Ron Beaufort
Lifetime Supporting Member
a true story - which MAY or may NOT have anything to do with your problem ...
many years ago I built a full-scale "pilot plant" project for the tech school that I used to work for ... I had specified several UNgrounded thermocouples to take the temperature of the water we were pumping around ... the dingbats in the school's purchasing department sent the specs "out for bid" to various vendors ... one vendor assured them that they could save the state several dollars apiece on the thermocouples by using the vendor's low-priced model ... the purchasing department took the bait – and I didn't notice the substitution until AFTER I had installed the thermocouples ... the temperature readings were "all over the board" with wild, random fluctuations ...
the only way that I could get things to settle down was to remove the thermocouples from the piping and let them just "hang in the air" ... by the way, some of the system's piping was PVC plastic pipe – some of it was galvanized steel ... anyway, with all of the thermocouples removed from the piping, the temperature readings were rock-solid and accurately reflected the ambient air temperature ...
when I installed any ONE of the thermocouples, the temperature readings remained steady ... BUT ... when any TWO thermocouples were allowed to touch the water in the pipes, the readings went nuts again ...
when I carefully checked the model numbers of the thermocouples I had received, I found out that they were a GROUNDED type – not the UNgrounded type that I had originally specified ... I demonstrated the problem to my department head (management) and let him take up the issue with the purchasing department ...
when I finally got a set of UNgrounded thermocouples installed, everything worked perfectly ...
yes, I realize that you're working with RTDs instead of thermocouples – and with an SLC-5/03 system instead of a PLC-5 ... but I still offer this story from my own experience in hopes that it might be useful in solving your problem – or possibly the problems of some other forum reader in the future ...
many years ago I built a full-scale "pilot plant" project for the tech school that I used to work for ... I had specified several UNgrounded thermocouples to take the temperature of the water we were pumping around ... the dingbats in the school's purchasing department sent the specs "out for bid" to various vendors ... one vendor assured them that they could save the state several dollars apiece on the thermocouples by using the vendor's low-priced model ... the purchasing department took the bait – and I didn't notice the substitution until AFTER I had installed the thermocouples ... the temperature readings were "all over the board" with wild, random fluctuations ...
the only way that I could get things to settle down was to remove the thermocouples from the piping and let them just "hang in the air" ... by the way, some of the system's piping was PVC plastic pipe – some of it was galvanized steel ... anyway, with all of the thermocouples removed from the piping, the temperature readings were rock-solid and accurately reflected the ambient air temperature ...
when I installed any ONE of the thermocouples, the temperature readings remained steady ... BUT ... when any TWO thermocouples were allowed to touch the water in the pipes, the readings went nuts again ...
when I carefully checked the model numbers of the thermocouples I had received, I found out that they were a GROUNDED type – not the UNgrounded type that I had originally specified ... I demonstrated the problem to my department head (management) and let him take up the issue with the purchasing department ...
when I finally got a set of UNgrounded thermocouples installed, everything worked perfectly ...
yes, I realize that you're working with RTDs instead of thermocouples – and with an SLC-5/03 system instead of a PLC-5 ... but I still offer this story from my own experience in hopes that it might be useful in solving your problem – or possibly the problems of some other forum reader in the future ...
