Elcometer CG30 Operating Instructions Manual page 10

To set the zero point:
1. Plug the transducer into the gauge ensuring that the connectors are fully engaged. Check that
the wearface of the transducer is clean and free of any debris.
2. Switch on the gauge.
3. The battery compartment cover on the top end of the gauge acts as a metal 'probe-disc'. Apply
a single droplet of ultrasonic couplant to the top of this disc.
4. Press the transducer against the probe-disc, making sure that the transducer is flat against the
surface (Figure 5). The display should show some thickness value, and nearly all the bars of
the stability indicator should be illuminated.
5. While the transducer is firmly coupled to the probe-disc, press the PRB-0 key on the keypad.
The gauge will display 'Prb0' while it is calculating the zero point.
6. Remove the transducer from the probe-disc.
When setting the zero point, the gauge will always use the sound-velocity value of the built-in
probe-disc, even if some other velocity value has been entered for making measurements.
Although the gauge will remember the last zero point, it is generally a good idea to set the zero point
whenever the gauge is switched on, as well as any time a different transducer is used. This will
ensure that the zero point of the instrument is always correct.
5.2 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 sound-velocities. For example, the velocity of sound
through steel is 5918 m/s (about 0.233 in/µs) and the velocity of sound through aluminium is 6350
m/s (about 0.248 in/µs).
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.
There are three methods of calibrating your gauge:
One-point CALIBRATION: This is the simplest and most commonly used calibration procedure -
optimising linearity over large ranges. One-point calibration is carried out using a known thickness.
Two-point CALIBRATION: This allows for greater accuracy over small ranges. Two-point
calibration is carried out using two known thicknesses.
Known velocity CALIBRATION: The sound-velocity of the material being measured is entered
directly into the gauge.
Note: One-point 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.
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Elcometer 206
Elcometer 208
Figure 5. Transducer pressed against probe-disc