Few questions about LabView and thermocouples ( rather complicated )

[Galjuntur]

Hello, I plan to read 20 thermocouples [here are only 16] ( type K - in the daq assistant there is J but it will be changed soon ). I will be using two devices, usb 6008 and usb 6251. I made a program in LV ( see attached ) and I've got some questions/statements which need to be confirmed  :

1. I plugged my devices, put DAQ Assistant on the panel, chose temperature->thermocouples->K and I marked channels by dragging a "clicked" cursor over all channels. So now I am able to read all channels, right ?

2. I added some thermometers and linked numeric indicators with them to have an accurate reading from the thermometers in numeric data.

3. I read some articles about plotting multi channels on one wave chart and I would like to apply it to my program. Basically I don't know how to make it working, I need to make an array and connect it to the wave chart. The thing is, I don't know how to do it.

4. I would like to get 1 sample per 1 second, I added time delay = 1000ms. Unfortunately, DAQ Assistant says it works like 1 k samples per 1 Hz.

I'm quite fresh in LabView and currently I'm watching tutorials and other stuff. I would be grateful for any tips.

BTW I cannot test if it works with connected thermocouples because I don't have any at the moment and the place where I would be able to test the program has very limited access 😞

[GerdW]

Hi Galjuntur,

which hardware do you use?

2. You can enable a numeric display for each "thermometer" style indicator, no need to add another indicator…

3. When you don't know how to create an array you shoul dgo through the free online basics courses offered by NI for LabVIEW beginners!

4. Change your DAQAssistent to read 1 Sample/s, no need fo an additional wait function…

[Galjuntur]

I use usb 6008 and usb 6251.

So 1 Sample on Demand = 1 sample/s . I'm silly because I didn't notice this "1" before Sample on Demand.

I forgot to add that these thermocouples will be amplified by external op amps.

[GerdW]

Hi Galjuntur,

when your TC readings are amplified with some arbitrary OpAmp electronics you probably can't use the DAQmx internal conversion from TC voltage to temperature. Read the voltages values and use you own scaling to temperature in this case…

[Galjuntur]

Oh my... that makes sense. I'll look further into it, thanks for mentioning this issue.

There is one thing still unclear to me if it comes to my program. I want to read continously my thermocouples so would these two usb devices measure the connected TC simulatenously ? I mean, if both would work at once ? I don't want to find myself in the situation when after clicking run, one device is running. Then I push stop and again run so at this time, second is working while the first one is idle. Did I write it quite clearly ?

I'm still trying to figure out how to plot my signals on one chart. I try to follow this topic https://forums.ni.com/t5/Example-Code/Two-methods-for-writing-multiple-plots-to-a-waveform-chart/ta-... but I still cannot get the wires continous ( program indicates that the wires got loose end )

Thanks for helping me.

[GerdW]

Hi Galjuntur,

those two devices need their own DAQmx tasks, atleast one task per device.

And such DAQmxTasks can be run in parallel - no need to fetch values in serial order…

When you have trouble with a VI you should attach that VI. Or atleast a BD image (or better snippet) for a first impression…

[Galjuntur]

Here's my improved VI file, according to your tips. In the previous post's link, there are two methods for plotting multiple channels on one chart. I tried to make an array or cluster but I think, I'm still missing some knowledge. 

If it comes to the scaling option because of TC amplifying, I hope that I will have to only make a scalling "equation" bewteen the daq output and the signal splitter ( outside the loop, right ?) . ( not adding scaling to every splitted channel )

[Bob_Schor]

If you read and study this excellent NI White Paper, you will be able to eliminate both the DAQ Assistant and the Dynamic Data Wires, making your code, especially constructing your plots, much easier.

Bob Schor

[Sam_Sharp]

Never mind the software side...

You should bear in mind that thermocouples produce very small voltage variations with temperature - I seem to remember when doing some calculations on this (a long time ago), the minimum possible sensitivity was on the order of 2-3 degrees (i.e. 1-bit change in measurement was 2-3 degrees...you get more noise than that!) for the low-cost DAQ devices like the USB-6008 so you can't really use the lower resolution devices with thermocouples without first amplifying the signal.

Oh...would you look at that - I even found the article about it here: http://digital.ni.com/public.nsf/allkb/8166630BD6965EEA86256FDD0052E89C

Dedicated thermocouple devices have 24-bit resolution and +/- 80mV input range! Even the USB 6251 only has 16-bit resolution and +-100mV input range.

Edit: Crap, just read the part about using external op-amps...don't mind me!

[Galjuntur]

Well, I would like to study this program wider but I'm short on time so I cannot learn everything quite quickly. I'm still trying to find solution to this plotting, it really cannot be so hard. If I fail, I will save data to a file and then plot everything manually.

Unfortunately, I don't have channels name in the saved file. All I have inside the file is time ( this can be easily skipped in the options ) and values. Above every column of values, there is "Untitled". So it can be impossible to make a use of it due to the lack of names. Anyone got any ideas ? But please, dont tell me I have to play again with arrays or something that is needed to solve previous issue with multiplotting. 

P.S. Why none of these methods work for multi plotting ? ( second program attached )