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Incorrect Scaling Behavior

I'm having issues with scaling data from a 0-5V pressure transducer.  I've input the manufactuer supplied calibration coefficients, as a custom linear scale (For reference slope is 62.9 psi/V, and offset is -6.44 psi).  There are two (likely related) scaling errors going on.  

 

First is that the scaling doesn't appear to be applied correctly.  For example, at ambient pressure, the scaled value that SignalExpress reports is 7.27 psia.  The raw voltage sensed at this time is .336V.  Manually performing the scaling math gives a pressure value of 14.6 psia, which is exactly correct.  Any ideas why SignalExpress would be applying the scaling incorrectly?

 

Secondly, the scaled value doesn't appear to respond to pressure changes for some reason.  When I apply ~60 psi of air to the transducer, the scaled value remains 7.27 psia even though the raw voltage value increases to 1.11 volts (~63 psia when calculated manually).  

 

Anyone have an idea of what's going on here?

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Hello RJ88,

 

It sounds like a setting within the custom scale was set incorrectly. I would advise starting from scratch and creating a new custom scale according to these steps:

 

docs.google.com/a/umn.edu/forms/d/e/1FAIpQLScA4UZ1WziHqbEUDTAcKf0p2dO0endXGaEs9Pazq0oboSyHVg/viewform?c=0&w=1

 

Double check that you are using the correct device and channel when you enter in the custom scale, since reading from a channel that is not taking samples would create a circumstance where the data is stagnant like you're seeing. 

 

Best,

 

Zoe B.

Applications Engineer

National Instruments

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What exactly does your calibration sheet say? Does it give you a PSI value at 5v? Photo would be even better. Different mfg's have different ways of delivering thier calibration results.
Once that's figured out we'll be able to choose the best scaling method and go from there. I would not worry about any offset just yet because you'll want to appy your own to compensate for your specific system anomolies.
Also be sure your Input Min/Max ranges are in order. For instance if its a 0-300psi transducer you'll want your min=0 and max=300. If your min/max is valid you probably have a wiring issue... polarity maybe.
Lets start with that cal sheet first.

SCXI- 1000 Chassis w/ 1346 adapter
PCI 6281 DAQ card
SCXI- 1520 Bridge Board w/ 1314 Terminal Block (x2)
SCXI- 1180 Feedthrough Panel w/ 1302 Block
Signal Express 2014.
Win7 Enterprise
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