03-30-2009 03:31 PM
Hi guys, I've searched the previous posts for this question, and did find similar issues but not the exact hardware I'm using.
I've got a SCXI 1101 chassis, the SCXI 1102 card and the 1300 TB measuring a type J TC using a standard Daq assistant in a while loop in LV 8.6. I've grounded the negative side of the TC connection to chassis ground as recommended in the 1300 instructions.
I have a part in a press that i'm monitoring temperatures on. I'm sure the press has it's own ground (although I haven't actually seen a stud on a frame etc). We were getting noisy temperature signals past roughly 400F., so suspecting a ground loop, I isolated the test part from the press with pretty good results. It's not perfect however, as I am getting some intermittent noise when the part is jogged in and out of the press - I'm suspecting the isolation isn't perfect so we're making contact to the frame when the part moves
My question is this - I noticed that some other recommendations in this forum (*for other equipment*) were to remove the ground within the terminal block as the ground on the frame was interfering wiht it. Can I do this with a floating (non ground referenced) TC with my hardware?
I cannot guarantee that I can get the negative only lead on the TC to touch the panel, while the positive would be not touching - is there anything I can add to this cirucuit to eliminate the effect of the ground loop?
maybe I'm off base here - but it sure looks like a ground loop problem. It's tough to explain to coworkers why our Fluke handheld TC reader is reading stable while the $$ NI isn't when i'm not 100% sure what's causing the problem. I'm assuming the Fluke's TC's are floating and the ground from the press is the only ground??
Thanks
Darren
03-31-2009 01:49 PM
03-31-2009 02:42 PM
It appears to have fixed the problem. I tried logging without the grounding to the SCXI 1300 and the readings are stable and are matching the old Fluke.
Thanks for the response.
Darren
Doug L. Bear wrote:
I would completely agree with your theory of ground loops. And would say that because your thermocouple is attached to a grounded source, it is in fact not floating and should not be grounded as per the methods in the 1300 manual.