100 Ohm Platnum RTD at sub zero

Adriel

Member
Join Date
Jan 2019
Location
Haifa
Posts
8
Hello and thank you to all of you who volunteer your time to help others.


I have a question about 100 Ohm Platinum RTDs.


Using this with an Allen Bradley 1771-IR series C It appears that when the temperature drops to sub zero the readings are not accurate. I see mercury and thermister thermometers reading -6 for example but the RTD reading +2


I checked the accuracy of the RTD with a glass of crushed ice when I installed it - it read 32 degrees and comparing with other devices it appears to be accurate at temperatures above zero.


Unfortunately I do not have a data sheet or any specifications for this RTD. It is 100 OHM Platinum RTD Probe .25" diameter, 1.75" in length, 3 Wire 40' long.

It was originally intended to be used in an incubator so low temperatures were not required.

I am guessing the temperature range of this RTD is the issue but I have never seen an RTD that did not read sub zero temperatures in my limited experience with them. If that is the case I am supposing I need a different RTD for these temperatures. What do you think?


Thank you!


Adriel
 
Last edited:
Are you looking at the data in the PLC-5 via RSLogix or indirectly through some attached device?

Can you see if any of the underrange/overrange bits are set?

Can you share the PLC-5 program (zip it and attach to a post)?
 
It should be no problem the sensor will work in sub zero, however the software could be the problem as the line is different, and please be sure you have the fahrenheit program instead of the celsius.
 
Thanks to both of you.. Looking at the data directly in the PLC.

Used auto config to create file so nothing outside of what you have seen before.

After thinking longer on this I decided to check the physical location of the sensor and see if there were any issues that could be affecting the reading.

There was a very very small almost undetectable air leak from a heated compartment to the tube the RTD is in. Once that was sealed the temperature read properly. The tube was pretty well isolated and sealed before but a couple leaks the diameter of a pin was enough to throw off the temperature.



Thank You for your comments!
 
Adrriel, Thanks to you for reporting back that the problem was that the high temperature readings were correct - it was not an equipment/instrument problem, it was a problem with the ‘process environment’, a leak.

All too often the forum resders don’t get feedback.

Thanks for the update.
 

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