Dakota Ultrasonics CMX - Manual (Page 49)

CMX High Performance Material & Coating Thickness Gauge
45
7) Press the OK key to complete the probe zero function, or ESC key to cancel
the probe zero function.
8) Remove the transducer from the probe zero disk, and proceed to the
calibration section.
Note: The value that is displayed will change depending on the current velocity
setting in the CMX. Disregard the number that is displayed. It is not
important. What is important is accurately performing the steps outlined above
to insure reliability of the probe zero calculation.
6.3 Material Calibration
In order for the CMX to make accurate measurements, it must be set to the correct
sound velocity of the material being measured. Different types of materials 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
method - optimizing linearity over large ranges. The Two Point calibration allows for
greater accuracy over small ranges by calculating the probe zero and velocity. The
CMX provides four simple methods for setting the sound-velocity outlined below:
Known Velocity
If the material velocity is known, the user may wish to simply enter the velocity
number into the CMX, rather than have the CMX calculate the velocity value using a
known thickness on a material sample. The steps for entering the velocity are
outlined below:
Using a Known Material Velocity