The methods are:
1. Ultrasonic sound detection;
2. Compound specific detection; and
3. Tracer (dye) injection.
As a gas escapes from a pressurized system through a leak (hole), it causes a noise. This is the result of turbulence in the gas flow caused by the edges and/or shape of the leak point. The greater the pressure differential, the greater the turbulence and, therefore, the greater the noise.
This noise, similar to the sound of a dog whistle, is at too high a frequency (i.e., ultrasonic) to be heard by human ears. It can, however, be “heard” by the sensitive microphones used in ultrasonic leak detectors.
A similar phenomenon occurs with a vacuum leak, as a gas rushes into an evacuated system or vessel. In such an instance, the noise is actually generated inside the system (as opposed to outside, as with a pressure leak).
Depending on their sensitivity and complexity, these leak detectors range in price from about $150 to several-thousand dollars.
Simply put, the bigger the leak, the better an ultrasonic detector works.
These detectors employ chemical, electrochemical, ionization, and other types of sensors that undergo some type of detectable change when exposed to specific gases or families of gases. Examples of such include detectors for halogen gases, combustible gases, carbon monoxide, etc.
When these sensors are exposed to the right type of compound, they change their chemical or electrical properties and the detector circuit, in turn, senses this change and provides an indication to the user.
Ultimately, this type of detector is capable of sensing minute levels of particular compounds, down to 1 part per million (ppm) or less. However, the wide range of sensors means that not all provide such capability.
Some products are more compound-specific than others are and some are more sensitive than others. Performance is usually concurrent with price and practicality.
This type of technology has an inherent compromise. The more compounds a technology can detect, the less responsive it will be to each; the more compound-specific, the greater the sensitivity. Of course, this compromise also varies from one type of sensor to another and is not always so clear-cut.
Such a dye may be visible under normal circumstances, or sometimes only under specific conditions, as when exposed to an ultraviolet (UV) light.
Several factors can influence these units’ operation. Most important is the need for the tracer-dye to thoroughly mix with the contents of the leaking system. Subsequently, enough time must elapse to allow the dye to be leaked out.
Also, since this technology relies on visual indications at the source of the leak only, a line of sight to every point of a system or vessel is needed.
Depending on the equipment needed (dyes, UV lights, and injectors), pricing may run from less than $100 to more than $700.
Ultrasonic detection is good when the leaking compound is unknown. Compound-specific detection is best when looking for specific compounds. Tracers are good when dealing with miniscule leaks.
Of course, ultrasonic is not your best choice in noisy environments, compound-specific is not good for leaks of unknown content, and tracers are least effective in very dark or very bright areas.
The choice of method must ultimately be made with all of the above information in mind, and consideration of the specific need. Invariably one, if not all, of the above means will effectively detect virtually any leak of anything.
You also must consider training, confidence, patience, and budget. No one product can do it all, and the most sophisticated product is useless in the wrong hands.
Always remember that there is no magic about leak detection, nor should there be any mystery. Patience and knowledge are the only real secrets.