RTD/Pt1000 measurement

RTD specification and conversion

Temperature measurement with a resistance-dependent RTD sensor generally consists of two steps:

• Electrical measurement of the resistance, if necessary in several ohmic measuring ranges
• Conversion (transformation) of the resistance into a temperature value by software means according to the set RTD type (Pt100, Pt1000…).

Both steps can take place locally in the Beckhoff measurement device. The transformation in the device can also be deactivated if it is to be calculated on a higher level in the control. Depending on the device type, several RTD conversions can be implemented which only differs in software. This means for Beckhoff RTD measurement devices that

• a specification table of the electrical resistance measurement is given
• and based on this, the effect for the temperature measurement is given below depending on the supported RTD type. Note that RTD characteristic curves are always realized as higher-order equations or by a sampling points table in the software, therefore a linear R→T transfer only makes sense in a narrow range.

Application on the ELM350x

The ELM350x supports the measurement of resistances up to 2 kΩ in 2/3/4‑wire measurement and the conversion of Pt1000 RTD sensors up to 2000 Ω / 266 °C.

Although the ELM350x does not support a sole resistance measurement (without conversion to temperature), a resistance specification is given here because the temperature measurement is based on it.

¹) The offset specification does not apply to 2-wire operation, since the offset is increased on the device side. Therefore, a system-side offset adjustment is recommended, see "Note on 2-/3-/4-wire connection in R/RTD mode". The final targeting basic acuuracy within the 2-wire operation is mainingly dependent by the quality of this system-side offset adjustment.

³) Values related to a common mode interference between SGND and internal ground.

⁶) Calculated value according to equation in chapter "General information on measuring accuracy/measurement uncertainty" for quick estimation of usability over the specified ambient temperature range in operation (T_ambient). In real use, for example at a relatively constant ambient temperature T_ambient, a lower (better) achievable uncertainty is attained. A specific calculation according to chapter "General information on measuring accuracy/measurement uncertainty" is recommended, especially if the instrument allows a wider ambient temperature range in operation than 0...55 °C.

ELM3502 (20 ksps)

ELM3504 (10 ksps)

RTD measuring range

Fig.88: Chart: RTD measuring range

Data for the sensor types in the following table The values for the sensor types listed in the following table are shown here merely for informative purposes as an orientation aid. All data are given without guarantee and must be cross-checked against the data sheet for the respective sensor employed.

The RTD measurement consists of a chain of measuring and computing elements that affect the attainable measurement deviation:

Fig.89: Concatenation of the uncertainties in RTD measurement

The given resistance specification is decisive for the attainable temperature measurement accuracy. It is applied to the possible RTD types in the following.

On account of

• the non-linearity existing in the RTD and thus the high dependency of the specification data on the sensor temperature T_sens and
• the influence of the ambient temperature on the analog input device employed (leads to a change in T_measured on account of ∆T_ambient although T_sens = constant)

Basic accuracy for PT1000, 3-wire connection:

Basic accuracy for PT1000, 4-wire connection:

If further specification data are of interest, they can or must be calculated from the values given in the resistance specification.

The sequence:

• General: The conversion is explained here only for one measuring point (a certain input signal); the steps simply must be repeated in case of several measuring points (up to the entire measuring range).
• If the measured resistance at the measured temperature measuring point is unknown, the measured value (MW) in [Ω] must be determined:
  MW = R_{Measuring point} (T_{Measuring point}) with the help of an R→T table
• The deviation at this resistance value is calculated
• Via the total equation
  
• or a single value, e.g. E_Single = 15 ppm_FSV
• the measurement uncertainty in [Ω] must be calculated:
  E_Resistance(R_{Measuring point}) = E_Total(R_{Measuring point}) ⋅ FSV
  or: E_Resistance(R_{Measuring point}) = E_Single(R_{Measuring point}) ⋅ FSV
  or (if already known) e.g.: E_Resistance(R_{Measuring point}) = 0.03 Ω
• The slope at the point used must then be determined:
  ΔR_proK(T_{Measuring point}) = [ R(T_{Measuring point} + 1 °C) – R(T_{Measuring point} )] / 1 °C
  with the help of an R→T table
• The temperature measurement uncertainty can be calculated from the resistance measurement uncertainty and the slope
  E_Temp(R_{Measuring point}) = (E_Resistance(T_{Measuring point})) / (ΔR_proK(T_{Measuring point}) )
• To determine the error of the entire system consisting of RTD and the measuring device in [°C], the two errors must be added together quadratically:
  

The numerical values used in the following three examples are for illustration purposes. The specification values given in the technical data remain authoritative.

Example 1:

Basic accuracy of an ELM3504 at 35 °C ambient temperature, measurement of -100 °C in the PT1000 interface (4-wire), without the influence of noise and aging:

T_{Measuring point} = -100 °C

MW = R_{PT1000, -100 °C} = 602.56 Ω

= 86.238 ppm_FSV

E_Resistance(R_{Measuring point}) = 86.238 ppm_FSV ⋅ 2000 Ω = 0.1725 Ω

ΔR_proK(T_{Measuring point}) = (R(-99 °C) – R(-100 °C)) / (1 °C) = 4.05 Ω/°C

E_{ELM3504@35°C, PT1000, -100 °C} = (0.1725 Ω)/(4.05 Ω/°C) ≈ 0.043 °C (means ±0.043 °C)

Example 2:

Consideration of the repeatability alone under the above conditions:

T_{Measuring point} = -100 °C

MW = R_{Measuring point} (-100 °C) = 602.56 Ω

E_Single = 10 ppm_FSV

E_Resistance= 10 ppm_FSV ⋅ 2000 Ω = 0.02 Ω

ΔR_proK(T_{Measuring point}) = (R_{-99 °C} – R_{-100 °C}) / 1 °C = 4.05 Ω/°C

E_Temp(R_{Measuring point}) = 0.02 Ω / 4.05 Ω/°C ≈ 0.005 °C (means ±0.005 °C)

Example 3:

Consideration of the RMS noise alone without filter under the above conditions:

T_{Measuring point} = -100 °C

MW = R_{Measuring point} (-100 °C) = 602.56 Ω

E_Single = 37 ppm_FSV

E_Resistance= 37 ppm_FSV ⋅ 2000 Ω = 0.074 Ω

ΔR_proK(T_{Measuring point}) = (R_{-99 °C} – R_{-100 °C}) / 1 °C = 4.05 Ω/°C

E_Temp(R_{Measuring point}) = 0.074 Ω / 4.05 Ω/°C ≈ 0.018 °C (means ± 0.018 °C)

Example 4:

If the noise E_{Noise, PtP} of the above example terminal is considered not for one sensor point -100 °C but in general, the following plot results:

Fig.90: Diagram noise ENoise, PtP in dependence on sensor temperature