SCXI 1102 and 1303 RTD and TC wrong readings

[acanivell]

Hello

I am reading temperature from 6 diferent sensors, RTD and Thermocouples, using a SCXI chassis and a 1600 controller with these modules:

- SCXI 1102 with a terminal screw 1303

- SCXI 1581 excitation source with a terminal screw 1300

To read the following temperature sensors:

- 3 x PT100. 2 wire and 3 wire configuration. 300uA excitation, bridging 3 outputs of the 1581 module.

- 3 x thermocouples. Floating configuration. Type J. Built in compensation

I get readings wrong like this:

- PT100 with 2 wires read about  6C above the actual temperature. With a 3 wire configuration they reads 8C above the real temperature.

- TC read about 10C above the actual temperature.

Cold junction temperature is already wrong 25C while it must be 20C.

In terminal 1303, according to the manual, I set the bias resistor to 10ohms and the pull up to 10Mohm for TC and RTD channels.

I do not know what could have gone wrong in my configuration, could someone give me a hint why do I have this OFFSET in my readings?

Cheers

[acanivell]

After some trying, I mad the following improvements:

- Thermocoples: the CJC temperature of25C was probably right as it is inside the equipment, which could be warmer than the rest.

I switched the copper and the constantan line in the screw terminals so that copper goes to negative and constantane to positive. This improved the measurement, but I do not understand why NOTTHING IS SAID IN THE MANUAL ABOUT TC polarity connection!!!

In addition, I removed the pull up and pull down resistors completelly. This improved the measurement again. But AGAIN, I do not see why nothing is clearly said about this in the 1303 manual.

So now TC readings ARE FINE!

- PT100: this was my mistake, I was missing the wire resistance to compensate. As soon as I compensated the reading with the cable resistance, the reading was better.

But then,when I removed the bias and pull up resistors from the channels, the measurement was even better. Again, I do not understand why NOTHING about how to connect RTD sensors is said in the 1581 or 1303 manual.

All in all now PT100 readings are fine.

Though the problem seems tobe solved, I would appreciate comments on this experience.

[ninlmartijn]

Dear Acanivell,

thank you so much for your post on our website. None of our user manuals will explain you how a transductor works; it only explains you how the hardware works. You will find at page 2-6 of the manual from the SCXI 1102 module a schematic of how to connect a floating signal to the SCXI 1102. Please notice that the bias resistor is provided in the SCXI 1303 connector block (as explained in the manual from the SCXI 1303). But the schematic implies a + and a - pole.

Most common transductors are explained in knowledge bases on our website, for example this one. A specific KB about thermocouple and the SCXI 1102 / 1303 can be found under  

this link.

One fast and easy way how to connect signals (I'm using this if I have any doubts) is to create a task in the Measurement and Automation eXplorer (MAX) and then have a look at the connection diagram. In this case a chosed create task in MAX, acquire signals => analog input => temperature => thermocouple. Here you can chose your SCXI 1102 module as your input channel. If you chose the correct channel(s) and give a task name the task will be shown. Below the page you will find the connection diagram tab (this looks like the attachment).

Please don't hesitate to contactus when you have any further questions.

Best regards,

[acanivell]

Thanks Martijn for the quick response

Indeed NI support is without any doubt one of the most valuable things from NI. 

In any case, I was missing some information about RTD in the manual of 1303, which speaks mainly about TC. I agree that you need to know the fundamentals of RTDs and TC, but it would be helpful to refer in the manual that TC polarity must be carefully considered and RTD as well, as one is mostly floating and the other ground referenced.

It is a pitty as well that the MAX does not display in its connection wizard the internal excitation source connection when you read RTDs.

Anyhow, now problem is solved

Thanks Martijn

[ninlmartijn]

Dear Acanivell,

thank you for your response. The most important thing is that the problem has been solved, so I'm glad to hear that. I will forward your feedback to my colleagues in the US. Thank you for this feedback. For now it is best to look at knowledge bases on the web for transducer information; I know our website is big so sometimes things can be difficult to find, in that case don't hesitate to contact us.

The SCXI 1102 has no internal excitation capability; the excitation is running trough a second module (SCXI 1581), so from the SCXI 1102 perspective this is external. The connection diagram shows only connecting to one module.

Best regards,

[nwilliams]

Is there any possible way to hookup an 3-Wire RTD to the SCXI-1308 or the TBX 1303, I'm new the NI system and learning as I go thanks any help would be awsome

[ninlmartijn]

Dear nwilliams,

thank you verry much for your post on our webforum, it is best to start a new post instead of replying on an old one.

You need also to add some excitation. Another module which can be used is the SCXI 1503.

Best regards,