Using the Labview VI "Convert Thermocouple Reading (scaler).vi"

[B_Myers]

I am having a problem using this vi (6.1) with a DAQ700 pcmcia card and breakout board with CJC.

The problem is the output temperature varies +/- 3 degC.

I put probes on the input voltages from TC's for the Ref and CJC voltages input and they are very steady.

Does anyone have any idea on how this vi works and what I may be doing wrong?

Thanks in advance!

[RIO-Todd]

Are you getting around the correct temperature when using this VI. Meaning, do you get the correct temperature but it's not a stable reading?

The VI itself just takes the voltage from the thermcouple and CJC and then takes the voltage from the thermocouple through the NIST formula to calculate the temperature. When you say the voltage is stable, how closely are you looking at the voltage? Is this with a DMM or with the DAQCard-700? One thing to note is that the thermocouple voltage is going to be very small (single mV range) and also note that a very small change in voltage (less than a mV) will result in a change in temperature. So, make sure you're probing the voltage with a device that has the resolution to detect a very small change.

To get additional he
lp on the VI, you can look at the context help which gives an explanation on all of the inputs/outputs of the VI as well as explaining the functionality. Make sure that you have selected the right type of CJC sensor (IC or thermistor). Also, take a look at the voltage array that is going into the VI. Make sure you show enough significant digits to see about 5-6 digits after the decimal place and see if the voltage is truly stable.

I would also try to create a thermocouple virtual channel in Measurement and Automation Explorer (MAX) and see if you see the same flucuation. The virtual channel should use the same formula that the Convert Thermocouple Reading VI.

Finally, I would make sure you've configured the DAQCard-700 for differential readings. And I would also suggest that you connect a bias resistor between CH- and AIGND of the DAQCard-700. The DAQCard-700 user manual discusses connections for differential readings in chapter 3.

DAQCard-700 User Manual
http://digita
l.ni.com/manuals.nsf/webAdvsearch/883A43ACCFEB3C9286256B2E007A2F00?OpenDocument&vid=niwc&node=132100_US

I hope this helps.

Regards,

Todd D.
NI Applications Engineer

[Process_Systems]

I would suggest that you get some signal conditioning to bring the signal level of the T/C up to a larger value before trying to measure it with the DAC 700. According to the manual, it's lowest (most sensitive range) is +/- 2.5 volts. The total number of counts in the ADC are diveded over this input range. If you feed the input with a T/C signal (usually less than 100 mv., then you will get very lousy temperature readout resolution. This will go a long way towards producing the temperature ambiguity you are experiencing.

Hope this helps somewhat.

Dave