If you are in any job like mine, you have come into a startup facility with the latest and greatest analog sensor, just to find out the field device is showing something different than what you see on your PLC. So begins the great troubleshooting, which, inevitably, involves pulling out a bunch of manuals, possibly the back of a napkin (or, if you have no deadlines, a nice spreadsheet), calculating what the output should be, going to the field device to see what the inputs are ranged to.

Worse still if you are working with a field device that sits in-between your instrument and the PLC. Whether it is a converter or a repeater, there are dip-switches and other settings to play with. You get to pull things apart, toss your voltmeter on it, etc.

Having done the math on a napkin one too many times, I made a little cellphone app. It looks like something written on a napkin and is no substitute for a proper calibration and calculator, but it is portable and certainly saves me time. See more at http://psillc.blogspot.com/2017/11/scaling-loops-for-field-instruments.html

WARNING: If you want a pretty app, this is not for you, nor is it a substitute for a field manual or trained engineer. This is only for first-glance linear interpolations. Use at your own discrepancy and risk.