DAQ signal accessory and thermocouple reading fluctuations

[engrudy]

I have a DAQ signal accessory connected to a laptop via a 6024E DAQ card. I am attempting to connect a type K thermocouple to the thermocouple port on the signal accessory and take temperature readings. The VI that I have set up is a slightly modified 'continuous thermocouple reading' example VI. I am having problems getting accurate readings. The temperature fluctuates 30-40 degrees between readings. This is impossible do to the lack of thermal variations in the environment. Please help!

[markwysong]

I'm leaning towards the resolution of your board.

Is it the exact same amount (35.2 deg or something) every time, or does it change? Is it sometimes 10 degrees, sometimes 14, sometimes 30?

Mark

[engrudy]

The resolution is 12-bit, should be adaquit. The readings fluctuate: some times 5 degrees, sometimes 40 degrees. There is no distinct time frame between fluctuations either. Every reading fluctuates. The general trend of the temperature graph follows the expected temperatures, the individual readings are useless though. I have a feeling that the signal isn't being amplified enough for the DAQ to read properly, the DAQ card is capable of +/- 10V readings. Is this a possibility? If so, then the signal is not set up to be amplified in the signal accessory as I have tried other DAQ signal accessories (I have access to a bunch of them) in an attempt to determine if the equipment was faulty. Is this something that needs to be set up with software (Labview
or NI Measurement Explorer)? Other things I have tried is changing the properties of the DAQ card in NI-MAX. There is no option of assigning the DAQ signal accessory as an accessory in the properties dialog box, should a driver be there or something? Any help would be much appreciated.

Mark

[markwysong]

Just a couple of questions here. First, your DAQ card has the capability of amplifying these signals before it is digitized. The E-series cards can go down to +/-50 mV range, which should give you decent resolution for your thermocouples.

What signal conditioning are you using for your thermocouples? (Are you using SCXI?)

Since thermocouples are very low level signals, they are prone to noise. Therefore, they usually have filters on them to get rid of any AC interference. If you are using signal conditioning, see if there is a filter you can set. For example, the SCXI-1102 has a 4 Hz filter you can use, which is very appropriate for thermocouples, and will most likely help immensely with your noise problem.

Mark

[engrudy]

The only signal conditioning device present as of now is the DAQ Signal Accessory box. It has a thermocouple input jack, which leads me to believe that some type of signal conditioning is occuring internally. However, I have not found any supporting literature on what the intensity of this signal conditioning is. I understand that the SCXI system is far better for thermocouple applications, but I am trying to utilize what I have.

I am wondering if that +/- 50 mV range is adjustable in NI-MAX. The only range shown when setting up the hardware using NI-MAX is the +/- 10V range. Thanks for your help.

Mark

[markwysong]

The NI catalog describes the 6024E card as having a range down to +/- 50 mV. You do not set this range in MAX.

The Continuous Thermocouple Measurement VI that you refer has a control on the front panel for the Temperature Range you want to look at. This is where the range of the DAQ card gets set--the voltages cooresponding to the temperatures in this control, as well as the thermocouple type determine what range of voltages you need to read. This is sent down to the DAQ card, which selects the range most appropriate to read these voltages.

Is the DAQ signal accessory box from NI? I can't find it in the catalog. Does it have a Cold Junction Compensation (CJC) sensor on it? You'll need this to calculate the temper
ature correctly.

Mark

[engrudy]

Yes, the DAQ signal accessory is from NI. I know its a little hard to find (took me about an hour to run it down the first time), but if you search for 'DAQ signal accessory' on the online (www.ni.com) "Prices & Specs" search it will be the first result. It has a IC sensor for CJC compensation. The DAQ signal accessory has this channel labled as 0 or 5 (the thermocouple is channel 4). I have tried interchanging the two CJC channels with the same results.

As far as the temperature range control on the front panel, I have set it to the range I am trying to read. I have also checked to make sure that all of the wires on the wiring diagram correspond to the inputs with each of the controls, sub VI's, and installed equ
ipment.

[markwysong]

First, it mentions turning the noise generator off before taking any thermocouple measurements. Make sure it is off.

Second, are you calculating the CJC correctly? According to the manual, you read the voltage from either channel 0 or 5; the IC puts out 10 mV per degree C.

Mark

[engrudy]

The noise generator is off. The IC sensor fluctuates like the TC jack does (as I checked it using the MAX test panel for both channel 0 and 5). The average of the readings is right at what is expected, so I'm assuming that the IC sensor is doing its job. I just can't understand how a TC jack on this equipment can't get better readings than +/- 50 degrees between readings. I can send some screenies of what I'm getting on the graph or an excel file of outputs if that will help.

[markwysong]

Nah. I think we've isolated the problems.

It sounds like noisy inputs to the DAQ board.

As I said, TCs are very prone to noise on the line, and normally there's a filter on the TC to eliminate the noise, and just get the DC component of the TC--but you don't have that.

Therefore, to get the same effect, you're going to have to average many readings, which will in effect average out the AC component.

Another thing--if your panel (TC junction) is in a controlled environment (same temperature all the time), I would average the CJC IC voltage, and just continuously use that, rather than reading it every iteration.

Try something like this attached file.

Mark