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

[Wayne.C]

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]

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]

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

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

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

Regards

Ash Palmer

[Wayne.C]

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.

[K_Bull]

Hi there Ash,
What software are you using to measure the temperature? If you have MAX, are you able to replicate this issue in there and does this still happen if you change the measurement range? Do you get any errors what so ever?
Have you made sure of the working temperatures of the thermocouple is within range of what you are doing? Most of the thermocouples listed on www.ni.com measure only up to 482 degrees Celcius. So you might want to check the limitations of your K type thermocouple.
EDIT - the maximum temperature value is valid for the thermocouple cable as advertised on ni.com and this may vary with regards to different types.
Best Regards,

Message Edited by Kabul on 10-22-2007 03:23 PM

[AshP]

Thanks for the replies centerbolt and Kabul,

cenerbolt, I will now have to wait till the furnace is relit until I can check the voltage to 'com' on the TC but will take some readings so I have some sort of comparable readings, i.e. hot vs cold!  Regarding the wiring, nothing visually seems out of place, but i may change the TC ( see below on some other findings regarding the wiring!) to eliminate internal faults of that particular TC!

Thanks again!!

Kabul

Initally I was running LabVIEW 8.2.1 with DAQmx 8.5, I then applied the upgrade to LV 8.5 with DAQmx 8.6, this is when i noticed some problems, i then returned to 8.2.1 with DAQmx 8.5, obvioulsy now showing there's no fault with the software!!  With regards to replicating the fault, i have tried with a TC source and sending a signal to the DAQ this of course works perfectly fine.  My point of applying the TC source is where the normal point of termination of the Thermocouple.  I am unable to send the same signal to all 4 channels at the same time to replicate the furnace on, on zones 5 to 8, although i may try this at the DAQ!

Below is a portion of the code im using to acquire form the 9211.



I have done some further investigation today, i've attached a drawing which I hope is understanderble!
This is the current setup I have.  The thermocouples that are in use are a duplex type K. Initial setup is as per the top drawing, with several joints and what I suspect to be thermocouple pyro cable, if such a thing exists. 

In an effort to remove undesirable joints in the thermocuple cable, I have ran in a length of Type K  thermocouple extension cable, this is depicted via the bottom drawing!  At the point where these cables terminate onto the 24 Pair multicore cable, i measured two different resistance values!

I have now wired one leg of the duplex as per top drawing and the second one as per the second drawing!  I will have to wait till the furnace is re-lit and running >800oC to see if this changes anything!



Many thanks again for your advice!

Regards

Ash Palmer

[K_Bull]

Hi Ash,
I would recommend reading temperature in an ice water bath. The thermocouple generally is kept quite short and close to the signal conditioner/DAQ device. Here I see you are using a USB Daq to acquire a thermocouple signal over a distance of 40 meters? That is a long distance. If you do your ice water bath measurements with a short thermocouple wire (1m tops) and the 40m one, I am sure you will see a vast difference.
Also can you confirm the range (temperature) of your thermocouple?
Best Wishes,

[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