Data acquisition - scaling thermocouple

[TobyETO]

Hello All.

Very new here, Just started learning lab view.

Going through the core courses and making projects.

I am stuck with a thermocouple project...

Everything is starting to take shape, but I still cannot get the scaling right so the temperature is displayed in degrees C...

This set up seems to have come up a few times in core 1, but the scaling just seems to work... 

maybe sombody here could take a look at my Vi and suggest where i might be going wrong.

Help appreciated! Thanks in Advance!

Toby

[cbutcher]

A few possible points:

• You're sampling at 6kHz, although with the DAQ assistant rather than DAQmx I'm not exactly sure how many elements will be read at once (samples to read controls the buffer size, and may or may not control the read amount for DAQ assistant. It does not for DAQmx). This is fast for a thermocouple
• You're the lowpass filtering. Probably this is less necessary if you lower the sample rate a bit (you could still take mean values etc)
• All of the DDT (dynamic data type) wires hide the possibility of samples being lost somewhere. I don't know if this might be happening.
• The behaviour when wiring a DDT to comparison with a double might be anything (given the DDT has multiple samples probably, and the double is a single value - if you did this with an array and a scalar you'd get an array of booleans, here you display a scalar boolean - which value is compared or shown?)

I'd strongly suggest using instead DAQmx - see Learn 10 Functions in NI-DAQmx and Handle 80 Percent of Your Data Acquisition Applications for some starting guidance.

Beyond that, the scaling "should" work (probably, again, I'm not sure the behaviour with the blue wires 😮 ) so

• check that you do have your CJC set up correctly (apparently you're measuring it at 25 degrees C, is that really the case?)
• Is the thermocouple the correct type (it shows K for me)?
• Do you have some calibration information (may or may not be completely necessary, but likely will be more accurate). I don't see any calibration information, but that might be because it was uploaded/downloaded etc.

[TobyETO]

Thanks for your reply!

yes, the 6kHz setting was me just experimenting and i had not changed it back to a more appropriate level.

I have some filtering on because there is a lot of noise where I am. 

I have tried playing with the DAQ mx way of doing things too, this is used a lot on the core course.

i have attached a vi. that is basically taken from teh core course but set up to read from my thermocouple. I get the same issue there, it does not scale the readings to degrees C.....

the CJC is roughly right, the feed I am getting on my indicators is in the range of -2000000... so i dont think this is a CJC problem.

yes, the thermocouple is the correct type

i dont have any calibration information.

I have the thermocouple running through an instrumentation amp and then to NI USB-6000

The response i am seeing on MAX is correct visually. Also correct on the vi. the temp reading goes up as i would expect when the thermocouple is put in warm water.

It is just the scaling...

as far as I can tell, you are right! the scaling should just "work" but it does not...

[EamonnFPMcKernan]

You have a large negative temperature. Do you have the thermocouple wires swapped?

In the US for type K yellow is plus and red is minus. Here is the full colour listing:

https://www.madisonco.com/thermocouple-reference

[RTSLVU]

---

@TobyETO wrote:

 the feed I am getting on my indicators is in the range of -2000000...

..

---

Sounds like an open thermocouple. Disconnect the TC and see if it changes....

[cbutcher]

---

@EamonnFPMcKernan wrote:

You have a large negative temperature. Do you have the thermocouple wires swapped?

In the US for type K yellow is plus and red is minus. Here is the full colour listing:

https://www.madisonco.com/thermocouple-reference

---

But OP says the direction is as expected...

Do you couple the wires anywhere else (i.e. between the 'sensor' and the DAQ device/CJC)?

If you're changing the wire material (basically unavoidable if you're connecting to a non-TC specific extension cable) then that can cause offsets. Not sure if it can cause such a large offset...

[EamonnFPMcKernan]

I don't know what an OP is, but swapping the wires should be an easy test. Did you try this? Since you say the system responds to temperature changes I doubt it's disconnected, though maybe just one wire could be disconnected. Here is a table of voltage vs temperature if you have a multimeter handy.

https://www.thermocoupleinfo.com/type-k-thermocouple.htm

I use thermocouple wire for extensions, such as sold by Omega. Can you just hook up the thermocouple to your system for testing without the extension wire of the wrong material?

[cbutcher]

---

@EamonnFPMcKernan wrote:

I don't know what an OP is

---

Sorry - "original poster", or sometimes original post. I guess also a great many other things, but I'd guess those are the most likely in a forum.

[TobyETO]

So,

The TC is definitely fully connected.

I tried reversing the wires but this did not bring the figure down.

below is a screen shot of test panels in MAX, here the amplitudes seem quite fine...

So why when it goes through the vi does it go totally crazy?

[GerdW · Accepted solution]

Hi Toby,

in MAX you are reading voltage signals.

Are you doing the same in your VI? (Can't check due to its LabVIEW version.)

The USB6000 does not support TC measurements, but it can read voltages…