[ATR121] ATR121-AD with PT100 reading error

user11605 · Post by **user11605** » 20/07/2015, 16:01

We have the above unit operating with a PT100
SEn set to PtI
The relays operate at the correct temperature according to the display.
e.g. display =80 deg C
Relay prog to change at 80 deg C and the relay changes in line with the display.
The problem is that with a separate sensor (thermocouple) at the same location it is reading approx. 105 deg C
We have checked RTD
RTD wiring
and consider they are ok
Is there a reason for this type of offset??

Post by **Pixsys tecnico 1** » 20/07/2015, 17:16

20 degrees is a really high value, I think it isn't a PT100B "Beta385"

user11605 · Post by **user11605** » 20/07/2015, 17:49

Not sure I understand the 20 degree ref as a high value.
Do you mean as a difference ?

Post by **Pixsys tecnico 1** » 21/07/2015, 8:39

Yes, I mean as error in reading (difference between real and read).

user11605 · Post by **user11605** » 21/07/2015, 9:05

This is the spec for the RTD
Element: DIN IEC 751 Class B PT100 Probe:
316 stainless steel, 3mm or 6mm OD
Operating Range: -50 to +200°C
Cable construction: 3 core, FEP insulated Cable length:
2mtr standard

Post by **Pixsys tecnico 1** » 21/07/2015, 9:39

Hi,
disconnect the probe from the regulator and measure it with a multimeter.
At 105°C it must be about 140 Ohm. If it is about 130 Ohm the probe is reading 80°C so it is not working good.

Also check the cable you use: the three wires must have the same section and
electrical characteristics.

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