thermocouple reading limited to a value

[Johnrally]

Hi,

I am new here and do not know much about Labview.

T-type thermocouples are connected to SCXI-1000 chasis with 1102 module and 1303. I use 20 channels and 12 are left open. For those open channels, reading from Labview program is around 139oC.  However, the measured temperature from those occupied channels cannot exeed this temperature, no matter how hot I heated the test surface.

Is this due to the Labview program or the resistor of inside the 1303 or any other possible reason?

Thanks alot.

[Brad_K · Accepted solution]

Hi Johnrally,

By "139oC" do you mean 139 °C, 1390 °C, or something else?

1390 °C is well outside the temperature range of T-type thermocouples according to NIST. However, it's close to the upper limit for K-type thermocouples. Are you sure your program is specifying the correct thermocouple type?

If that doesn't answer your question, please post a snippet of code to show what your program does. Also, I recommend double-checking the SCXI configuration in MAX, to make sure the SCXI-1303 is configured correctly (and is not configured as an SCXI-1300).

Brad

[Johnrally]

Thanks, Brad.

It's 139 °C not 1390 C. So it should be in the range of T-type thermocouple. I checked the Installation Guide of SCXI-1303. The resistor and Open-thermocouple detection are mentioned in the manual. Do you think the issue I described is due to the open-thermocouple detection?

[Johnrally]

From the manual, I got this paragraph:

The SCXI-1303 circuitry helps you detect an open thermocouple. To detect
whether any thermocouple is open, check whether the corresponding SCXI
module channel is saturated.

Is the problem  I described belonging to this channel saturation? I am poor in electronics, sorry.

[Steven_K.]

Hi Johnrally,

That does not sound that it is an open thermocouple. The value would be wildly different from the range of the thermocouple. The open thermocouple detection circuit will read a 5 volt signal if it detects and open circuit versus the millivolt range for a working thermocouple.

For more information about the detection circuit look at these Knowledge Base and Dev Zone articles:

http://digital.ni.com/public.nsf/allkb/C2C7AB329ABC608986256D3D004C86AE

http://zone.ni.com/devzone/cda/tut/p/id/12334

Like Brad said, please check the settings for you configuration. If that does not work please post a screenshot of your code. Thanks.

[Broken_Arrow]

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@steven K. wrote:

Hi Johnrally,

That does not sound that it is an open thermocouple. The value would be wildly different from the range of the thermocouple. The open thermocouple detection circuit will read a 5 volt signal if it detects and open circuit versus the millivolt range for a working thermocouple.

{...}

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Actually, the open thermocouple feauture gives you a reading well within the range of the thermocouple. Open thermocouple on my nearly idenetical system to the OP reads about 450 F, or 232 C.

[Broken_Arrow]

Johnrally, have you tried taking out the pullup resistors in the 1303 or changing them to 10 ohms?

[Johnrally]

To Broken Arrow,

I have tried to pull out the 10M Ohm resistor clusters and replace them with the 10Ohm ones. However, the open channel reading is almost the same as 139 C. It seems that it is not the open T.C detection issue.

[Johnrally]

Hi, Steven,

Thank you for your suggestion. This is the Greencopy of the labview program I am using. I have no idea which part will cause the problem.

[Steven_K.]

Hi Johnrally,

Are you measuring the open channels while you measure the channels with the thermocouples? Since the DAQ card uses multiplexing, this could affect your thermocouple measurements. Please see these Knowledge Base articles:

Inaccurate Readings When Some Channels of the SCXI-1102/SCXI-1100 Are Open Circuits

Why Do I Experience Incorrect Readings When Sampling Open SCXI Channels?

Thanks,