05-01-2012 10:46 AM
I'm getting strange readings on my NI9211 (used with USB carrier) when trying to take a measurement. I have the NI9211 and a Eurotherm 3216 controller connected to the same unsheilded grounded thermocouple (diagram attached). I realised this isn't best practice, but I have no alternative, and have used the set up successfully in other systems.
I'm using this set up for 3 thermocouples & 3 controllers sharing 3 channels on the same NI9211. With one TC connected to both the readings are fine, when I connect a second is when the readings are changed. I always get the correct reading on the Eurotherm.
It was suggested to me I should use resistors to link the negative legs of the thermocouples to the common on the NI9211. I've tried this with limited success. It seems to only work at certain temperatures.
Any ideas how I can correct it?
05-02-2012 05:47 AM
Firstly, as you know, having an unshielded grounded thermocouple is not advisable.
To check that the other channels on your thermocouple module are working as expected with just the one thermocouple connected, please use the Measurement & Automation Explorer software. Click here for a tutorial on this.
This will work out whether your module is faulty. My main suspicion would be interference between the three thermocouples.
Have you made a connection to your COM port on your 9211? This acts as a ground.
05-02-2012 07:23 AM - edited 05-02-2012 07:27 AM
Thanks for the reply. It works correctly with only one connected not all three.
I haven't got anything connected to the COM terminal, as I understand the ground is made at the "hot end" of the thermocouple as it is directly in contact with a metallic object.
I should explain also, all three thermocouples are of the same type, and attached to the same object in different locations but would be electrically linked.
05-08-2012 11:25 AM
As your thermocouples work fine in individual channels just not at the same time, there appears to be some issues with interference between channels.
Have a look at the manual for this module as it details quite a lot of information.
When using multiple thermocouples, a small offset error occurs, however this can be increased when thermocouples with a higher lead resistance are used. This offset error can be measured using the Autozero channel on your module so I would definitely recommend measuring this to find out the offset that you are seeing.