I am having a variety of problems sensing process temperatures accurately in an ice rink application including temperature drift and cross talk between channels.
First about the system: I have a SNAP-AIRTD-8U with 4 100ohm 3 wire PT sensors attached. The sensor leads are soldered to 3 wire, 16 awg unshielded cables. The cable lengths are 160 ft, 100 ft, 100 ft and 5 ft. The cable leads connect to a screw terminal strip and then 20 awg wires, about 18 inches long, connect from the terminal strip to the SNAP-AIRTD-8U. These are stranded wire with the ends soldered where they insert in the terminal strip and the SNAP-AIRTD-8U.
I need this system to repeatably measure temperatures around 20F (20F =/- 10F). The absolute accuracy is not as important at the repeatability, meaning that if a sensor reports 24F for the conditions, I need it to always report 24F for the same conditions, even if a mercury thermometer would give a different value.
Here are the problems.
moving the cables changes the reading
The cables run from my machine room to points on the refrigeration system. For half the run, they run next to each other before splitting up to their final destinations. If I grab a cable in the middle of the run and move it a few feet, the temperature reading will immediately change a few degrees and mostly stay at the new value.
abrupt changes on one sensor changes all the readings.
One of the sensors measures outside air temp (OAT). I first noticed that all the readings seemed to drift with time of day so I tried an experiment. When I put the OAT sensor in a cup of ice water, the reported temp drops quickly as expected but the other three readings also immediately drop a few degrees. Stirring my coffee with the outside air sensor has the opposite affect on all the temperature readings.
cable temperature sensitivity
Also as an experiment, I collected up some of the snow that we have a lot of at the rink and covered about 10 feet of the cable bundle. This caused a 6F drop in all the readings. When the snow melted, readings returned to the former values.
I clearly have some design flaw in my system. I am on the forum hoping someone can point me in the right direction to fix this.
Thanks in advance and I am sorry if this question has been asked and answered before. I searched and couldn’t find it.