open thermocouple detection in SCXI 1328

[Angelantoni]

Hi

Am using scxi 1125 signal conditioner and scxi 1328 terminal blcks to acquire temperature from thermocouple, i acquire the voltage from thermocouple ends and then convert them in to temperatures in my appliction, i wanted to know if it is possible to check if a value acquired is due to open thermocouple or any other error's. can any one tell me how to check if a thermocouple is in opened? or sending proper values?

thanx
Arun

[Tom_W_[DE]]

Hi Arun-

Open thermocouple testing is not possible with the SCXI-1328 by conventional means. Open thermocouple testing with other modules (i.e. SCXI-1303, etc) is available because internal pull-up and -down resistors ensure that the voltage reading on the channel will be ~5V if the connection is open.

The SCXI-1328 does not have this functionality built in, so open thermocouple testing will not be readily available.

Please see this KnowledgeBase: Open-Thermocouple Detection for more information.

Thanks-

[Angelantoni]

Hi Tom

Thanx for the answer. am actually concerned that there is no way to detect open thermocouple in 1328, is there any other alternative like placing an additional circutry, because it is very important for me to detect the open thermocouple's.

Thanx for the help
Arun

[Tom_W_[DE]]

Hi Arun-

I am sorry to say that there isn't an easy way to perform this detection using your module. The reason for this is that the SCXI-1125 does not provide excitation channels and does not provide a +5VDC pin.

You could provide this voltage from an external module for use with the technique mentioned in the above-linked KnowledgeBase article, but you won't be able to perform open thermocouple detection with the 1125/1328 directly.

Thanks,

[Angelantoni]

Hi Tom

Am posting after a long time, i wanted to know if i get some extra modules which does provide +5v signal, will i be able to use the same terminal block(1328) and be able to detect open thermocouple?
How will the connection be in that case? i will also need an external resistance in this case right?

thanx for your reply
Arun

[Angelantoni]

And also if i acquire data with no sensor's connected to the terminal blocks, i get +5v or -5v as voltage between the + and - pin of terminal block, how is this possible i should get a very low voltage in this case right?

Regards
Arun

[Tom_W_[DE]]

Hi Arun-

If you have another module/terminal block combination that exposes a 5V and GND (or -5V) pin you can wire an external resistor to the positive pin of your signal to pull up the pin and provide open thermocouple detection. You will also need to wire a pull-down pin to the appropriate pin (GND for RSE or -5V for differential).

As for a broken connection with your current configuration, you can expect to see a floating signal if there is no connection between the two pins. This can range between small signals to very large signal readings. The important thing to remember is that without pull-up and pull-down resistors this reading will be totally unpredictable and will not be a reliable way to check for open thermocouples.

I hope this helps!

Thanks-