10-24-2013 03:56 PM
Using NI 9237 w/ RJ50 connections.
What is the effect or error in only using a 4-wire connection to a full-bridge (Ex+, Ex-, Ai+, Ai-), and not wiring the +/- Sense pins, as detailed in the manual?
I generally calibrate the strain gage V/V readings with the cable (to length) to lbf, using our certified cal stand, so I have a linear fit with cable effects encapsulated in the calibration curve. I'm guessing if I used the sense pins, it would merely remove the effects of the cable and subsequent calibrations of the same gage would be equal (within error) regardless of cable length.
10-25-2013 12:02 AM
According to the DevZone on Strain Gauge Measurements:
"The extra sense wires serve to regulate the excitation supply through negative feedback amplifiers to compensate for lead losses and deliver the needed voltage at the bridge."
Remote sensing should eliminate the gain errors caused by the resistance of the EX+ and EX– leads (since the sense +/- are high impedance). Thus it sounds like you understand this properly. Without the sense +/-, you must assume that the exact excitation voltage at the card is equal to the voltage at the excitation of the bridge (which isn't technically true). The other option for the 9237 is shunt calibration which is to correct for errors from the resistance of both excitation wiring and individual resistors wiring.
The difference between remote sense and no built in sensing is explained here.