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Unistream AI8 Input Non-Linear


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During calibration of a pressure sensor (4-20ma) I was unable to achieve an precise calibration 0 to 30.00 PSIA within 0.7 PSIA at the lower or upper range when using the scaling functions.

The sensor was calibrated from the manufacture within 0.1% accuracy.

I decided to check the Analog Input with a calibrated Fluke Processmeter from: 4.000 ma to 20.000ma.

The Hardware was configured for 4-20ma input and default scaling 0 to 8191 inputs with strong filtering enabled.  Unilogic version 1.22.13

Here are the results:

Fluke(ma) Actual Count Expected Count Count Diff.
4.0000 -45 0 -45
6.0000 954 1024 -70
8.0000 1954 2048 -94
12.0000 3953 4096 -143
14.0000 4957 5119 -162
16.0000 5951 6143 -192
18.0000 6951 7167 -216
20.0000 7949 8191 -242

The counts are non-linear corresponding to the 4-20ma inputs, i would expect the counts difference to be constant.

I was not able to check all the channels, but could this be just a bad channel or are the values within the specification of the AI card?


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  • MVP 2014

I did quick fiddle with the data in Excel.  It looks like an offset of -45 counts and then a linear scaling error of -2.4% over the range of the input.

Fluke(ma) Actual Count Expected Count Count Diff. Data without offset Percentage Error
4.000 -45 0 -45 0  
6.000 954 1024 -70 -25 -2.44%
8.000 1954 2048 -94 -49 -2.39%
12.000 3953 4096 -143 -98 -2.39%
14.000 4957 5119 -162 -117 -2.29%
16.000 5951 6143 -192 -147 -2.39%
18.000 6951 7167 -216 -171 -2.39%
20.000 7949 8191 -242 -197 -2.41%




I would not expect this as normal behaviour.  The spec sheet gives accuracy of 0.4%.   However, apart from the offset and scaling errors, the data looks quite clean.  Usually a bad input won't even perform this well.

Were you able to test with the field wiring completely disconnected from the analogue module (that includes the commons as well as the signal lines)?

I notice you are a few versions behind on UniLogic, the current version is 1.28.34.  I doubt the issue is version related, but I would suggest upgrading at some point.  I'd be pushing into a more thorough diagnosis of the hardware and field wiring first.

Hope this helps,



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I have two 24V power supply's both with connected to the same 0V common (and system ground) in addition both supply's are connected to two-stage AC line filter.  The analog sensors and analog inputs modules  have one of the two  24V power supply and other powers the Unistream HMI and discrete IO.

I checked the power supply's and are with in a (+) 0.4Vdc of the 24Vdc.

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