Quick scaling question

Having a brain block, If i go from a 4-20 ma. scale to a 3-15psi how do i scale again. like say if I have 10ma. how many psi should i have?


Thanks!

mA times 0.75 equals psi ...

so for your specific example:

10 mA times 0.75 = 7.50 psi ...

how did you come up with the .75 number?

Hello bigrthanur

This is the way I think Ron calculated it.

value at 20mA - value at 4mA / measurment range (16mA)

15-3/(20-4)=0,75

Regards

Henny

Just take the difference between your higher and lower pressure(12 psi).

Then the difference between the upper and lower current range(16 mA).

Divide the pressure difference by the current difference and this will give you a scaling factor of 0.75 psi per mA.

Jon.

If you are wondering how Ron derived the numbers, the equation is the algebra 101 line equation, y=mx+b

X is the raw input.
Y is the scaled value.

M is the slope and it is defined as: EUMax-EUMin/Input_Max-InputMin
In this case,
(15psi - 3 psi) / (20mA-4mA) = 12psi/16ma = .75psi/mA

b is the y intercept offset = EUMin - (InputMin*M)


So for an input of 10mA,

y =    m          x    +          b



y = (( .75psi/mA )*10mA) + [3psi - (4mA * .75psi/mA)]

y =     (7.5psi)         +  3psi - 3psi

y = 7.5psi

Note that the b term is zero in this particular case, it often is zero, but not always. Whenever a transducer has a non-zero lower limit, don't automatically assume that b will be zero.

Also note that I included the units, psi, and mA as part of the numbers in the calculation and that the mA units all cancel out,
leaving the answer in only the units I wanted, ie psi. I find that this practice helps me keep from making mistakes in the process.

how did you come up with the .75 number?

Click to expand...

be careful what you ask for ...

go to the "Sample Lessons" area on my website ... I've posted about 40 pages of detailed "scaling" instructions on there ... it ended up being so long that I broke it into two PDF files so that more people could download it without a time out ...

you can also read a lot of the original material right off the forum here:

http://www.plctalk.net/qanda/showthread.php?p=101766&postcount=1

but it's usually easier to print from the PDFs ...

Henny and Jon R have the right idea - in a much condensed form ... and Alaric - who posted while I was typing ...

Thanks you guys all answered my question in more detail than i thought would be givin.......

THANK YOU!

You can download a very useful scaling tool from http://www.rce24.co.uk/tech24/downplc.htm

Go to PLCS.net Downloads section, click on the Misc selection and download the Excel file 'scaling chart.xls'. This is good for calibration charts to be used in the field.