IMC CANSAS - Pt100; RTD Measurement

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UNI8: universal 421

6.20.4.2 Pt100/ RTD measurement

DSUB-plug: ACC/DSUB(M)-UNI2

Pt100. RTD, platinum resistor thermometer. Along with thermocouples, Pt100 can be connected directly

in 4-wire-configuration. The 4-wire measurement returns more precisely results since it does not require

the resistances of both leads which carry supply current to have the same magnitude and drift. Each

sensor is fed by its own current source with approx. 1.2 mA.

6.20.4.2.1 Pt100 in 4-wire configuration

The Pt100 is supplied by 2 lines. The other two serve as Sense-leads. By

using the Sense-leads, the voltage at the resistor itself can be determined

precisely. The voltage drop along the conducting cable thus does not cause

any measurement error.

The measurement inputs +/-IN carry practically no current.

The 4-wire configuration is the most precise way to measure with a Pt100.

The module performs a genuine differential measurement.

6.20.4.2.2 Pt100 in 2-wire configuration

Use the software to set a Pt100 4-wire configuration, because the connection is made in the same way as

for the 4-wire case. The difference is that +IN(A)/sense(F) and –IN(B)/-VB(D) must be jumpered inside the

connector.

Note that the total cable resistance contributes to measurement error, and that this method is the most

imprecise and not to be recommended.

6.20.4.2.3 Pt100 in 3-wire configuration

The Pt100 is supplied by 2 lines. The other one serve as sense-lead. By using

the Sense-lead, the voltage at the resistor itself can be determined

precisely. The voltage drop along the conducting cable thus does not cause

any measurement error.

It is important, that the connection between +IN(A) to Sense

and -IN(B) to -VB(D) is made directly at the module.

3-wire configuration is not always as precise as 4-wire configuration. When

in doubt, 4-wire configuration is preferable.

The cable resistance that has to be compensated (simple line) is not

supposed to be more than 7 (ca. 140 m cable length with 0.35 mm

consider overview of cable resistance as function )

519

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