Sunday, June 23, 2013

Calibration and Re-Arranging

Every instrument has at least one input and one output. For a pressure sensor, the input would be
some fluid pressure and the output would (most likely) be an electronic signal. For a loop indicator,
the input would be a 4-20 mA current signal and the output would be a human-readable display.
For a variable-speed motor drive, the input would be an electronic signal and the output would be
electric power to the motor.
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
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.

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