Accuracy, Stability, and Repeatability.
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Tolerance/Accuracy is calculated as: |
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Class B
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change in t=+/- (0.3+0.005|t|)
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Class A
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change in t=+/- (0.15+0.002|t|)
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1/3 Class B
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change in t=+/- 1/3 x (0.3+0.005|t|)
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1/5 Class B
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change in t=+/- 1/5 x (0.3+0.005|t|)
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1/10 Class B |
change in t=+/- 1/10 x (0.3+0.005|t|) |
|t| = absolute temperature
in °C. Where elements have a resistance of n x 100 Ohms then the basic
values and tolerances also have to be multiplied by n
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These three terms are often confused, but it is important to understand the
difference.
- Accuracy. IEC standard 751 sets two tolerance classes for
the accuracy of RTDs: Class A and Class B:
Class A: Δt = ±(0.15 + 0.002 • | t | )
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| Figure 3. Lead wires
have resistance that is a function of the material used, wire size, and
lead length. This resistance can add to the measured RTD resistance, and
improper wire compensation can result in significant errors. The common
configurations of RTDs are two (A), three (B), or four wires (C). |
Class B: Δt = ±(0.30 + 0.005 • | t | )
where:
| 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. 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.
Response Time.
Response time varies according to the application.
It is the sensor's ability to react to a change in temperature, and depends
on the sensor's thermal mass and proximity to the material being tested. For
instance, an RTD sensor in a thermowell will react more slowly than the same
sensor immersed directly into a process. RTD specifications will list the
sensor's time constant, which is the time it takes for an RTD to respond to
a step change in temperature and come to 63% of its final equilibrium value.
Response times are calculated in water flowing at 0.2 m/s and in air flowing
at 1 m/s. This gives a useful comparison of RTD sensor configurations.
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