Electricity + Control August 2017

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20 Electricity + Control AUGUST 2017
TEMPERATURE MEASUREMENT
Resistance thermometers change their elec-
trical resistance as a function of tempera-
ture. This physical effect makes it possible
to measure the temperature of a process with a
Pt100. The resistance is determined by electronics
(e.g. temperature transmitter) by using a constant
current and measuring the voltage drop. Accord-
ing to Ohm’s law (R = U/I), the resistance [R] and
voltage [U] are proportional to each other at a con-
stant current [I]. There are three possible ways to
connect the Pt100 to the transmitter: In a 2-, 3- or
4-wire connection.
Figure 1: Pt100 in 2-wire connection.
Pt100 in 2-wire connection
With a 2-wire connection, the resistance of the ca-
ble is added as an error in the measurement. For
a copper cable with a cross-section of 0,22 mm2,
the following guide value applies: 0,162 Ω/m
0,42°C/m for Pt100. For a version with Pt1000 the
influence of the supply line (at 0,04°C/m) is smaller
by a factor of 10 in relation to the basic resistance.
The lead resistance becomes still less significant
in relation to the basic resistance R25 with an NTC
measuring element (e.g. R25 = 10k). Due to the
sloping characteristic curve of the NTC, the influ-
ence at higher temperatures increases dispropor-
tionately.
Figure 2: Pt100 in 3-wire connection.
Pt100 in 3-wire connection
The influence of the lead resistance is compensat-
ed to the greatest possible extent with a 3-wire
connection. The requirement for this is that the
lead resistances are the same, as can be assumed
with a 3-wire connection. The maximum length
of the connection lead depends on the conduc-
tor cross-section and the compensation options
of the evaluation electronics (transmitter, display,
controller or process control system).
Figure 3: Pt100 in 4-wire connection.
Pt100 in 4-wire connection
The 4-wire connection completely eliminates the
influence of the connection lead on the measuring
result since any possible asymmetries in the lead
resistance of the connection lead are also com-
pensated.
3 Ways to Connect Pt100 to
the Transmitter
Roland Sachs, WIKA
Pt100, Pt1000 and NTC are the most-used measuring elements in resistance ther-
mometers. I would like to use this space to consider the question of connection
types in greater detail.
With a 2-wire connec-
tion, the resistance of
the cable is added as
an error in the measure-
ment.
The influence of the lead
resistance is compen-
sated to the greatest
possible extent with a
3-wire connection.
The 4-wire connection
completely eliminates
the influence of the
connection lead on the
measuring result.
Take Note!
1
2
3
Resistance
thermometers
change their
electrical
resistance as
a function of
temperature.