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.
If you have a set sensitivity it is like having a yard stick instead of a measuring tape. You can measure the length of a 2X4 with the yard stick with low speed and accuracy, or you can use a tape that is fast and accurate. In the case of the AccuTrak® you have a measuring tape that depending where you are with the sensitivity, is like having a tape that you can extend or retract to fit your needs. If you are doing compressed air leak detection you walk into the area and set the sensitivity (up or down) so that you get 2 to 5 LEDs. (If the area is quiet you may not get any LEDs) This setting is your background. Anything above this level indicates a leak or other ultrasonic sound. Following this sound and letting the intensity guide you will eventually find the leak. However as you get closer to the leak the intensity will increase (this is the reason that you cannot tell how much a leak is leaking because you measured it in db or any other unit) and you may run out of “stick” i.e. max the display. At that point you lower the sensitivity, which reduces the display indication which in turn allows you to continue getting closer to the leak until you get right over it.
Having the ability to select the sensitivity and having the ability to change the volume in addition to that gives you flexibility to adapt in any situation.