USB 9211A Acquisition Problems....

[AshP]

I am currently using a NI USB 9211A to measure 4 type k thermocouples from a furnace, i call these zones 5, 6, 7 and 8
 
The problem that I am seeing is that if all 4 zones are connected to the DAQ the temperature readings are all over the place and do not correspond to the furnace temp.
 
If zones 5, 6 and 7 are connected to the DAQ, it reads correctly and sensibly.  As soon as zone 8 (-) connection is attached to the DAQ the readings again go all over.
If zone 8 is the only thermocouple attached to the DAQ this then reads correctly.
 
Has anyone come across this problem or have any ideas as to what would cause/correct this problem?
 
Many thanks
 
Regards
 
Ash Palmer

[centerbolt]

Ash,
You need to figure out if problem is with zone 8 TC or input channel on 9211.
1 - Physically swap zone 5 & 8 thermocouples at 6211 terminal block.  I'm guessing that you will get crazy readings with all TC's connected. 
2 - Next unwire the zone 8 TC.  If things settle down then I would  inspect the wiring for zone 8 TC.  Make sure it is not shorted to something in furnace.

[AshP]

Also, forgot to add these symptoms only appear at higher temperatures of approx >800o C, the set point for the zones were approx 900 to 920o C

 

I have ran another just now, the temperatures of the zones are 750o C or less and zones 5 to 8 work correctly!?

 

Any help/advice or an insight into this would be greatly appreciated!

 

Regards

 

Ash Palmer

[centerbolt]

Ash,
All four channels share the same common. 
My suspicion (sp?) would be that TC 8 is making contact/shorted to something in the oven at higher temps and coupling some sort of signal into the 9211 that is upsetting the module.  It sounds like you might have an electrically hot TC.  You could unhook TC 8 and use a good voltmeter to measure potential from TC leads to COM on 9211 and see what you get.  Scanned through specs for module and it would not take much voltage to cause this kind of problem.
 

[AshP]

Thanks for the reply.
 
As you can probably appricate I have been going round in circles trying to figure out the source of this problem!
To confirm, testing this morning proves the fault follows the wire, i.e. swapping zones 5 for 8, still recieveing crazy readings!
Upon disconnecting this zone the readings return to normal.
 
I had spent some time previously trying improve the wiring connections, which seem sound, only that I am unable to check the connections at the T/C due to the furnace been on, fortunately I have a shutdown commening end of this week so I can further investigate the connections!!  What seems strange is that Zone 8 reads perfectly fine when it is the only T/C connected to the DAQ!?
 
Many thanks for your advice!
 
Does anyone know if all 4 channels within the DAQ commoned in some way or are they independant channels?  I have seen the 9211 manual which shows the open circuit detection and Filtered differential amp with the isolated ADC (page 10)
Thanks again, any further ideas?
 
Regards
 
Ash

[AshP]

Kabul,
Thanks for your reply.
To confirm temperatures, the furnace in question runs at a maximum temperature of approx 950 oC.  We use a type K thermocouple, the ranges of these are -250 oC to 1250 oC.  I am using a Type K thermocouple extension cable in places.
In a previous post I have mentioned  that I am monitoring zones 5 through to 8, with these 4 Im having problems.  I am also monitoring zones 1 through 4 in the same manner, i.e. USB 9211A, same wiring setup and transmitted down the same multicore thermocouple cable.  These 4 zones work perfectly!!
 
Regarsd
 
Ash

[thomas.baudouin]

Hello,I was wondering if an explanation was found for the incorrect behaviour with the thermocouples.One of my customer is facing a very similar issue.Thanks a lot,Message Edité par thomas.baudouin le 07-25-2008 09:25 AM