General Applications - NDT Systems Nova Eclipse TG930 User Manual

probe. The transducer has a thin wafer of a piezoelectric material at its
surface. Piezoelectric materials convert short pulses of electrical energy into
tiny mechanical displacements, and, conversely, from mechanical
displacements into electrical impulses. The probe acts both as transmitter
and receiver
When coupled to the surface of the test object – call it the entry surface – the
probe receives a short-duration electrical pulse from the pulser in the NOVA.
Fundamentally, the "clock" starts with this pulse. The ultrasonic sound-beam
begins its transit within the test object, induced by the electrical-to-
mechanical impulse generated by the transducer. The ultrasonic impulse
travels through the test object at a velocity characteristic of the material.
When it reaches a boundary – for example, the side of the test object
opposite the entry surface – the back surface – it reflects (echoes) back
toward the entry surface. As it impinges on the entry surface, the transducer
senses the tiny mechanical echo displacement and converts it into a small
electrical signal. In the receiver section of the NOVA, the "clock" is signaled
to stop. The time interval between the start and stop is converted into a
thickness measurement. The characteristic velocity of the test material –
distance divided by time (e.g., inches or millimeters per microsecond) –
divided by the round-trip transit time (microseconds) yields total distance
traveled. Since the total transit path is twice the actual thickness, it is
automatically divided by one-half and displayed as a decimal thickness. On
test objects with smooth, parallel surfaces, the displayed thickness is true
within the resolution of the last decimal place displayed.
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Nova Eclipse TG930 - Users Manual
Functional Diagram
This round-trip time of the echo is linearly correlated to the test material's
thickness – the thicker the material, the longer the round-trip time. The
NOVA's sophisticated circuitry precisely measures the round trip-time and
compensates for the characteristic ultrasonic velocity of the material being
gaged. The measured thickness is displayed, virtually instantaneously, on
the digital display and in the units selected, either inches or millimeters.

1.2 General Applications

The NOVA TG930 gages a wide range of thicknesses on metals, plastics,
ceramics, glass or virtually any other material which satisfactorily conducts
ultrasound, and has relatively parallel (or concentric) surfaces. Its high-
resolution capabilities make the NOVA TG930 uniquely applicable to gaging
very thin materials – for metals, as thin as 0.005 inch (0.13 mm) – with
resolution of 0.0001 inch (0.001 mm). However, it can also be effectively
used to gage thickness up to 20 inches (500 mm).
Small, local thickness variations, such as those caused by corrosion,
erosion, and certain types of internal flaws may also be detected. [9xx
Model-dependent: Later sections of this manual will detail a unique
feature of the NOVA that produces a cross-sectional view of the flawed
surface and the corresponding thickness reduction. Also included is
Nova Eclipse TG930 - Users Manual
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