14
CHAPTER FIVE
MAKING MEASUREMENTS
The steps involved in making measurements are detailed in this section. The
following sections outline how to setup and prepare your
ZX-3
for field use.
An automatic or manual zero must always be performed. The auto zero is an ‘off
block’ electronic zero that does not require a zero reference standard. This will most
always be the zero option of choice, as it makes the zeroing process very easy and
convenient to perform. However, the manual zero option offers better accuracy in
terms of a reference point. If the manual zero option is enabled, the probe zero must
be measured on the reference disk (battery disk) attached to the top of the
instrument. The zero compensates for variations in the transducer. In either mode
the sound velocity must be determined, and is used to convert the transit time to a
physical length. The sound velocity can be selected from a material chart in the
manual, selected from a short list of common materials in the
ZX-3
, or for greater
precision determined from a sample of the test material that has been mechanically
measured. To enter the velocity from a table, look up the material on the chart in the
appendix of this manual and refer to the section below on Calibration to a Known
Velocity. To determine the velocity of a single sample, refer to the Material
Calibration section on page 16.
When measuring curved materials, it’s more accurate to calibrate from two test
points, one at the minimum limit of the target thickness and one at the maximum limit.
In this case the reference disk mounted to the
ZX-3
is not used. This is called two-
point calibration and is described on page 19.
5.1 Probe zero
Setting the zero point of the
ZX-3
is important for the same reason that setting the
zero on a mechanical micrometer is important. It must be done prior to calibration,
and should be done throughout the day to account for any temperature changes in
the probe. If the
ZX-3
is not zeroed correctly, all the measurements taken may be in
error by some fixed value. The zero can only be performed with the measurement
mode set to pulse-echo (P-E). Therefore, if the
ZX-3
is to use the echo-echo (E-E)
measurement mode and a manual zero is being performed, the
ZX-3
will argue by
briefly displaying the message “NO PRB0”.
Important note:
The internal zero setting of the ZX-3, used for the auto zero mode,
can be reset at anytime by performing a “manual zero”, and immediately followed by
performing an “auto zero”.
The
ZX-3
is equipped with two zero options:
1)
Off Block Zero (Automatic Probe Zero)
– When this feature is enabled the
ZX-3
will do an electronic zero automatically, eliminating the need for a zero
disk or reference standard.
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