open thermocouple detection on the 1120

[rohrdt]

I am using an SCXI 1120 module with a 1320 terminal block to take temperature readings from k-type TCs. When we disconnect a TC the reading for that channel goes to around 272 C, which is in our measurement range. How can I get that reading to go out of range when the TC is removed?

[Matt Claessen]

Hi,

"rohrdt" wrote in message news:50650000000800000025B40000-1068850981000@exchange.ni.com...
> I am using an SCXI 1120 module with a 1320 terminal block to take
> temperature readings from k-type TCs. When we disconnect a TC the
> reading for that channel goes to around 272 C, which is in our
> measurement range. How can I get that reading to go out of range when
> the TC is removed?

I have some experience with thermocouple measurements, using SCXI,
I did not use LabView however, but a custom (Delphi) program.

Open TC's yield a signal that has the full measurement range, with the
maximum counts your MIO module has (in my case 32767).
I cannot imagine that you want the full range to end inside your
measurement range, like you say, so I guess
something is wrong
in your gain settings.

HTH

regards, Matt

[LesHammer]

Thermocouples are simply a voltage source.
When the TC is removed you are measuring the
"static" in the air. To the DMM a voltage is
a voltage. It may be in the "good" range or not.

You could take a resistance measurement on the
channel before doing the TC measurement. An
open circuit reading would indicate the TC is
disconnected. It takes longer, but it's one way
to be sure the TC is there.

Les.Hammer@CompleteTest.com

[Matt Claessen]

Hi,

"Les Hammer" wrote in message news:50650000000500000058450100-1068850981000@exchange.ni.com...
> Thermocouples are simply a voltage source.
> When the TC is removed you are measuring the
> "static" in the air. To the DMM a voltage is
> a voltage. It may be in the "good" range or not.
>
> You could take a resistance measurement on the
> channel before doing the TC measurement. An
> open circuit reading would indicate the TC is
> disconnected. It takes longer, but it's one way
> to be sure the TC is there.
>

You are right there Les, and that is why these dedicated
TC cards have a feature called 'open TC detection'.
The circuitry will detect whether a TC is open or not,
and will apply e.g. 5V over the contacts if it detects

an open (=broken) thermocouple.

Short circuits are a different matter, though.

regards, Matt

[LesHammer]

> these dedicated TC cards have a feature called 'open TC detection'.

That's good to know. Simplifies the program design a bit.

> Short circuits are a different matter, though.

Especially considering that a short circuit may be just a thermocouple in a different location. (As in, a TC is just two wires connected together anyway.)

Les

[rohrdt]

Hi Les, how do you propose taking a resistance measurement on the channel if the channel has been configured in MAX for temperature? Can you reconfigure channels programmatically using LV?

[LesHammer]

As Matt stated, that particular module has .5 Volts tied to it to give open circuit detection. In that case I would not attempt to measure its resistance. I'm more familiar with DMM & switchboxes. For those, the configuration is all in the programatic commands sent to the DMM - so changing from "Thermocouple Measurement" to "Resistance Measurement" is done at the program level.

But, as Matt stated, for the module given here, there should not be a need to do that. Open circuits should just go to a max reading.

Les

[rohrdt]

The problem is that this module doesn't have open thermocouple detection. That's why I asked the question to begin with. But when I read your comment about the resistance measurement I was curious to the details of how exactly to do this.

[GlennXS]

The subject is of great interest to me.  I'm programming C# and using the Measurement Studio Class Library.  Does anybody know of a MS/Class Library feature that addresses (tests) Open TC Detection?

[Neal M]

Hello GlennXS,

All of the open thermocouple features I know of in NI hardware is taken care of in the driver.  In addition to this, the hardware needs to support this functionality as well.  The previous discussion should work well, but I do not know of a library that does this for you.  All it really is is just 2 separate tasks (a resistance measurement and then a thermocouple measurement).  You should be able to program this yourself using the DAQmx calls relatively easily.  I would suggest looking at the example programs (C:\Documents and Settings\All Users\Documents\National Instruments\NI-DAQ\Examples).

If you give me more details about the hardware you have and what you have tried, then I can try and help you out more.