Connection of resistance-type thermometers

Two-wire measurement

When two-wire temperature sensors are used, the resistances of the sensor and wiring are added. The resulting systematic error must be taken into account when the signal evaluation unit is calibrated. A jumper must be connected between terminals T2 and T3 for this purpose.

Wiring errors

The errors that are generated by the wiring comprise approximately 2.5 K/?. If the resistance of the cable is not known and cannot be measured, the wiring errors can also be estimated using the following table.

Temperature drift dependent on the length and cross-section of the cable with PT100 sensors and an ambient temperature of 20 °C, in K:

Example: on a PT100 sensor with a cable length of 10 m and a conductor cross-section of 1 mm² the temperature drift equals 0.9 K.

Three-wire measurement

To minimize the effects of the line resistances, a three-wire circuit is often used. Using the additional cable, two measuring circuits can be formed of which one is used as a reference. The signal evaluation unit can then automatically calculate the line resistance and take it into account.

Connection of thermocouples

Based on the thermo-electrical effect, a differential temperature measurement will be performed between the measuring point and the signal evaluation unit.

This principle assumes that the signal evaluation unit knows the temperature at the clamping point (T2). For this reason, the 3RS11 temperature monitoring relay has an integral compensator that determines this comparison temperature and builds it into the result of the measurement. The thermal sensors and cables must therefore be insulated.

The absolute temperature is therefore calculated from the ambient temperature of the signal evaluation unit and the temperature difference measured by the thermocouple. Temperature detection is therefore possible (T1) without needing to know the precise ambient temperature of the clamping point at the signal evaluation unit (T2).

The connecting cable is only permitted to be extended using connecting leads that are made from the same material as the thermocouple. If a different type of conductor is used, an error will result in the measurement.

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Operating principle

Once the temperature has reached the set threshold value ?1, the output relay K1 changes its switching state as soon as the set time t has elapsed (K2 responds in the same manner to ?2). The delay time can only be adjusted with digital units (t = 0 on analog units).

The relays return to their original state as soon as the temperature reaches the set hysteresis value.

Temperature overshoot

Closed-circuit principle

Temperature undershoot

Closed-circuit principle

Range monitoring (digital units only)

Once the temperature has reached the upper threshold value ?1, the output relay K1 changes its switching state as soon as the set time t has elapsed. The relay returns to its original state as soon as the temperature reaches the set hysteresis value.

K2 responds in the same manner to the lower threshold value of ?2.

Closed-circuit principle

Principle of operation with memory function (3RS1042, 3RS1142) based on the example of temperature overshoot

Once the temperature has reached the set threshold value ?1, the output relay K1 changes its switching state as soon as the set time t has elapsed (K2 responds analogously to ?2). The relays only return to the original state when the temperature falls below the set hysteresis value and when terminals Y3-Y4 have been briefly jumpered.

Closed-circuit principle

Characteristic curves

For resistance sensors

Short-circuit and open-circuit detection as well as the measuring range are limited, depending on the sensor type.

Measuring ranges in °C for resistance sensors

? Detection possible
-- Detection not possible

¹⁾ NTC type: B57227-K333-A1 (100 °C: 1.8 k?; 25 °C: 32.762 k?).

For thermocouples

Characteristic curves for sensor types J, K, T, E, N

Characteristic curves for sensor types S, R and B

Measuring ranges in °C for thermocouples

? Detection possible
-- Detection not possible