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1.3.2 Absolute measurement / Relative measurement / Differential measurement
Absolute measurement
(Absolute ZERO) Condition for the absolute measurement is the performing of a
reversal measurement for determining the absolute zero (to the centre of
the earth)
After this procedure the instrument will display the effective deviation from
the centre of gravity. This means the value is an absolute angle of the
measured surface.
Example: if the object to be measured is absolutely level, the display is "0"
Relative measurement
(Relative ZERO) A number of measurements do not require the absolute ZERO as described above.
Example: Angular deviation between two objects. (Lines, surfaces, guide ways) The
measurement instrument is placed on an object and the displayed value is
changed to "0". Then the instrument is relocated to a second object and the
displayed value of the angle is the angular difference between the two
surfaces. If the display is also "0", then both surfaces are parallel.
Most important is that always the instrument is placed on both positions in
the same direction.
Differential measurement
A differential measurement is a measurement with two instruments Measuring
instrument (A) and Reference instrument (B), measuring the angular difference
between the two. This means e.g. if the angular change in both instruments is the
same the displayed value (Difference A - B) does not change. In principle this is a
special relative measurement.
I the following measuring tasks the differential measurement is especially used
• Measurement on object with vibrations imposed.
• Measurements on unstable systems or objects
Example: Measuring the flatness of a machine bed. The reference instrument is
placed on a stable part of the machine where the measurement is not
interfered. With the measuring instrument the measurement is taking place
without removing the reference instrument.
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