Wrong polarity of measured strain with SCXI-1520?

[LeifS]

I have connected a bridge calibration unit (HBM K3607) to an input on SCXI-1314T/SCXI-1520 and try to read the strain using MAX.

The reading is correct but with inverted polarity.

I have connected a DVM to the input (RJ-50, pin2 as AI+,pin3 as AI-) to verify that I have a positive voltage.

 

I also tried the exactly same setup on an NI-9237 in a CompactRIO and that gives me correct (positive) polarity.

Should it be like this or have I missed something?

[LeifS]

Any one having both a C-series bridge module (NI 9237) and a SCXI 1520 that can check if I'm right or wrong?

Just hook any resistance bridge up to the module, check polarity and then switch to the SCXI-1520 to see if polarity changes ...

Please help...

/Leif

[Rexbo]

Actually, I am experiencing the opposite.  When I use our strain calibrator (Vishay 1558) on a SCXI-1520 channel configured for strain I get a positive output with tension in strain.  With the NI 9237, I get negative output with tension in strain.

I created a service request (7058005) in 2004 because according to the wiring diagram in the user guide, I thought tension should have a negative output.  NI's explanation was they reversed the polarity when converting from mV to microstrain, making tension positive.

I suspect NI is not making this correction in the 9237 module.  I plan to check with them today.

[LeifS]

Thanks for your answer.

I'm not really sure how you define tension in regard to which resistances in the bridge that increases and which decreases.

Have you measured the polarity on the input pins (AI+ and AI-) ?

When I use a bridge that gives positive polarity on theinput pins, I also get a positive reading on the NI9237 and negative on SCXI-1520.

Do you mean that NI define tension as negative polarity on the input pins?

Kind regards

Leif

[Rexbo]

Let me eat some of my words. 

I set up both a 9237 and a SCXI-1520 with a full bridge setup in MAX (FBI).  Using a Vishay 1558 strain calibrator, in FB mode, I get negative microstrain for positive voltage across the signal lines (+ to +S; com to -S) for either module.  So if I input - microstrain on the calibrator, I get a positive strain reading in MAX.

When I change the 1558 to half bridge, two active arm configuration, and MAX to HBII configuration I can no longer measure the input voltage, because only +S is available.  When I input - microstrain on the calibrator, I get a negative strain reading in MAX.  However, the wiring on the strain calibrator changes.  I think that may be the source of my problem.

If you are getting different readings in MAX with a 9237 and a SCXI-1520, you must have something different in the wiring.

Rex