DAKOTA ULTRASONICS DX-1 Manual page 13

At this point, the DX-1 has successfully calculated it's internal error
factor, and will compensate for this value in any subsequent
measurements. When performing a "probe-zero", the DX-1 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 DX-1 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.
In order for the DX-1 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 DX-1 provides three simple methods for
setting the sound-velocity, described in the following pages.
Calibration
DX-1 Ultrasonic Micrometer
13