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2) The
key has to be pressed multiple times to tab to the numeric digit to be changed.
The
/
keys have to be used to increase/ decrease numeric values on the display
until the sound velocity of the material being measured is matched. An auto repeat
function is built in, so that when the key is held down, numeric values will increment/
decrement at an increasing rate.
3) The
key has to be pressed to confirm or the
key hast to be pressed to cancel
the calibration.
To achieve the most accurate measurement results, it is generally advisable to cali-
brate the instrument to a sample piece of known thickness. The composition of mate-
rials (and thus, its sound velocity) sometimes varies from lot to lot and from manufac-
turer to manufacturer.
Calibration to a sample of known thickness ensures that the instrument is set as closely
as possible to the sound velocity of the material being measured.
5.5 How to perform measurements
The instrument always stored the last measured value until a new measurement is
made. In order for the transducer working in the right way there may not be any gaps
between the contact area of the sensor and the surface of the material being meas-
ured. This is accomplished with the coupling fluid, commonly called "couplant". This
fluid serves to "couple" or transfer the ultrasonic sound waves from the transducer, into
the material and back again.
Therefore a small amount of couplant should be applied onto the surface of the mate-
rial, before measurements are performed. Typically, a single droplet is sufficient.
After the couplant is applied, the transducer has to be pressed firmly against the area
being measured. The coupling status indicator should appear on the display as well as
a digit number. If the instrument has been "zeroed"
properly and if it has been set to the correct sound velocity, the actual thickness of the
material directly beneath the transducer will be indicated as a number in the display.
If the coupling status indicator doesn't appear or if it isn't stable or if the numbers on
the display doesn't seem to be correct, it has to be checked whether there is an ade-
quate film of couplant beneath the transducer and whether the transducer is placed flat
onto the material.
If conditions persist, sometimes it is necessary to select a different transducer (size or
frequency) for the material intended to be measured.
While the transducer is in contact with the material, the instrument will perform four
measurements every second, updating its display as it does so.
If the transducer is removed, the display will hold the last measurement performed.
Note: Occasionally a small film of couplant will be drawn out between the transducer
and the surface, as the transducer is removed. If this happens, the instrument may
perform a measurement through this couplant film, resulting in an erroneously meas-
urement. This is comprehensible because one thickness value is observed while the
transducer is in place and the other value is observed after the transducer is removed.
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TU_US-BA-e-1714
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