Nova Eclipse TG950 - Users Manual
32
5.4 Ultrasonic Velocity
Velocity Measurement:
To accurately measure the Longitudinal
Ultrasonic Velocity characteristic of any particular material, it is necessary to
have a sample of the material having known composition, and, if a metal,
the heat treatment condition. Case-hardened, coated, or plated samples
are not acceptable. Both front and back surfaces should have a fine finish,
such as that produced by finish surface grinding. The surfaces should be
flat and parallel. The ideal nominal thickness of the sample should be
approximately 0.3 inch (7.5 mm) to 1.0 inch (25.4 mm). The precise
thickness should be determined to at least 0.xxxx inch (0.xx mm) accuracy
using an appropriate micrometer measurement tool.
Couple the probe to the surface of the reference sample, and, using the
Up/Down ARROW keys, scroll or toggle to the known thickness. As the
displayed thickness is adjusted to the known thickness, note that a velocity
value is displayed immediately below the thickness readout. If the gage has
been set to English units (inches), the velocity value will be displayed as a
number having four decimal places. If millimeters were set, the velocity will
have three decimal places. The units of velocity are inches per
microsecond (in/
m
s), or millimeters per microsecond.
Velocity Calibration:
Though infrequently used, if the ultrasonic velocity of
the test material has been previously established or recorded, and
reference standards are unavailable, velocity calibration will produce
accurate measurements. To set velocity, while in the Measurement
Display, momentarily depress either of the Up or Down ARROW keys
(VEL+ or VEL
-
).
Velocity
, with the currently stored velocity value, is
displayed immediately below the thickness readout. The ARROW keys can
then be used to toggle or scroll velocity until the display precisely reads the
known velocity value.
Since many factors can affect ultrasonic velocity, the Velocity Measurement
procedure should produce significantly more accurate results than using
published velocity data. If approximate gaging accuracy is acceptable, the
use of published data, such as shown in the Longitudinal Ultrasonic Velocity
Table, below, can be considered.
This approach is reasonable for simpler, more-rugged applications; for
example, the gaging of mild steel or plain carbon steel alloys. Published
velocity data will be found to differ somewhat in different publications. This
Nova Eclipse TG950 - Users Manual
33
is due to chemical or physical variations within the material that are within
the range of acceptable parameters for a particular alloy.
Longitudinal Ultrasonic Velocity Table
(At Room Temperature)
Material
Velocity - in/
m
s
Velocity - mm/
m
s
Acrylic Resin
0.105
2.67
Aluminum
0.249
6.32
Naval Brass
0.174
4.43
Phosphor Bronze
0.139
3.53
Cast Iron
0.181
4.60
Copper
0.183
4.66
W indow Glass
0.267
6.79
Inconel
0.225
5.72
W rought Iron
0.232
5.90
Magnesium
0.248
6.31
Monel
0.237
6.02
Nickel
0.222
5.63
Mild Steel
0.232
5.90
4340 Steel
0.230
5.85
CRES 303
0.233
5.66
Titanium
0.239
6.07
Zirconium
0.183
4.65
