Turbulent flow has a high content of ultrasound. This is sound which is above the human hearing range, but can be heard with the AccuTrak, and traced to its source. It is important to remember that 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.
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 Raynolds 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.
Ultrasonic leak detectors "hear" leaks, therefore the sensitivity can not be accurately stated in terms of cc/sec, parts per million, or ounces per year. The proper specification for these types of detectors is decibels. The amount of sound pressure created by the leak will determine its ability to be detected ultrasonically.
When comparing to gas specific detectors which are extremely sensitive, remember, the instrument was tested under controlled laboratory conditions, and the ability to actually locate small trace gases, especially in a windy outdoor environment is extremely limited. It is physically impossible for an ultrasonic detector to locate a leak such as .5oz per year because there is no turbulent gas flow involved. A leak of .5 oz. per year is equivalent to a loss of 1 pound in 32 years!
Realistically, an ultrasonic detector will detect most of the leaks you encounter on a regular basis. It is the only instrument which will pinpoint a vacuum leak, or detect any pressurized gas in any system. It is also more accurate for detecting larger leaks because its sensor will not become saturated or false alarm from the presence of gas in the atmosphere. Because ultrasonic detectors do not "sniff" out the gas, they can easily locate a leak under windy conditions.
