ULTRA SOUND
As the name ultrasonics implies, this term deals with that branch of
acoustics whose frequency waves are above the highest frequencies
audible to the human ear. Ultrasonic vibrations (sound waves) are
measured in terms of Hertz (Hz). One Hz is one wave cycle per second.
The human ear is generally assumed to hear sounds with a frequency of 16
Hertz up to a limit of 20 kilohertz (20,000 cycles per second). However,
most sounds which we hear, are in a very limited range considerably
below this theoretical limit.
Today the most generally accepted
definition of ultrasonics refers to sound waves with a frequency greater
than 16 Khx. The present upper limit of detectable ultrasonic
frequencies is approximately 100 megahertz (100,000,000 cycles per
second).
As the frequency of the sound wave
changes, the way in which the sound wave propagates also changes. Low
frequency sounds tend to propogate spherically with equal intensity in
every direction. Higher frequency waves particularly those over 20
kilohertz tend to propogate more directionally like a beam. This makes
the location easier to find. As the wave frequency increases it becomes
more and more easily dampened, requiring the detector to be closer to
the source to hear the vibration.
The Sonic 3000 has been set to translate
frequencies between 30 and 50 kilohertz. Although leaks and most
vibration sources produce a very broad band of ultrasonic noise
experiments have shown that peak amplitudes for many of these problems
areas are around 45 kilohertz. This frequency range allows the detector
to work with directional waves which are not easily dampened.
MECHANISMS THAT PRODUCE
ULTRASOUND
A number of different mechanisms produce translatable
ultrasonic sound in the 45 kilohertz region. These mechanisms are:
1)
Turbulent fluid flow
2) Liquid movement
3) Mechanical movement
4) Sound generators
5) Electrical discharge
Turbulent Flow
Sometimes called sonic or choked flow, turbulent flow is the most widely
recognized source of ultrasonic vibrations. Turbulent flow occurs with
any fluid whether a liquid or a gas. This type of flow is one of the
three basic flow modes. Laminar and molecular are the other two. Of the
three, however, only the turbulent flow of a fluid across a pressure
boundary creates acoustic waves. These waves can be transmitted through
the medium of the fluid itself, through the containment structure, or
through the air surrounding the containment structure. Thus, depending
upon the situation, turbulent flow can be detected in a variety of ways.
Turbulent flow often occurs through holes with a diameter of .015 inches
to .0005 inches. It is generally assumed that the smallest detectable
flow through leaks this size is 1 x 10-2 standard cc/sec. (a
rate equivalent to a pound of Freon leaking out of a container every 3
months). However, because of the Sonic 3000's high sensitivity even
smaller leaks can sometimes be found. Besides instrument sensitivity two
other controllable factors, viscosity and velocity, can improve test
results. Lower viscosity fluids tend to create greater turbulent energy
and as a result, pressurization with a gas-like helium may allow the
location of leaks which could not be found with air. In the same way,
great velocity (or its complement, a greater pressure differential)
causes increased turbulent energy. For practical purposes pressure
differentials of 5 psi are at the lower limit of delectability. The
Sonic 3000 has been able to find laboratory leaks at pressure as low as
½ psi. Higher pressures can cause the acoustic waves to have a higher
and more constant amplitude, making leak testing easier and more
reliable.
There has been considerable discussion as
to which type of leak configuration is more likely to cause acoustic
vibrations. Several authors have held that labyrinth type leaks such as
threaded fittings or folded metal edges would diffuse the turbulent
vibrations to such an extent that turbulent flow would be undetectable.
Actual tests however have proven that even with pressure differentials
as low as 10 psi labyrinth leaks can be detected.
Liquid Movement:
Besides turbulent flow, liquid movements such as cavitation,
flashing of a liquid to a vapor, and bubble bursts can also produce high
energy ultrasonic noise. Cavitation can be especially useful for finding
small vacuum leaks which are usually considered difficult to find
ultrasonically. By applying a high surface tension liquid, (such as
30VS) on the area to be tested, the ultrasonic energy produced as the
film is broken by the vacuum creates a signal which is detectable at a
distance of several feet. In the same way, small bubbles from synthetic
bubble fluids (such as 16-OX)
applied across a pressure boundary create strong ultrasonic signals as
they burst and reform. Bubbles which are often too small to be seen can
be heard easily ultrasonically. This technique enables leaks to be
detected in the range of 10-6 standard cc/sec. Soap and water
will not work for this technique. Soap or detergent solutions for a
great number of bubbles when applied. This makes it difficult to
distinguish real from apparent leaks. In addition, soap solutions form
large rather than small bubbles and as a result, they do not produce
much ultrasonic energy not do they burst very often. For these and a
number of other reasons soap or detergent solutions have been banned by
a number of regulatory agencies such as ASTM and ASME for any leak
detection use.
Mechanical Movement
Another widely used source of ultrasonic energy results from contact
between metal parts and stress. Stressed material which results in
stretching, shearing, abrading or other types of deformation releases
high levels of ultrasonic energy. Ultrasonic sound resulting from
friction is often used to monitor machines to prevent shutdowns and
predict maintenance needs. Examples of problems which are easy to detect
are: Bearings with pits, cracked races, loose parts, lubrication
failure, misalignment, malfunctioning valves or gears. By detecting
defects such as insufficient oil film, worn bearings, misalignment, or
defective gears before significant increases in vibration or temperature
incipient failures can be prevented. See
How to Detect Machine Problems
Sound Generation
A technique for testing unpressured containers involves the use of
the ultrasonic sound
generator (3000SG) and our detector matched to the same frequency.
When used in a closed container with walls which reflect rather than
dampened noise the ultrasonic signals will pass through small leaks and
can be detected. This technique has been used successfully on items such
as: welded seams, airplane compartments, refrigerators, automobile
windows, condenser tubes and large tanks. The sound generator comes with
the master kit.
Electrical Discharge
The fourth type of ultrasonic noise that can be detected is
generated by electrical discharges (corona), sparks, and flashovers.
When an electrical spark jumps from one object to another, the heated
air expands rapidly and produces an airborne shock wave. (This effect is
similar to that of "thunder" which accompanies a lightning stroke). The
strong agitation of the air produces ultrasonic noise. Therefore, the
detection device can be used to locate electrical defects, i.e.,
high-voltage corona discharges, arcs in cables, on trolley arms, carbon
brushes, transformers, motors, contactors, insulators, reactors,
distribution systems and other electrical installations subject to
electrical leakage or breakdown of insulation. See
How to Detect Electrical Leakage
ANSI Approved Specification
ASTM E-1002 Standard Test Method for Leaks Using Ultrasonics
available from the
American Society for Testing and Materials
or from American Gas & Chemical Co. |
