Understanding IR Imagery of HTHW Systems
Thermal infrared (IR) imagery is imagery that shows heat. It is often
in the form of a grayscale picture whose scales (shades of gray) indicate
the differences in temperature and emissivity of objects in the image.
As a general rule, objects in the image that look lighter are warmer
and those that look darker are cooler. Bright white objects are the
warmest in the images. Black objects are the coolest.
When surveying High Temperature Hot Water (HTHW) Loop Systems, we record
infrared imagery on digital videotape and may later copy it to a VHS
videotape or a JPEG digital image file. We may modify the image to enhance
its value to the end user, such as creating a false-color image or adjusting
the brightness and contrast of a gray scale image. The digital images
are captured directly to BMP or JPEG format and placed on a CD-ROM.
The digital videotape and CD-ROM usually have the highest resolution
and contain the most easily viewed infrared images.
The infrared picture only shows objects which emit infrared wavelengths
in the 3000-5000 nanometer (3-5 micrometer) range; objects in the visible
light wavelengths of 400 to 700 nanometers (i.e., normally visible to
the human eye) are only detected because they also emit heat. An example
of this would be a street light that can be seen in the imagery. Any
object with a temperature above absolute zero (0 Kelvin or –273
degrees Celcius) emits infrared radiation.
How HTHW Leak FindIR™ can help you....
Underground High Temperature Hot Water (HTHW) Loop System lines are
almost always readily visible with infrared imaging, even when no notable
problems exist. This is due to the fact that no matter how good the
insulation, there is always heat loss from the lines which makes its
way to the surface. Problem areas are generally quite evident, having
brighter white IR signatures that exceed the norm.
Typically, line faults appear as an overheated line or as a large hotspot
in the form of a bulge or balloon along the line. Overheated lines often
occur when the HTHW line is located in a conduit or tunnel. If there
is a leak in the line it will heat up the whole conduit with escaping
heat. If a line is buried directly in the ground with an insulating
jacket, a leak will usually saturate the insulation, rendering it largely
ineffective and will begin to transfer heat into the ground around the
leak, producing the classic bulge or balloon-like hot area straddling
the line. Some leaks may show up as an overheated manhole or vault cover.
If manholes or vaults that contain system control apparatus which are
leaking, will often heat the covers to warmer than normal temperatures.
Unless these leaks are severe enough to significantly raise the manhole
temperature above their normally slightly elevated temperatures, these
leaks can be difficult to identify. Some leaks actually appear cold
on the ground since water that has made its way to the surface can start
evaporating – which cools the ground.
HTHW line imagery can be a little misleading, unless one understands
and interprets the relative brightness/temperature of a given line correctly.
A HTHW line that is the same temperature from one end to the other that
passes under different surfaces and materials can exhibit numerous grayscale
or pseudocolor variations. For example, five different apparent temperatures
will result from the same temperature line that runs under a grass-covered
field, an asphalt parking lot, a concrete loading dock, a gravel-covered
area and bare earth pathway.