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Scaling analog input (4-20mA)

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Hello there, 

I am having some issues when I am trying to scale the 4-20mA from a temperature sensor ( -50 to 150 degrees ) , I am getting wrong values. I have done this before using sensors from 0 to 200 degrees without any problem at all, but in this case, I think the linearization block struggles with negative values?
Can somebody help me please  ??


Thank you 

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  • MVP 2023

Agree with Flex.

Also, as someone in the HVAC game, I would make sure that your sensor is the temp range that is says it is.  I often find variations of the same item that have different ranges, and you can only know for sure by matching the actual model numbers on the particular unit, right down to checking labels on the PCB.  I have even had manufacturers change this without making it known in their literature.

And for researching and trial purposes, I suggest you also look at my post here which has my calculator: 

The calculator is good for seeing what results should be, but I have used it for compensating for errors, and also finding the different ranges that I mention at the top of this post by playing with numbers and comparing that to actual results.

Also, don't forget that the Linearisation function does NOT have upper and lower limits.  The conversion will occur outside your ideal input range if your sensor is capable of putting out larger or smaller amounts than normal. Quite often you need to use a compare to set up high and low limits.  This is especially confusing the first time you encounter the problem, without realising the simple fix. If you are setting up different span systems via logic that then goes into a number of linearisations, it is definitely needed




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  • MVP 2023

Have you got jumpers in their correct locations for 4-20ma?

And looking at your ladder, why have you got MB101 and a sub call AFTER the linearisation?  This makes no sense at all.....the linearisation only does a conversion.  As if often quoted here, Unitronics does not charge for ladder rungs!  Separate things out.

cheers, Aus

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  • MVP 2023

Also, not sure why you need MB 6, but you also have two networks in that ladder rung. Don't do that.

And to further Ausman's point, MB 100 and MB 101 aren't doing anything. If they are used elsewhere, just use SB 1, as they are both always on.

Place those subroutine calls on a ladder rung by themselves.

I don't see which I/O module you're using, but if it's truly a 14-bit module, the correct linearization is 3277-16383, not 3286.

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