Full-Bridge Strain Vs. Custom Voltage with Ex: outputs opposite polarities of eachother (SCXI 1314)

[OKors]

Please see my Sig for current hardware specs.

I have a manufactured load cell that I'm plugging into an SCXI-1314 wired for full bridge. When I condition and calibrate the load cell as a Full-Bridge Strain channel (shunt cal) my signal output is POSITIVE in compression.

If I take the same load cell plugged into the same channel in the 1314 and select Custom Voltage with Excitation and give it a mV/V slope curve in the Custom Scale the signal output is NEGATIVE in compression.

Why is this? I fear I'm overlooking something very rudementary.

Other than the polarities  being opposite of eachother the signals appear just fine.

[OKors]

Here's a drawing of our wiring/cabling convention for load cell/scxi interface.

[Brad_K]

Hi OKors,

The strain scaling formulas are listed in chapter 4 of the SCXI-1520 User Manual and Specifications. They all multiply the result by a negative number, so there is an inverse relationship between voltage ratio (as returned by a Custom Voltage with Excitation channel) and strain. When you use a custom scale with a positive slope, the voltage ratio no longer gets inverted. You can use a negative slope in the custom scale if you want.

Brad

[OKors]

Remembering to invert the slope of a custom scale isn't so hard to do, but for the unsuspecting user couldn't that could be a concern? 

I know it took me a day of troubleshooting and a post in the forums before I figured out why my data didn't make any sense.