Trying to get an accurate reading from thermocouple

[iinWill]

I have the NI 9201 connected to NI cDAQ-9174. On the NI 9201 i have an OMEGA TAC80B-K Thermo-to-Analog Converter. I have it connect through banana plugs with the red wire to AI0 and COM. On the OMEGA I have a K type thermocouple connected. When I run my VI I get weird reading of negative values on the Temp wave chart. I tested the OMEGA with a THERMOMETER and it works. I don't know what the issue is here. I am trying to get the temperature reading. 

[ghd06002]

I think, because you're using an analog input card instead of a thermocouple input card you need to scale the voltage to temperature yourself before plotting

[johnsold]

The TAC80B generates 1 mV per degree output.  This is no longer a thermocouple signal but a voltage linearly proptionals to temperature.  Just measure the voltage.

The problem with the NI 9201 is that it has a +/-10 V input range and 12-bit resolution which is just barely good enough.  The accuracy is not very good for small signal measurements.

Lynn

[iinWill]

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@johnsold wrote:

The TAC80B generates 1 mV per degree output.  This is no longer a thermocouple signal but a voltage linearly proptionals to temperature.  Just measure the voltage.

The problem with the NI 9201 is that it has a +/-10 V input range and 12-bit resolution which is just barely good enough.  The accuracy is not very good for small signal measurements.

Lynn

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Do you have a VI that you can show me? 

[johnsold]

Any of the standard Analog Input examples should be suitable as a starting point. 

Lynn

[iinWill]

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@johnsold wrote:

Any of the standard Analog Input examples should be suitable as a starting point. 

Lynn

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I am a beginner in LabView. I am not sure of how to convert the voltage to temperature (C). Is it possible for someone to explain how I would approach the VI?

[johnsold]

According to the Omega documentation the output voltage in millivolts is equal to the temperature in degrees. So 103 mv = 103 degrees. The only thing you need to "convert" is the label or caption on the indicator.

Lynn

[iinWill]

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@johnsold wrote:

According to the Omega documentation the output voltage in millivolts is equal to the temperature in degrees. So 103 mv = 103 degrees. The only thing you need to "convert" is the label or caption on the indicator.

Lynn

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When I run the VI the temperature isn't correct, and it isnt working to the thermocouple. When i place on a cold metal plate, it still reads 40c even though i know its below 20c. The temperature isn't consistent to the temperature of the room. Is there something wrong with the VI?

[johnsold]

Do you have a voltmeter or oscilloscope? If so, check the output of the Omega device with that.  Verify your connections between the Omega and DAQ device, including the ground.

What does your VI read if you connect a 1.5 V battery to the inputs?

Lynn

[iinWill]

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@johnsold wrote:

Do you have a voltmeter or oscilloscope? If so, check the output of the Omega device with that.  Verify your connections between the Omega and DAQ device, including the ground.

What does your VI read if you connect a 1.5 V battery to the inputs?

Lynn

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I took a multimeter and a 1.5v battery. It measure 0.93Vdc. I ran my VI and it measured 0.947358V. I then use the multimeter and measure the Omega while its unplugged from the DAQ:

Switch to C: -0.016Vdc

Switch to F: -0.030Vdc

All tested while its ON. 

I then ran the VI while its connected to the Omega. It displayed 0.125 on the num indicator. I use the multimeter and it measure 0.125Vdc. I think something is wrong with the Omega's readings. Also i was using the Acq 0-20mA Current Sample.vi from NI LabView examples.