mvip@mitech-ndt.com
5 When the progress bar shows complete, remove the transducer from the probe disc. If
necessary, repeat this procedure for times.
6 At this point, the instrument has successfully calculated its internal error factor, and will
compensate for this value in any subsequent measurements. When performing a “probe zero”, the
instrument 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 instrument 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.
7. Press
while in probe zero mode will stop the probe zero operation and return to the
measurement mode.
5.4 Sound Velocity 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 inherent sound
velocities. 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 T
wo-point
calibration allows for greater accuracy over
small ranges by calculating the probe zero and velocity.
Note
:
One
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.
5.4.1 Calibration to a known velocity
Note: This procedure requires that the operator knows the sound velocity of the material to be
measured. A table of common materials and their sound velocities can be found in Appendix A of
this manual.
In the Set Velocity dialog,
press F1/F2 and
/
keys
to adjust the velocity value
up or down, until it matches
the sound velocity of the
material to be measured.
You can also press the
key to select among the
preset
commonly
using
velocities.
5.4.2 Calibration to a known thickness
Note: This procedure requires a sample piece of the specific material to be measured, the
exact thickness of which is known, e.g. from having been measured by some other means.
1
Perform a Probe-Zero on the standard 4.00 mm disc.
2
Apply couplant to the sample piece.
3
Press the transducer against the sample piece, making sure that the transducer sits flat
against the surface of the sample. The display should show some thickness value, and the
coupling status indicator should appear steadily.
4
Having achieved a stable reading, remove the transducer. If the displayed thickness