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How Ultrasonics Work
How to Use the Sonic 3000
Leak Testing Large Containers

 

 
  Sonic 3000 Ultrasonic Detector
How Ultrasonics Work
 
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 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.

 
 

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