Strange Behavior of Scales for a AI Voltage

[Gan Uesli Starling]

So I'm calibrating a NI-9205 as voltage on a range of 2-10 VDC (the differential of 4-20mA through 500R). And one channel is behaving oddly. It clips at 42% of its 50 PSI scale, but only while measuring via scale, not while measuring volts. I'm setting pressure via a dead weight standard, pressing air into a Specter sensor which outputs 4-20mA. I'm measuring it as V across 500R because I need to filter it with a Sallen-Key active filter (a retro-fit to eliminate pump ripple from out of the signal). So, anyhow, Ch 0 on the NI-9205 is fine in all ranges from 2 to 10 VDC while measuring volts. Let me turn on the scale, however, and then it's nice and linear right up until 21 PSI where it clips.

The proper voltage is there. I am double checking it with a Fluke DMM. I can add weights on the dead weight standard right up to 50 PSI, and while set to show Volts and not scale, then NI Max shows the right volts. At each PSI level, I can switch back and forth between Volts and scale on NI Max and both are good right up to 20 PSI. Above that, however, and even though Volts keeps on climbing, scale clips and holds a t21.345388 PSI.

I have tried remaking the scale. I have tried that same channel with other scales. Those other scales too (all for ranges of 2-10 VDC but different PSI ranges) all clip at 40% of their full scale on this channel only.

I have swapped out the NI-9205 and the new one does just the same.

Any ideas?

Edited September 7, 2016 by Gan Uesli Starling

[hooovahh]

Does it behave this way on other computers?  Maybe resetting MAX config?  Not quite sure really but if you tried another piece of hardware and it behaves the same it would make me think it is not the hardware, but some software or config on that PC.  What kind of scale is used?  Is it a linear, table, mapped range, etc?  I guess it probably doesn't matter but I'd try to see if I can make any scale work on the full range.

[ShaunR]

Well.  21.3xxxx  is the maximum over-range of a 20mA current loop, IIRC. I suspect you told max it is a current loop rather than just applying a scaling factor to your voltage.