Checkline MX-3 - Manual (Page 17)

MX-3 Ultrasonic Thickness Gauge
13
At this point, the MX-3 has successfully calculated it's internal error
factor, and will compensate for this value in any subsequent
measurements. When performing a "probe-zero", the MX-3 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-3 will remember the last "probe-zero" performed, it is generally a
good idea to perform a "probe-zero" whenever the gauge 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-3 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 gauge is not set to the correct sound-velocity, all of the measurements
the gauge 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-3 provides three simple methods for
setting the sound-velocity, described in the following pages.