signal mistery

[Roddy]

Hi all,

I have a semi interesting problem that is actuall causing a lot of trouble:

The setup:

-the test station is 50 feet from the cDAQ.

-I am using twisted shield omega TC wire.

-I am using 6 type K omega TC.

-the PC is about 15 feet from the cDAQ.

The VI:

gather data from 6 TC and 3 voltages.

the voltage data is fine,but  I am using an isolated module.

it take the average (median) from 100 point every 10 sec.

The problem:

Like I said the voltage is fine, but every 20 minutes (exactly) there is a change in temp (+- 2 degrees); I will post a picture of my data but for now the data looks like a sine wave beween 48 and 52 degrees every 20 minutes. At first I though it was the station heaters but that was discarded when it happened while the heaters were off. Then I though it was the AC of the lab but that wasn't either.

Possible cause of the problem:

-50 feet of TC wire; but the voltage wire is 50 feet too and no problem.

-the wire are run in the celing and who knows what is up there (high voltage cables, magnetic fields, vibrations)

-the different metal of the wire, tc and cDaq that can be acting as TC

-I haven't grounded the unit yet, but I don't think is going to make a difference... I hope so...

-my VI?

Any of you have any idea where this can come from?

[DavidMaidman]

After properly grounding the unit you could try to include a signal conditioning unit close thermocouple so that you are transmitting larger voltage signals across the 50ft reducing the effect of any noise you are picking up.

Hope this helps

[johnsold]

David M has given some good advice.

What you described sounds like a grounding problem.  Something turns on or off every 20 minutes and changes the current through a ground conductor to shift the voltage at the cDAQ enough to appear as a 2 degree shift.

Lynn 

[Wayne.C]

Roddy,

Your post mentions a sine wave.  The frequency of this sine wave should be a clue as to the source.

[AnalogKid2DigitalMan]

What are you measuring the temperature of and what type of environment is that object contained in?

If in ambient, could it be the air handlers cycling thus adding a little bit of heating or cooling to the object?

-AK2DM

[Roddy]

I actually tried to groun 3 of the 6 TC but there was minor-to-no-at-all improvement.

[Roddy]

The enviroment is kinda difficult to describe; it is and R&D lab so there is a bunch of stuff in the celing (industrial style drop ceiling tiles)--I guess that the cables are subject to vibration, air system, high volagte cables, etc...

[Roddy]

I really don't know what to think...

if they were cause by electromagnetic field, I would expect noise every 20 minutes but wthat i get it is a semi perfect sine wave from 48 to 52 back and forth...

[johnsold]

Please post a VI showing the results your are seeing.  Run your VI.  After the data is collected showing the shift at the 20 minute point and the sine wave, Make Current Values Default and save the VI.  Then post that VI.

Often someone will recognize a pattern or see something in the way your VI is put together which will help you solve your problem.  It is very difficult to adequately describe such situations in words.

Lynn 

[wd8ivl]

I wonder if what you are seeing is thermocouple action between the thermocouple and it's connection to the cDAQ board. 

If that connection point doesn't have TC compensation, what you could be seeing is caused by a swing of the room temperature where the thermocouple to cDAQ board is located.

Just my $0.02

Mike