Olympus EPOCH 650, User Manual, Page: 288 / 362

DMTA-10055-01EN, Rev. A, February 2015
Chapter 11 276
signal (largest echo) on screen by automatically adjusting the gain level of the
system. AGC adjusts this largest echo signal on screen to a predetermined height
(based on the selected probe), to maintain a consistent echo height and shape.
• Zero Cross Detection Algorithm
The Corrosion Module uses a unique measurement mode called the Zero Cross
detection algorithm. This measurement mode, unlike Peak, Edge, or First Peak
mode, uses the first point in time when the echo of interest crosses the baseline
(zero cross) to make the measurement. By using the zero cross point, the
measurement is not affected by variation in amplitude, which is a major source of
inaccuracy in traditional flaw detector corrosion applications. To be recognized as
the “echo of interest,” an echo must be higher than a fixed screen height
threshold. To ensure prominent echoes achieve this criteria, AGC is used to
maintain dynamically optimized amplification of the A-scan.
• V-Path Correction
The EPOCH 650 Corrosion Module includes V-path correction for all dual probes
in its standard probe library. Dual transducers are typically constructed with a
small angle of incidence in both the transmit and receive crystals. This small angle
is known as the “roof angle.” Because of the roof angle, sound is introduced to a
test piece at a small refracted angle. Therefore, the actual depth of a given
reflector is slightly smaller than the measured depth unless the measurement
includes a correction for this roof angle. The angle correction is called “V-Path
Correction,” and is achieved when the roof angle of a given dual transducer is
known and compensated for during measurement.
11.9.2 Corrosion Module Screen
The EPOCH 650 ultrasonic flaw detector Corrosion Module has a modified screen
layout compared to standard flaw detector mode. Also, many of the normal flaw
detection functions are limited or unavailable when in the Corrosion Module (see
Figure 11-36 on page 277).
Measurement Boxes – The measurement boxes are fixed in Corrosion Module to more
limited selections applicable to corrosion measurement only.
Gain Control – The Gain control is set to Auto when AGC is active
Measurement Region – The measurement region is defined by the gate at the top of
the A-scan window. Gate level adjustment is not available in Corrosion Module
because the measurement mode is fixed to “Zero Cross.”