To calibrate an instrument means to check and adjust (if necessary) its response so the output
accurately corresponds to its input throughout a specified range. In order to do this, one must
expose the instrument to an actual input stimulus of precisely known quantity. For a pressure
gauge, indicator, or transmitter, this would mean subjecting the pressure instrument to known fluid
pressures and comparing the instrument response against those known pressure quantities. One
cannot perform a true calibration without comparing an instrument’s response to known, physical stimuli.
To range an instrument means to set the lower and upper range values so it responds with the
desired sensitivity to changes in input. For example, a pressure transmitter set to a range of 0 to
200 PSI (0 PSI = 4 mA output ; 200 PSI = 20 mA output) could be re-ranged to respond on a scale of 0 to 150 PSI (0 PSI = 4 mA ; 150 PSI = 20 mA).
In analog instruments, re-ranging could (usually) only be accomplished by re-calibration, since
the same adjustments were used to achieve both purposes. In digital instruments, calibration and
ranging are typically separate adjustments (i.e. it is possible to re-range a digital transmitter without having to perform a complete recalibration), so it is important to understand the difference.