Rtd/Pt1000 Measurement - Beckhoff EPP3504-0023 Short Manual

Product overview
3.2.4

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 EPP3504‑0023
The EPP3504‑0023 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 EPP3504‑0023 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.
Note on 2-/3-/4-wire connection in R/RTD mode
With 2‑wire measurement, the line resistance of the sensor supply lines influences the measured value. If a
reduction of this systematic error component is desirable for 2‑wire measurements, the resistance of the
supply line to the measuring resistance should be taken into account, in which case the resistance of the
supply line has to be determined first.
Taking into account the uncertainty associated with this supply line resistance, it can then be included
statically in the calculation, in the EPP3504-0023 via CoE object 0x80n0:13 [} 67].
Any change in resistance of the supply line due to ageing, for example, is not taken into account
automatically. Just the temperature dependency of copper lines with approx. 4000 ppm/K (corresponds to
0.4%/K!) is not insignificant during 24/7 operation.
A 3‑wire measurement enables the systematic component to be eliminated, assuming that the two supply
lines are identical. With this type of measurement, the lead resistance of a supply line is measured
continuously. The value determined in this way is then deducted twice from the measurement result, thereby
eliminating the line resistance. Technically, this leads to a significantly more reliable measurement. However,
taking into account the measurement uncertainty, the gain from the 3‑wire connection is less significant,
since this assumption is subject to high uncertainty, in view of the fact that the individual line that was not
measured may be damaged, or a varying resistance may have gone unnoticed.
Therefore, although technically the 3‑wire connection is a tried and tested approach, for measurements that
are methodological assessed based on measurement uncertainty, we strongly recommend
fully‑compensated 4‑wire connection.
With both 2‑wire and 3‑wire connection, the contact resistances of the terminal contacts influence the
measuring process. The measuring accuracy can be further increased by a user‑side adjustment with the
signal connection plugged in.
24 Version: 1.2 EPP3504-0023