17
en
Though the gauge will remember the last zero point, it is generally a good idea to set the zero point
whenever the gauge is switched on, as well as any time a different transducer is used. This will ensure that
the zero point of the instrument is always correct.
6.2 CALIBRATION
In order for the gauge to make accurate measurements, it must be set to the correct sound-velocity for the
material being measured.
Different types of material have different sound-velocities. For example, the velocity of sound through steel
is 5918 m/s (about 0.233 in/µs) and the velocity of sound through aluminium is 6350 m/s (about 0.248
in /µs).
If the gauge is not set to the correct sound-velocity, all of the measurements the gauge makes will be
erroneous by some fixed percentage.
There are three methods of calibrating the Model CG60 and Model CG60DL gauges:
One-point
CALIBRATION: This is the simplest and most commonly used calibration procedure - optimising
linearity over large ranges. One-point calibration is carried out using a known thickness.
Two-point
CALIBRATION: This allows for greater accuracy over small ranges. Two-point calibration is carried
out using two known thicknesses.
Known velocity
CALIBRATION: The sound-velocity of the material being measured is entered directly into
the gauge.
Note: Although the gauge has a through-paint/coating feature, one-point and two-point calibrations must be
performed on material with the paint or coating removed. Failure to remove the paint or coating prior to
calibration will result in a multi-material velocity calculation that may be different from the actual material
velocity intended to be measured.
CG60_CG60DL_TMA_0498_00_01_23225_A6L.fm Page 17 Tuesday, November 22, 2011 12:19 PM
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