Trying to get an accurate reading from thermocouple

[johnsold]

OK.  The first test shows that your battery is almost dead. But more importantly it shows that the multimeter and the VI both give about the same result for a stable voltage source. The second test shows that the Omega gives reasonable readings on the voltmeter, so it is probably working.

The last test shows that the DAQ and the multimeter are still in agreement.  Is the Omega connected to the AI input and to COM? Is the thermocouple connected to a ground? If so, what is the voltage between that ground and the 9201 COM?

Your signal is a voltage signal.  The Acq 4-20 mA VI is probably designed to measure a current and may scale the result based on what it expects from a 4-20 mA system.  Find a continuous voltage acquistion example.

Lynn

[iinWill]

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@johnsold wrote:

OK.  The first test shows that your battery is almost dead. But more importantly it shows that the multimeter and the VI both give about the same result for a stable voltage source. The second test shows that the Omega gives reasonable readings on the voltmeter, so it is probably working.

The last test shows that the DAQ and the multimeter are still in agreement.  Is the Omega connected to the AI input and to COM? Is the thermocouple connected to a ground? If so, what is the voltage between that ground and the 9201 COM?

Your signal is a voltage signal.  The Acq 4-20 mA VI is probably designed to measure a current and may scale the result based on what it expects from a 4-20 mA system.  Find a continuous voltage acquistion example.

Lynn

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Omega is connected to the AI input AI0 and COM. The thermocouple is in the attached picture.The thermocouple is also plugged in through the module by the thermocouple connector, the yellow adapter you can see on the second pdf. The voltage between the COM port and ground is 0.077Vac.  Is it possible if you could show me a link to an example for the VI? the examples I found in NI seems too complicated. 

[johnsold]

That all seems reasonable.  I do not have any other ideas at the moment.

Lynn

[RavensFan]

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@iinWill wrote:

 The voltage between the COM port and ground is 0.077Vac.  

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That actually seems kind of high to me.  That would be like a 77 deg AC signal.  The AC voltage sounds like about the right magnitude if it was picking up 60Hz  interference somewhere.

More importantly, you need to measure the voltage between ground and COM on the DC setting.

[iinWill]

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@RavensFan wrote:

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@iinWill wrote:

 The voltage between the COM port and ground is 0.077Vac.  

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That actually seems kind of high to me.  That would be like a 77 deg AC signal.  The AC voltage sounds like about the right magnitude if it was picking up 60Hz  interference somewhere.

More importantly, you need to measure the voltage between ground and COM on the DC setting.

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I get a 0Vdc reading from ground to COM port. The DAQ isn't active, if that makes a difference. 
Anyone knows how to fix this? Still haven't a issue with the reading. Would it be the NI 9201 issue? 

[iinWill]

Here are some examples of the readings i get from my VI. They aren't correct, because when I try holding the thermocouple to warm it up, the temperature doesn't output the right temperature. It continuous gives random numbers that doesn't correspond to the thermocouple. 

[johnsold]

When I plot the data you posted, it certainly looks like you may have power line interference.  The time column in the data file was not saved with sufficient precision to get an accurate indication of the timing.

What is the sampling rate?

It is almost certain that you have a grounding problem. Try connecting a resistor - maybe 1 to 10 kohms - between Ch- and ground.

Lynn

 all the data

  a data subset

[iinWill]

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@johnsold wrote:

When I plot the data you posted, it certainly looks like you may have power line interference.  The time column in the data file was not saved with sufficient precision to get an accurate indication of the timing.

What is the sampling rate?

It is almost certain that you have a grounding problem. Try connecting a resistor - maybe 1 to 10 kohms - between Ch- and ground.

Lynn

 all the data

  a data subset

Thanks for the reply. I use a NI cDAQ-9174, which I thought was grounded, if not then would i clip a ground cable from the chassis to ground it? 

[Eric-E]

Regarding only the last few comments, the chassis is not connected to earth ground via the power cable. Unless you connect a grounding line to the cDAQ chassis the chassis ground and earth ground are different. So you could ground the chassis if you're concerned about grounding issues. However, I believe that Lynn was suggesting using common mode rejection for the differential measurement. This will give a return path to ground for the bias currents. See the below source for more information (particularly table 1):

http://www.ni.com/white-paper/3344/en

[iinWill]

Would I connect a resistor from the CH- (COM) to a seperate ground cable? I never wired a ground cable to the DAQ. Would I simply have two cables on CH+ (ai0), one to the thermocouple converter (Omega) and the other as a ground, so that would be touching metal? Did I wire my DAQ to the OMEGA thermocouple correctly? I have the ai0 connected to the + output on the OMEGA and the COM to - output on the OMEGA. I'm confused, is the ai0 port on the NI9201 a + or - output/input? So the COM port suppose to be grounded or connected to the - output on the OMEGA thermocouple?