NI 9213 CJC Channel

[asdfasfda]

Hello users,

Let met start with a short introduction. I'm the testbench engineer at GBS International (The Netherlands). I'm currently working on a improved version of the software and noticed something odd yesterday. When one of our doors was open the temperature rises (it's still winter here, so that's impossible) I'm using a NI 9213 16Ch. thermocouple module and remembered something about CJC. The module is wired to thermocouple connectors (all copper). A quick test showed me that heating up the module itself is no problem ( I'm using the built-in CJC sensor) But when i'm heating up the TC connectors the temperature drops.

I put a thermocouple on channel 15 and attached this to the connector panel. (With the 16 TC connectors, too measure the CJC there)

So i've looked to set one of the NI 9213 channels as CJC channel but up till now no success. I know it's possible to create a virtual channel in MAX but i've got my reasons to do everything programmatically. (Software runs on multiple machines and multiple DAQ's, everything is initialized on startup). I'm retrieving the physical channels from the module (all used modules) and automatically processing them into tasks.

Does anyone know how to set up a CJC channel programmatically? The Create channel AI temp TC asks a global channel?

Things i've done and tried so far:

> Wired a string with the name to the CJC input, but this is different on every DAQ and is not working.

> Create a seperate task with channel 15 as input and wired this to the CJC channel (Not working)

> Type Cast the physcial channel to a global channel -> results in a software crash

Attached is a VI to test with.

[jackstokley]

Hi,

Have you seen the below document which explains how to read from a CJC sensor?

http://digital.ni.com/public.nsf/allkb/FFAEB301361522D9862572CA00196D7B

Jack

[asdfasfda]

Hi Jack, 

Thanks for your input, but I don't want to read the CJC temperture. I want to set channel 15 of the module as my CJC channel. The input required is a Global channel, but my NI9213/AI15 is a physical channel? 

Best regards,

Y. Verburg

[RainingTacos]

CH15 is a thermocouple channel, not a CJC channel.  The CJC channel is an internal channel that is used to determine the absolute temperature of the thermocouple attached to the connector inputs.  Without the CJC you would only have the relative temperature between the thermocouple junction and where the thermocouple is connected to the module.  There is no way to connect an external CJC to the 9213.

[deceased]

If you were able to use a thermocouple reading as your CJC value, what value do you use for the CJC channel's CJC.

If I remember correctly the CJC value in those modules comes from an onboard RTD.

0XDEAD

[deceased]

Anyway... back to the problem at hand.

What type thermocouples are you using?

Do you connect directly to the module or is the cable extended?

Is the thermocouple wired the right way around?

0xDEAD

[Michael_78]

'The module is wired to thermocouple connectors (all copper). A quick test showed me that heating up the module itself is no problem'

The thermocouple must be only connected to the correct compensation cable and compensation connectors until they reach the terminals on the module. The CJC sits just behind the terminal block. What are the copper connectors you are refering to?  If they are not directly on the 9213 this is your problem.  Maybe share a photo of the connector arrangement?  What thermocouple type are you using?  It is also worth noting that the accuracy of the 9213 can be affected by it's orientation, I have seen situations where the top channels read slightly different to the bottom channels, heat from the chassis rises and causes the gradient.  NI make an isothermal c series module but it is more expensive than the 9213.  Your other option is to read the raw mV signals, read an external CJC RTD and apply the temperature calculation in software. 

[asdfasfda]

@ deceased

I want to use the internal RTD for channel 15, and use the reading from channel 15 as CJC for the other channels. The 9213 is inside a Rittal box, we're the following connectors are placed. From the module to the panel connector is copper wire. 

Cable connector

Panel connector

Main reason I used the copper ones, so that we're able to switch thermocouple types. (Mainly we use K-type)

I'm perfectly able to read from the module, but when the inside temperature differs with the outside temperature, i'm getting a offset. 

@ Michael78

Orientation is with the connector facing forward. Maybe the raw mV signals are an idea...

Meanwhile I maybe found another solution. It's possible to create your own Global Channel programmatically, and maybe set that as CJC channel. But i've not tried that yet. 

[asdfasfda]

The 9213 module is inside a Rittal box and wired to this Connector with copper wire. The connector is copper to be (relativly) flexible with the type of thermocouples. We mainly use K-type. 

From the help:

So it's possible to use 'another virtual channel already in the task'. But the input only accept a global channel. 

I also found this: Link But i'm not sure how to implement this. 

[deceased]

Every dissimilar metal join is effectively a new thermocouple.... but as long as you keep all the joints at the same temperature as the CJC in the module you will keep the error from this to a minimum.

I would buy Ktype extensions (where the 2 leads are the same alloy materials as the probe) and route the signal back to the module directly. No need to introduce additional thermocouples to your thermocouple.

0xDEAD