thermocouple measurement directly into USB DAQ

[heineken]

hi , with regards to the above matter..

i'm having complications using the DAQ to take measurement directly from the thermocouple..

im currently using the acq thermocouple sample.vi to take the temperature reading..

but what does the cold junction constant means ?

i am unable to figure out what it implies

using the default settings of constant and 25 degree CJC value

the temeprature reading fluctuates a lot ranging from negative few thousdand degrees to slightly above 200 plus degrees..

thanks in advance

[tzj]

CJC (Cold Junction Compensation) is required for all thermocouple measurements as a reference. Thermocouples measure a temperature between two points and not absolute temperature. Many thermocouple devices have another built in temperature sensor (thermistor for example) to supply this value (this is the CJC). Your built in CJC could be causing problems. If you bypass this and put in a set value (I see you put in 25 degrees) and still have problems, then there is something else wrong. Are your thermocouples insulated? What kind of application/temperatures are you dealing with? Have you tried attaching a different thermocouple? Also it is important that you have the right type selected/plugged in.

[jon_mcbee]

What hardware are you using?  Is it the 9211 from NI?

[heineken]

im using a K-type thermocouple with a USB DAQ 6008 cause its the only DAQ available for me..

by the way what do you mean by the right type selected/plugged in ?

the 25 degrees is the default value set by the DAQ assistant.. should i change it ?

thanks again in advance

[jon_mcbee]

The 6008 has 12 bit resolution and a range of +/- 1 volt, this means the smallest change in voltage that it will measure is about 0.048 Volts.  The sensitivity of a K-type thermocouple is 41 µV/°C this means that you will only be able to measure a change of temperature that is about 10 °C.  Also, if you have any noise in your setup it will make it look like the temperature is jumping around in huge increments.  I dont know that this is your problem but it is definitely something to keep in mind.

[tzj]

It might seem obvious, but when you set up the channels you have to select the thermocouple type. I've skipped this many times.
I'm not too familiar with the device you are using, but if the readings are reading strangely it could be a problem with the thermocouples rather than the DAQ. Try switching them out, making sure they are wired correctly, maybe even use another thermocouple type. Also see if just putting in a thermocouple with the probe end not attached to anything works.

The USB 6008 has a resolution of 12 bits, which is too low to accurately take data from a thermocouple. I am using a PCI 4351 DAQ board with a 2190 terminal block and things work fine. I would first off see if the thermocouples are working by disconnecting the probe end and seeing if it can hold a steady temp, check the wiring, and seeing about getting different DAQ hardware.

[StevenA]

The usb 6008 also does not have a built in CJC sensor.  Unless you make another temperature measurement to know what the temperature of wire conections to your 6008 (the Junction of the TC with the instrument) then if that temperature varies, your measurement will also vary.  The daq assistant provides a default of 25 C because no other active channel is avalible.  The 6008 device is really not a well suited device for direct connection to a Thermocouple. 

[tzj]

I guess, to sum it up, you will need new hardware. the USB 9211A will work for your application (assuming you have no problems with the thermocouples themselves)
http://sine.ni.com/nips/cds/view/p/lang/en/nid/202128
24-bit resolution is what you are looking for, it only has 4 channels though.

[jon_mcbee]

I agree with tzj, the USB 9211A is just the tool for this job.  Good luck.

[heineken]

i have chances of getting a 6211.. would that be a better option than the 6008 ?