Relating 4 to 20 mA signals to instrument variables

Calculating the equivalent milliamp value for any given percentage of signal range is quite easy.
Given the linear relationship between signal percentage and milliamps, the equation takes the form
of the standard slope-intercept line equation y = mx + b. Here, y is the equivalent current in
milliamps, x is the desired percentage of signal, m is the span of the 4-20 mA range (16 mA), and
b is the offset value, or the “live zero” of 4 mA:

This equation form is identical to the one used to calculate pneumatic instrument signal pressures
(the 3 to 15 PSI standard):

The same mathematical relationship holds for any linear measurement range. Given a percentage
of range x, the measured variable is equal to: