Hi,

If I connect a PT100 directly to the dataTaker, correct values are obtained, no problem. We have a PT100 on a test rig connected to a PLC as a 3 way system, that is, the 2 red wires connect to the same point.

If we connect the dataTaker off the same connections, the values are incorrect on the dataTaker and the PLC. How can I overcome this problem.

Many thanks,
Nick.

Hi Nick,

Connecting one sensor to two reading devices is just far too hard. You have two devices trying to power one device and two very different acquisition systems trying to read it.

I would recommend you put in another sensor.

Cheers,
Roger

Hi Nick,

I work in the manufacture of temperature sensors. The 3 wire config pt100 is a compensated loop system where by the plc measures the resistance of the two common wires ie red wires in order to subtract this resistance from the sensor reading in order to compensate for the length of cable used.

Average cable used is 7 strands 0.2 mm which has a resistance of approx 0.3 ohms/meter and with the PT100 0.39 ohms = 1 deg C. When you have it connected to the PLC, the PLC has compensated but when you further connect it to the logger this is most likely only compensating the cable loop from the PLC to the logger and losing out on the process to PLC loop.

If you wanted to verify this measure the loop resistance between the the PLC and logger and see if that is how much the probe is out in temperature using 0.39 ohm/C. Two ways around this put in the offset of the loop that is not being compensated from the PLC to logger into the logger thus manually setting compensation or use a PT100 duplex 3 wire sensor one sensor two outputs no change in dimension and very little difference in cost.

Regards
stephen