Electricity + Control August 2017

TEMPERATURE MEASUREMENT

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.

R esistance 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.

Take Note!

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.

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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).

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Figure 1: Pt100 in 2-wire connection.

Resistance thermometers change their electrical resistance as a function of temperature.

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 mm 2 , 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 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.

20 Electricity + Control

AUGUST 2017