Superior Signal Company LLC AccuTrak
®
VPE
User’s Manual
© 2015
5
PRINCIPLE OF OPERATION
The principle of operation of the AccuTrak
®
VPE is based on the turbulent
flow of fluids and gasses. Turbulent flow has a high content of ultrasound.
This sound is above the human hearing range, but it can be heard with the
AccuTrak
®
VPE, and then traced to its source.
Imagine air leaking from a tire. Because this is such a large leak, your ear
can detect this sound, however your ear hears only about 1/3 of the actual
spectrum of sound which exists. The sound of small leaks is mostly ultrasonic
which your ear can not detect.
It is important to remember this example: A piece of straight tubing
connected to a gas supply and left free to exhaust into the atmosphere
will not generate sound if the volume of gas through it is such that
turbulence does not take place. Yet for that same flow, an opening as
small as 0.005 of an inch could generate enough sound to be heard
several feet away.
For a leak to happen there must
be an opening in the system that
carries a gas or fluid. Normally,
these openings are not clean
smooth
holes,
but
passages
through cracks with many jagged
edges and internal chambers.
Fluid or gas escaping through an
"orifice" like this is forced into
turbulence, random circular-like
motions. Inside a tube where a
gas may be flowing, the flow is
normally laminar which means
that a given layer of gas does not
mix with layers above it or below it. This condition happens in a straight long
tube when the velocity of the fluid is not high. A gas leaking out of a straight
and long tube will not generate as much sound as if it were leaking out of a
small crack because the flow is not turbulent.
The intensity of sound generated at a leak is a very complex function of the
viscosity, the temperature, the speed the fluid is moving, the Reynolds
number, the pressure differential across the leak, and the physical
dimensions and characteristics of the orifice. This is why it is possible for a
smaller leak to generate more sound than a larger one.
