Electromagnetic Testing

Read Out Mechanisms - Recorders
Data recorders might be required to meet the inspection criteria. Recording is sometimes
accomplished on analog paper strip charts or on magnetic tape formats. With most modern
equipment providing recording capability some form of digital media would be used. The
data could be stored internally in some test systems, but more often than not the data are
exported to an external storage device. Most of these digital recording media can retain the
files created for offline analysis and long term historical use. Early digital systems were
write once - read many devices. The more recent recording media can be erased and
reused. The advantage of digital systems is that all of the raw data created by a
multifrequency test system can be viewed in multiple display formats at the same time.
Tubing exam data are often reviewed using both the X-Y and strip chart modes to optimize
discontinuity detection and sizing.
The s trip chart format is often used where the discontinuity's location down the length of a
rod or tube is critical. The strip chart length is indexed to time or distance and signal
response deviation from the baseline indicates various material conditions. The amplitude
of the X-Y lissajous response in Figure 6.15 (6.66 V) is an 1nd1cat01' of the volume of the
discontinuity. The phase angle with respect to the/( axis (114 degrees) represents
discontinuity depthl(in this case, 41%) and discontinuity origin (tube outside diameter),
indicating whether the discontinuity originated on the inside or outside surface of the tube
(13). Many comp uter-based systems have multiple display modes available for the same
raw data set.
Charlie Chong/ Fion Zhang