PT100 periodically reading 0°C

Hi,

I have a Logo set up with 8 analogue inputs, 1 EM with 2 4-20mA inputs then 3 EM's all RTD/PT100 modules with 6 PT100's connected.

I have gradually expanded the system to its current configuration without any problems. But since I have installed the final PT100 (AI8) have have noticed periodically that the reading from this PT100 will read 0°C.

I didn't notice at first as this PT100 is measuring outside temperature and when checking it has been plausible that it was 0°C outside, however with the temperature now warming up i have noticed that occasionally the sensor will read 0°C when it shouldn't be. Then it will revert back to a correct figure.

There doesn't seem to be any pattern to this and it does not happen on any of the other analogue inputs.

Just wondered if anyone else has had similar or if anyone could suggest the cause/a fix.

Thanks

Rick

I'll double check that, the terminal rail connections are Wago spring terminals so confident they are OK, but it is quite tight for space especially at the EM so a good chance there is potential for a loose connection there.

But would a loose connection return a reading of 0°C? Would it not return a a minus value?

Hi ixo65,

yes it is standard PT100 setup. Also the reading of the sensor to the left (AI7) is also correct. Its quite strange and wondered if it was a capacity issue (not sure if that's the right phrase, but if i am pushing the limits of the Logo).

I will check the connections but will also swap AI8 with another AI to see if the problem moves with the senor or stays with the input and take it from there.

Thanks for your input.

Hello Luwarick,

Exactly 0°C as an error message is a bit strange. If the error occurs again, then check the values ​​at the analog inputs via the logo menu. It might not be a program error.

Regards Ella_68

(translated by google)

When I said capacity I was referring to memory/processing capacity. Its quite a large program (well large for me) with a second Logo and a KTP touch screen HMI networked to it and wondered if that was having a negative impact.

Yes I will open a support request, good call.

Thanks

Rick

Hi Ella,

I havent yet check what is actually being read, its a bit of a pain hooking up and going online as it means loosing all the data from counters etc.so so far have avoided doing so. Had not thought of reading from the Logo screen. Excellent idea!

Thanks

Rick