Connection for resistance sensors

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/О©. 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 3RS15 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

When the temperature reaches the set limit value П‘1, the output relay K1 changes its switching state once the set time t has elapsed. The delay time can be adjusted. K2 responds in the same manner to the lower threshold П‘2.

The output relays return immediately to their original state (the RESET response is configured at Auto RESET) once the temperature reaches the respective hysteresis value.

Both thresholds П‘1 and П‘2 can be parameterized for overshooting or undershooting. This makes it possible to use a limit value for issuing an alarm signal to announce that a limit value is about to be overshot or undershot. The other limit value can be used for disconnection or to implement two-point or three-point control.

Note:

The "Temperature monitoring mode" parameter can be used to set the desired monitoring type (monitoring for overshooting or undershooting or range monitoring).

With the closed-circuit principle selected

Temperature overshoot

Temperature undershoot

Range monitoring

Memory function

The digitally adjustable temperature monitoring relays for IO-Link have a memory function. The memory function is illustrated below by the example of a temperature overshoot.

When the temperature reaches the set limit value П‘1, the output relay K1 changes its switching state once the set time t has elapsed (output relay K2 responds to П‘2 in the same way).

The temperature monitoring relays for IO-Link respond as described below:

• With temperature monitoring relays for IO-Link the memory function is activated as standard (RESET). The output relays only return to the original state when the temperature falls below the set hysteresis value and when one of the following steps is performed:
  • Brief jumpering of the Y3/Y4 terminals
  • Set the rotary knob to "RUN" position and press the right-hand arrow key
  • Perform a RESET through IO-Link
• If the Y3/Y4 terminals are permanently jumpered, the memory function is deactivated (Auto RESET). The output relays return immediately to their original state once a previously occurred fault has been rectified and the temperature falls below the respective hysteresis value.

With the closed-circuit principle selected

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 for resistance sensors

вњ“ Detection possible
-- Detection not possible

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

For thermocouples

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

Characteristic curves for sensor types S, R and B

Measuring ranges for thermocouples

вњ“ Detection possible

-- Detection not possible