Checkline MX-5DL - Manual (Page 18)

Dakota Ultrasonics
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At this point, the MX-5 DL has successfully calculated it's internal error
factor, and will compensate for this value in any subsequent
measurements. Whe n performing a "probe-zero", the MX-5 DL will always
use the sound-velocity value of the built-in probe-disc, even if some other
velocity value has been entered for making actual measurements. Though
the MX-5 DL will remember the last "probe-zero" performed, it is generally
a good idea to perform a "probe-zero" whenever the tool is turned on, as
well as any time a different transducer is used. This will ensure that the
instrument is always correctly zeroed.
Calibration
In order for the MX-5 DL to make accurate measurements, it must be
set to the correct sound-velocity for the material being measured. Different
types of material have different inherent sound-velocities. For example, the
velocity of sound through steel is about 0.233 inches-per-microsecond,
versus that of aluminum, which is about 0.248 inches-per-microsecond. If
the tool is not set to the correct sound-velocity, all of the measurements the
tool makes will be erroneous by some fixed percentage. The one point
calibration is the simplest and most commonly used calibration procedure -
optimizing linearity over large ranges. The two point calibration allows for
greater accuracy over small ranges by calculating the probe zero and
velocity. The MX-5 DL provides three simple methods for setting the
sound-velocity, described in the following pages.