Scaling analog 4-20ma

Ones_Zeros · Apr 17, 2017

Hey guys i was wondering to best way to figure out the scaling of a 4-20ma signal
say for example I scale a device -10 at 4ma and 75 at 20ma
mathematically how would i check to see what i should be getting at
8ma, 12ma, & 16ma

thanks,

Mark- · Apr 17, 2017

Hello,

output = ((input value - input low) / (input high - input low)) * (output high - output low) + output low

I normally make a spreadsheet for proofing this type of function.

boneless · Apr 17, 2017

max - min = span (75 - -10 = 85)
span / 16 = value / mA (85/16 = 5.3125)
(analog value - 4) * value/mA + min

8mA: (8-4) * 5.3125 + -10 = 11.25
12mA: (12-4) * 5.3125 + -10 = 32.5
16mA: (16-4) * 5.3125 + -10 = 53.75

This is basic math

.

Bit_Bucket_07 · Apr 17, 2017

If you'd like more of an explanation in plain English:

First, determine what percentage of full scale is represented by your milliamp reading.

For instance, at 8 mA: 8-4 (4 mA being the zero offset) /16 (16 mA being the measurement span)

8-4/16=0.25

And so, at 8 mA, you are at 25% of full scale.

Next, multiply the percentage of full scale that you have measured by the span of your engineering units and then subtract the zero offset.

0.25*85 (85 counts being the span of your engineering units) -10 (10 being the zero offset)

0.25*85-10=11.25

And so, at 25% of full scale (8mA), your engineering units should equal 11.25 counts.

deanfran · Apr 17, 2017

Or just cheat and use the FORECAST function in Excel.

Lynx777 · Apr 17, 2017

This is what I like to use, plus it works well in a spread sheet or a calculator.

y=mx+b
m=(y2-y1)/(x2-x1)
b=y1-(x1*m)
x1=input min.
x2=input max.
y1=scaled min.
y2=scaled max.
x=input value
y=scaled output value

RussB · Apr 17, 2017

I believe designing these kinds of formulae were learned in junior high when I was in school back in the 1960's. Is this no longer part of learning math?

Lynx777 · Apr 17, 2017

RussB said:
I believe designing these kinds of formulae were learned in junior high when I was in school back in the 1960's. Is this no longer part of learning math?

When I came through (70's) they taught us the math but left out the application part. It wasn't until I was in the service that all the bits fell together.

RET · Apr 17, 2017

RussB said:
I believe designing these kinds of formulae were learned in junior high when I was in school back in the 1960's. Is this no longer part of learning math?

Learning is not needed nowadays, when someone on the internet will eventually give you an answer. Which is mostly why I don't bother helping most people out.

Ones_Zeros · Apr 17, 2017

Thanks for the feedback everyone, feedback helps me to learn!!

I do greatly appreciate it!!

DLMUK · Apr 18, 2017

RET said:
Learning is not needed nowadays, when someone on the internet will eventually give you an answer. Which is mostly why I don't bother helping most people out.

Why are you a member of the forum then?

JZerb · Apr 18, 2017

RussB said:
I believe designing these kinds of formulae were learned in junior high when I was in school back in the 1960's. Is this no longer part of learning math?

in my learning here of PLC programming and the like ive been doing alot of reading about analog scaling 4-20ma signals just recently. i got out of high school in 05' and ill be honest i dont remember learning anything of the sort thats been mentioned in this thread as far as formulas goes. FWIW.

Paullys50 · Apr 18, 2017

JZerb said:
in my learning here of PLC programming and the like ive been doing alot of reading about analog scaling 4-20ma signals just recently. i got out of high school in 05' and ill be honest i dont remember learning anything of the sort thats been mentioned in this thread as far as formulas goes. FWIW.

As previously mentioned by Lynx777: y = mx + b equation of a line. A simple exercise in basic Algebra. I'd be surprised if you haven't seen it. Perhaps a picture will look more familiar:

JZerb · Apr 18, 2017

Paully's5.0 said:
As previously mentioned by Lynx777: y = mx + b equation of a line. A simple exercise in basic Algebra. I'd be surprised if you haven't seen it. Perhaps a picture will look more familiar:

well now that you mention it, that does look familiar now that i see it drawn. perhaps it wasnt the lack of me learning it in high school and moreso the lack of me needing to use it ever since 10th grade math until now.

sparksy@gage · Apr 20, 2017

I use a app on android.
PLC TOOLS 1.2 by Mani
simply put in raw min and raw max, and scale min and max
then input the raw input you want converted.

Scaling analog 4-20ma

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