Thermocouple readout fluctuations

[Tom Jenkins]

I have found RTDs to be much less susceptible to noise and wiring and so on than T/Cs. In the temperature range you are looking at RTDs would be a better choice. This may be out of the question since you already have the material, but it's something to think about when you upgrade the system.

 

[leitmotif]

MGVOL

Boy are you right. I was surprised that the little current (total 8 amp on 120 V line) would do this. As I said before I read in many places about this happening. Well now I KNOW it does happen and I got a feeling for the conditons (magnitude ??) to avoid. Kinda like reading about smashing thumb with hammer - you know about it -- But AFTER you do it you REALLY KNOW AND UNDERSTAND it.


To all

THanks for help

Dan Bentler

 

[Lancie1]

leitmotif,

Its too late now for your class, but, doing some thinking "outside the box", a great solution (because this was a training classs) would have been to "multiplex" the heater and the thermocouples. That is, connect the PLC so that it can turn the 8 Amp heater off as needed. Now when you need to read the T/C's (every 10 seconds perhaps?) you just disable the heater output, take your reading in a noise-free environment for 1 second, then switch the heater back on

Maybe this is not usually very practical, but I have had to do it that way a few times in the real world. It works if your control can stand to use non-continuous temperature readings (most can actually). You just hold the old reading for 10 seconds and use it until the next one shows up.

 

[Unregistered]

leitmotif,

Its too late now for your class, but, doing some thinking "outside the box", a great solution (because this was a training classs) would have been to "multiplex" the heater and the thermocouples. That is, connect the PLC so that it can turn the 8 Amp heater off as needed. Now when you need to read the T/C's (every 10 seconds perhaps?) you just disable the heater output, take your reading in a noise-free environment for 1 second, then switch the heater back on

Maybe this is not usually very practical, but I have had to do it that way a few times in the real world. It works if your control can stand to use non-continuous temperature readings (most can actually). You just hold the old reading for 10 seconds and use it until the next one shows up.

LANCIE

BRILLIANT
Confirms more minds often help AND solutions do not come easily to me at 8pm after looking at it all day.
Thanks

Dan