how to connect cDAQ chassis to strain gage

[ZSH2010]

Hi, when I read the guide of wiring strain gage to a cDAQ chassis (http://www.ni.com/white-paper/7130/en) I found fig 10 confusing. 

What are the things between the chassis and the strain gage?  

Are the three wires going to only one channel of the cDAQ chassis? (in Fig 10, to channel 0?)

Why there are four terminals (CH+ EX+ QTR EX-) on the strain gage, but there are only two wires coming out from my strain gage? what should I do?

PS. I have a cDAQ USB 4431 and a strain gage from Vishay (http://www.vishaypg.com/docs/11297/250lw.pdf)

Thank you very much!

Elly

[PeterGC]

ZSH2010,

I agree, those diagrams are a bit confusing.  I'll see if I can get those edited for clarity.

To maintain the cDAQ formfactor, the 9237 has 4 RJ50 ports that break out to individual connectors.   That parts between the strain gauge and the module are those break out modules.

http://sine.ni.com/nips/cds/view/modelpopup/p/pcat/9015/lang/en

The three wires are only going to one channel (ch0), I think they wanted to show that you could wire this the same way on all channels, but it's a bit confusing.  Typically a straing gauge will have excitation channels signal channel.  For your strain gauge (which looks like you are trying to use quarter bridge configuration), you will connect your strain gauge in the configuration described in figure 4.

http://digital.ni.com/public.nsf/allkb/892C84122A6501AE86257547007E5C53

The 4431 can measure accelerometer data as well as current.  It is not recommended for strain measurement.  I would recommend the 9237 cDAQ module with a 9944 or 9945 quarter bridge completion adapter.  This will allow you to connecto your strain gauge directly to the input module and not have to create your own external completion circuitry for your quarter bridge.  

Sincerely,

[ZSH2010]

Hi Peter,

Thank you for your reply!

I just remember that we also have a NI 9219, can it work with our strain gage? If so, do we still need the "9944 or 9945 quarter bridge completion adapter" or additional external circuitry ? 

Thanks a lot!

[PeterGC · Accepted solution]

ZSH2010,

The 9219 will work.   You will need to use a 2 wire resistance measurement vs quarter bridge measurement.  For information on how to use the 9219 for 2wire resistance measurements take a look at page 11 and 15 of the specifications document 

http://www.ni.com/pdf/manuals/372407a.pdf

This will be a bit less accurate than the quarter bridge accessory for the 9237, but should still get the job done.

Regards,