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Energy auditors use thermography—
or infrared scanning—to detect
thermal defects and air leakage in
building envelopes by measuring surface
temperatures via infrared video and still
cameras.
These tools see light that is in the heat
spectrum. Images on the video or film
record the temperature variations of the
building’s skin, ranging from white for
warm regions to black for cooler areas.
The resulting images help the auditor
determine whether insulation is needed.
They also serve as a quality control tool,
to ensure that insulation has been installed
correctly.
The U.S. Dept. of Energy says, a thermographic
inspection is either an interior
or an exterior survey. The energy auditor
will decide which method would give the best results under certain weather
conditions. Interior scans are more common,
because warm air escaping from a
building does not always move through
the walls in a straight line.
Heat loss detected in one area of the
outside wall might originate at some
other location on the inside of the wall.
Also, it is harder to detect temperature
differences on the outside surface of the
building during windy weather.
Because of this difficulty, interior surveys
are generally more accurate because
they benefit from reduced air movement.
Thermal images, or thermograms, are
actually visual displays of the amount of
infrared energy emitted, transmitted, and
reflected by an object. Because there are
multiple sources of the infrared energy, the
DOE says it is difficult to get an accurate
temperature of an object using this method.
A thermal imaging camera is capable of
performing algorithms to interpret that data
and build an image.
Although the image shows the viewer
an approximation of the temperature at
which the object is operating, the camera
is actually using multiple sources of
data based on the areas surrounding the object to determine that value rather than
detecting the actual temperature.
This phenomenon may become clearer
upon consideration of the formula Incident
Energy = Emitted Energy + Transmitted
Energy + Reflected Energy where
Incident Energy is the energy profile when
viewed through a thermal imaging camera.
Emitted Energy is generally, what is
intended to be measured.
Transmitted Energy is the energy that
passes through the subject from a remote
thermal source. Reflected Energy is the
amount of energy that reflects off the surface
of the object from a remote thermal
source.
If the object is radiating at a higher temperature
than its surroundings, then power
transfer will be taking place and power
will be radiating from warm to cold. So
if there is a cool area in the thermogram,
that object will be absorbing the radiation
emitted by the warm object. The
ability of both objects to emit or absorb
this radiation is called emissivity.
Under outdoor environments, convective
cooling from wind may also need to be
considered when trying to get an accurate
temperature reading.
Thermographic scans are also commonly
used with a blower door test running.
The blower door helps exaggerate
air leaking through defects in the building
shell. Such air leaks appear as
black streaks in the infrared camera’s
viewfinder.
Thermography uses specially designed
infrared video or still cameras to make
images (called thermograms) that show
surface heat variations. This technology
has a number of applications. Thermograms
of electrical systems can detect abnormally
hot electrical connections or
components. Thermograms of mechanical
systems can detect the heat created
by excessive friction. Energy auditors
use thermography as a tool to help detect
heat losses and air leakage in building
envelopes.
Infrared scanning allows energy auditors
to check the effectiveness of insulation
in a building’s construction. The
resulting thermograms help auditors determine
whether a building needs insulation
and where in the building it should
go. Because wet insulation conducts heat
faster than dry insulation, thermographic
scans of roofs can often detect roof leaks. In addition to using thermography
during an energy assessment, you should
have a scan done before purchasing a
building; even new construction can
have defects in thermal envelopes. A
thermographic scan performed by a certified
technician is usually accurate
enough to use as documentation in court
proceedings.
The energy auditor may use one of
several types of infrared sensing devices
in an on-site inspection. A spot radiometer
(also called a point radiometer) is the
simplest. It measures radiation one spot
at a time, with a simple meter reading
showing the temperature of a given
spot. The auditor pans the area with the
device and notes the differences in
temperature. A thermal line scanner
shows radiant temperature viewed
along a line. The thermogram shows
the line scan superimposed over a picture
of the panned area. This process
shows temperature variations along the
line.
The most accurate thermographic inspection
device is a thermal imaging camera,
which produces a two-dimensional
thermal picture of an area showing heat leakage. Spot radiometers and thermal
line scanners do not provide the necessary
detail for a complete energy assessment.
Infrared film used in a conventional camera
is not sensitive enough to detect heat
loss.
Preparing for a Thermographic Inspection
To prepare for an interior thermal scan,
the property owner should take steps to
ensure an accurate result. This may include
moving furniture away from exterior
walls and removing drapes. The most
accurate thermographic images usually
occur when there is a large temperature
difference (at least 20° F [14° C]) between
inside and outside air temperatures. In
northern states, thermographic scans are
generally done in the winter. In southern
states, however, scans are usually conducted
during warm weather with the air
conditioner on. “The use of thermographic cameras in
restoration projects delivers higher levels
of accuracy and control,” said Chris
Netherton, managing director of the National
Flood School in the United Kingdom.
“Thermographic technology has been available for years, but it has traditionally
been extremely expensive.
“As with most technology, the equipment
has become much more cost-effective
and will become increasingly important
to flood restoration professionals. “The equipment enables users to pinpoint
problem areas. An example is a
leaking pipe buried in concrete. Traditionally,
much of the affected area
would need to be dug out to identify the
leak, but with a thermal imaging camera,
the problem can be quickly and accurately
traced, with only the affected
area requiring access.
“Another application in which they can
be used is to trace complex electrical
faults. Many hours of testing might need
to be carried out to establish the problem,
but a thermal imaging camera can quickly
locate the defect.”
Every object whose surface temperature
is above absolute zero (-273 °C)
radiates energy at a wavelength corresponding
to its surface temperature. Utilizing
highly sensitive infrared cameras, it
is possible to convert this radiated energy
into a thermal image of the object being
surveyed. ❑ |
