0.05°C over a five-year period.