IMC CANSAS - PT100 (RTD) - Measurement (Type: LEMO); Probe-Breakage Recognition

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SC16, SCI8, SCI16 voltage, current and temp. modes 375

The Pt100 reference current source is connected to the internal reference of the differential amplifier,

which has block-to-block isolation. It thus assumes the common mode voltage of all other connected

channels by turns, as long as they are operated in mode other than Pt100.

6.18.3.3 PT100 (RTD) - Measurement (Type: LEMO)

With the LEMO terminals at the IP65 housing SL it's only possible to connect Pt100 sensors directly in 4-

wire-configuration. A (supplementary) reference current source feeds the sensor. How to connect a

Pt100 sensor is shown below.

Connection scheme of a PT100 sensor with LEMO terminals

6.18.3.4 Probe-breakage recognition

SC modules come with the ability to recognize breakage in the probe lines. In case of a breakage within a

short time (only a few samples), the measurement signal generated by the module approaches the

bottom of the input range in a defined pattern. The actual value reached depends on the particular

thermocouple. In the case of Type K thermocouples, this is around 270°C. If the system is monitoring a

cutoff level with a certain tolerance, e.g. "Is the measured value < -265°C", then it's possible to conclude

that the sensor is broken, unless such temperatures could really occur at the measurement site.

The bottom of the input range will be displayed:

if at least one of the thermocouple's two lines breaks

if a channel is parameterized for "Thermocouple" and measurement starts without any

thermocouple being connected.

if the cold junction compensation is missing

For Pt100 in case of a short-circuit.

For Pt100, note that in a 4-wire measurement a large variety of combinations of broken and shorted

leads are possible. Many of these combinations, especially ones with a broken Sense lead, will not return

the default value stated.

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