⠿ NDT
(^08) ⠿ MARINE MAINTENANCE TECHNOLOGY INTERNATIONAL | APRIL
The old saying ‘all the
gear but no idea’ still
applies... professional
training is vital
Allinson says, “There are many marine
uses, from electrical systems to any
rotational machinery, AC, liners inside
pumps, liquid leaks and ingress. For
fiberglass hulls we can tell a lot by external
thermographic inspection. The biggest value
is in prediction.
“After fiberglass is damaged, there is
a loss of thermal mass. You are basically
looking for anomalies with the IR camera,
then in most cases you would follow-up
with another inspection such as ultrasound.
To inspect hull thickness, for example, the
ultrasound probes are about 2cm wide so
covering a whole hull is impractical – you
would use that to check areas that showed
up on the infrared scan.”
High-end equipment
Ian Johnstone, sales and marketing director
of Armstrong Optical, which markets
equipment from leading manufacturers,
says, “There are export limitations for the
high-end thermal imaging equipment. He
explains that if the equipment stays on board
there are few problems, but high-speed,
high-sensitivity detectors have obvious
military applications and drug dealers and
criminals in the past have used them to find
cannabis hot-houses.
“One market leader for both high-end
and low-end infrared cameras, has radically
changed the market. The cheap infrared
cameras are now mass market, bringing the
entry-level price way down. These have their
uses, but they shouldn’t be considered to be
professional equipment,” says Johnstone.
“There are three bands within the
infrared spectrum – usually we are using
the long-wave end of the spectrum. Military
use is more in the middle of the band and
just outside the visible light band. Composite
images using different bands can be very
revealing and contain much more than the
sum of the parts. For example, on a PCB
we might find heat from an overheating
component – by combining IR with a visible
light image, we can say exactly which
component is the problem,” he says.
“As with digital cameras, there is
an ongoing increase in megapixel (MP)
capacity,” says Johnstone, adding that it
is more important to discuss ‘effective
megapixels’. While digital cameras can
interpolate the missing light information
falling between receptors, thermographic
sensors are physically moved a fraction so
that these areas are now covered, which
creates a combined image from more than
one picture. This means each pixel would
be used four times. In practice, some
thermographic cameras use the natural
shake of the operator’s hand to achieve this.
Pixel shifting has become faster, too, says
Johnstone, who cited a high-end camera,
which can do this at full-frame video rate.
Pixel density has also increased rapidly.
Whereas early thermography cameras gave
an image less than 128 pixels wide, now
images of 1024x768 equal to 1MP resolution
are on the market. Johnstone says, “Our
cameras increase this to 3.2MP by shifting
the detector.”
From camera to camera, thermal
sensitivity ranges from 0.1°C in basic
units, to the most expensive models, which
detect 0.02°C. A 2% latitude in accuracy is
normal across some devices’ temperature
measurement ranges. That can be reduced to
a latitude of ±0.5%, with an accuracy within
0.5°C through changes in signal processing
from the receptors. A 12bit processor
will give 8,000 gray levels (temperature
indications), while a 16bit processor can
produce 140,000 levels –
a 16-fold increase in resolution.
High-end equipment now is capable
of picking up a human on an island at a
distance of 18km – that level of equipment
is going to cost around US$400,000. \
RIGHT: A motor
clearly showing
overheating housing
FAR RIGHT: Images
from thermography
can also capture the
expected general
temperatures of a
mechanical system
ABOVE: Moisture
intrusion can be
detected in fiberglass
components with
thermography. Here,
a sailboat keel shows
water inside the
laminated structure