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Thermocouple measurements that after a period of time suddenly go bad

The TC's are isolated, the sampling rate is around 20 Hz, the PWM is 15KHz. There is RC filtering on the PWM signal to provide me with the effective DC value (based on duty cycle) so we aren't even having the PWM signal actually into the measurement hardware.

Message Edited by LV_Pro on 04-15-2005 11:31 AM

Putnam
Certified LabVIEW Developer

Senior Test Engineer North Shore Technology, Inc.
Currently using LV 2012-LabVIEW 2018, RT8.5


LabVIEW Champion



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Message 11 of 14
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Looking at the signals (TC, etc.) isn't showing cross talk or noise being picked up, but of course if it is a transient event who knows

I wouldn't think that a noise problem would manifest itself as good readings for several hours, then sudden extreme garbage.

And if something like that did come up, it wouldn't be fixed by a power cycle of the SCXI box, UNLESS IT WERE COMING FROM THE SCXI POWER SUPPLY ITSELF !!



You mentioned it's a new installation - do you have any spare parts to throw at it?

Can you run for several hours with just the TC module installed?

If it's in the "wonky" mode, can you look at the power out of the supply and see if it's good? I don't know if you have a switcher or xformer/rectifier, but maybe a capacitor gets hot and the switching supply goes wonky until you kick it.

Steve Bird
Culverson Software - Elegant software that is a pleasure to use.
Culverson.com


Blog for (mostly LabVIEW) programmers: Tips And Tricks

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Message 12 of 14
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Not to beat a dead horse, but I don't believe that the T/C inputs on this unit are isolated. In an industrial environment, it is really a necessity to have isolated inputs. Have you tried my suggestion of disconnecting the inputs one by one to see if the problem goes away?
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Just wanted to provide an update. We have swapped in a new SCXI-1001 chassis and it has run 24/7 for a week with out the measurements going "flooie". Seems like we have finally nailed it down. Just gotta love those intermittent problems!


Putnam Monroe
Certified LabVIEW Developer
Putnam
Certified LabVIEW Developer

Senior Test Engineer North Shore Technology, Inc.
Currently using LV 2012-LabVIEW 2018, RT8.5


LabVIEW Champion



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Message 14 of 14
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