There are three fundamental means of leak detection. These methods each have many variations.

The methods are:

1. Ultrasonic sound detection;

2. Compound specific detection; and

3. Tracer (dye) injection.

Ultrasonic sound detection

This method relies on the fact that the movement of a gas from one pressure to another through an orifice creates a high-frequency (40-KHz or higher) sound.

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.


  • Not gas specific; can be used to find leaks of any gas or of unknown gases.

  • Good directional indicator of leak location, since ultrasound travels in straight lines and does not penetrate barriers.

  • Works on both pressure and vacuum leaks.

  • No danger of reactions with leaking compounds, since the sensor is simply a microphone.

  • Minimal, if any, maintenance is required.

  • Available as portable, handheld units; can provide access to most locations.

  • Will provide some indication of leak size; high-dollar versions may closely measure leak size.


  • Subject to high levels of background interference from many ambient noises (motors, fans, traffic, wind, etc.). Oftentimes background interference is equal to or greater than the leak source level.

  • Cannot easily locate hidden leaks as ultrasound is highly directional and will not pass solid barriers.

  • Probe tips are prone to false signals from direct contact, especially dragging on or along any surface.

  • Limited sensitivity is based on viscosity of gas and pressure differential. For example, it is difficult to find small leaks at low (30-psi or less) pressures. Also, the same-size leak will not sound the same for all gases. Some gases flow more readily and at greater volume because of their densities.

    Simply put, the bigger the leak, the better an ultrasonic detector works.

  • It’s difficult for inexperienced users to effectively operate these units at peak capability.

Compound-specific detection

This method employs any number of sensors that react or respond to compounds based on their chemical property. This is contrary to ultrasonic detection, which is based on a gas’ physical properties.

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.


  • They have potentially very high sensitivity.

  • Most detectors are sensitive enough to find all repairable leaks and will lead to, or pinpoint, leaks effectively, since the detector will sense the presence of a compound and where it may be in the system. The method can follow increasing concentrations back towards the leak source. Remember that the concentration of a leaking compound will invariably be greatest at the source.

  • Not nearly as prone to false signals as ultrasonic detectors, since ambient rarely contains the same compound as the leaking system or chamber. Background contamination can invariably be compensated for, as it is less than the leak source level.

  • Some detectors provide very compound-specific detection, ensuring that a signal occurs only when the leaking gas is sensed, not because of other detectable compounds (as no other compounds are sensed).

  • Will indicate leak size with a fairly high degree of accuracy. Accuracy improves as price increses.

  • Available as both portable and 110-220-V-powered models. Will provide access to most locations.

  • Easily used by novices.


  • Usually higher priced than ultrasonic detectors of similar grade.

  • Proper and routine maintenance is very important to keep these units working properly and effectively.

  • As compared to ultrasonic detectors, compound-specific detectors have limited applications. This will vary widely as some technologies detect hundreds of compounds, and others less than half a dozen.

  • Some technologies are more difficult to use in certain applications, as detectable compounds other than the leaking one may be present.

  • Continuous replacement of sensors is required. Costs and frequency vary from $10 to $500 and from days to years.

Dye injection

This method of leak detection involves the injection or addition of any number of dyes or colorants into the suspect system or vessel, with the intent that such a tracer-dye will leak out with the contents, providing a visual indication of the leak location.

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.


  • When used properly, all leaks will be indicated.

  • Will indicate even the minutest leaks.

  • Works equally well with gas and liquid leaks.

  • Provides precise leak location indication.

  • Gives no false signal or indications.

  • Easy to use, even for novices.

  • Can be comparatively low cost.


  • Question of tracer-dye compatibility with system/vessel contents.

  • Dye may be present but may be overlooked or not seen in a “line of sight” location.

  • UV cannot be used in sunlight unless dark covering or shield is used.

  • Set up time could be long.

  • Search time could be long.

  • Gives poor indicator of leak size.

  • Requires practice to gain proficiency, if not success.

  • Continued cost of supplies such as dyes, and sometimes UV bulbs.

In conclusion

Each method has advantages and disadvantages as compared to the other two.

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.