Effect of chained sensitivity

Also relevant for the use of RTD sensors is the occurring concatenated accuracy (measurement uncertainty) and sensitivity (smallest readable temperature change) from sensor and used measuring device (Beckhoff RTD analog input) in an intended temperature range. So the following are concatenated (connected in series)

• the measurement uncertainty ±[°C] and sensitivity [Ω/°C] of the sensor and
• the measurement uncertainty ±[Ω] and sensitivity [Ω/digit] of the measuring device

Example:

• For a given temperature measuring range ∆T different RTD sensors are available, which have different resistance values and sensitivities in the intended temperature range. One of them is to be chosen.
• Various Beckhoff RTD measuring devices (terminals, box, module) are available for the resulting resistance range ∆Ω of the sensor, which also have different sensitivities and measurement uncertainties.

Thus, the total sensitivity and the total measurement uncertainty of the sensor + measuring device configuration may vary depending on the selected measuring terminal and sensor. Via the technical data of the sensor and the measuring terminals used, the values of the overall configuration and thus the optimum combination can be determined.

Measurement uncertainty calculation The mathematics behind combined measurement uncertainties can become very complex, in this example the simple linear approach is chosen as the worst case.

Example sensitivity:

• A Pt1000 sensor is used at a measuring temperature of 100 °C.
• The Pt1000 sensor has a sensitivity of 3.78 Ω/K and a resistance value of 1385.1 Ω at this temperature point.
• The sensor is used with an ELM3502 for data collection. This has a resolution of 8388607 digits over 2000 Ω and thus 238.42 µΩ/digit.
• The theoretical total sensitivity of the configuration is thus:
  238.42 µΩ/digit / 3.78 Ω/K = 0.063 mK/digit.
  Due to the signal noise of the device and sensor, this value is practically only achieved with very strong filtering!

Continuation to the measurement uncertainty:

• The measurement uncertainty of the terminal in the 2 kΩ measuring range is ±120 ppm^*) at an ambient temperature of 23 °C, i.e. ±0.24 Ω.
• Converted to Pt1000 temperature, the measurement uncertainty would be
  ±0.24 Ω / 3.78 Ω/K = ±0.063 K or ±63.49 mK.
• If the Pt1000 sensor under consideration is a sensor of accuracy class A, it has a temperature tolerance of ±(0.15 + 0.002 ⋅ T). At a measuring temperature of 100 °C, the possible deviation (measurement uncertainty) would thus be ±(0.15 + 0.002 ⋅ 100 °C) = ±0.35 °C.
• The total uncertainty is therefore ±0.063 °C + ±0.35 °C = ±0.41 °C.

*) Example value, please observe device specification