New improvements to ultrasonic leak detection bring environmental and commercial benefits.

- Richard Doran of Primalec explains the latest advances.

Legislation governing action that must be taken to prevent or rectify leaks of refrigerant is getting tougher all over Europe

Although leaks from vehicle air-conditioning systems are inherently smaller than those from industrial aircon or refrigeration systems, they are no less subject to the law, and major penalties can be imposed on anyone failing to find and rectify leaks of refrigerant. It is also a fact, obvious to us all, that refrigerant leaks inhibit performance or stop systems working, and make customers unhappy – potentially a more likely and immediate penalty to the automotive aircon servicing business.

There several ways of finding leaks, and it is wise to have more than one in your armoury . The best known is probably the use of uv-fluorescent dye compounds, injected into the system, which show leaking refrigerant as a bright yellow-green fluorescence when the system is scanned with UV light – my own company has nearly 20 years experience with its Glo-Leak and ultra violet technology. UV-fluorescence is not affected by other vapours or by air movement. You can also locate the source of a 100% leak, if the test fluid was previously installed.

Then there are various forms of electronic leak detection. Corona discharge detectors pull air through the electro-magnetic field around a wire carrying a current. The presence of refrigerant in the air changes the measured current in the wire and triggers an alarm. The snag is that the presence of any abnormal gas in the air triggers the alarm, so it is not specific to refrigerant and can waste time in industrial atmospheres.

The ceramic diode in heated diode detectors triggers an alarm when halogenated gases are encountered, which is fine with R134a but cannot be used with hydrocarbon refrigerants, because of the explosion risk. However, the diode needs quite frequent replacement, which is a nuisance and expensive.

Infrared detectors rely on the fact that refrigerants absorb infrared radiation, and sound an alarm when this happens. Very accurate and not easily fooled by contamination, hand-held infrared leak detectors are comparatively new (although the technology has been used in industrial gas analysis for decades), and quite expensive.

And then there is ultrasonic detection, again not a new technology, but one which has recently been refined and improved to a degree which makes it about the most cost-effective means available of tracking down leaks, even micro-leaks, of refrigerants or other gases.

Ultrasonic detection principles

As a leak passes from a high pressure system to the lower pressure of the atmosphere, it creates turbulence, and the turbulence generates ultrasonic sound at frequencies far above those audible by the human ear. The frequency range of normal human hearing – which differs from person to person, and is reduced with age – is from about 100 Hz up to between 15 KHz and 17 KHz, or up to as much as 20 KHz in some cases. The ultrasonic frequencies of gas leakage is normally stated as being between 39 KHz to 41 KHz, although some authorities give a much wider range. The key point is that you can’t hear it, and therefore cannot detect refrigerant leaks by listening for them – unless, of course, you have an ultrasonic leak detection kit.

Until very recently, ultrasonic leak detectors used to bleep when they found an ultrasonic sound source, which might or might not have been a leak. So ultrasonic sound emitted by, for example, a bearing beginning to fail would produce the same bleeping alarm as escaping gas. Since most mechanical systems emit ultrasonic sound, and since an air-conditioning system is mechanical, such detectors could waste a lot of time when leaks were being sought. Portable ultrasonic detectors were also expensive.

The next generation of ultrasonic detection systems electronically reduced the frequency of the detected sound, so that the audible sound that the user would hear in a headset would be different if it came from a failing bearing or from leaking gas. While better, that is not the complete answer, because the sounds you heard would not be identifiably those of a bearing or a leak – they were just different to each other. What you really need when looking for a gas leak is a noise that sounds like a gas leak. Simply converting the ultrasound frequencies to audible frequencies is not the whole answer.

The latest ultrasonic detection systems therefore use heterodyning, a technology dating from the beginnings of radio and amplification, whereby two alternating currents that differ in frequency are mixed in a nonlinear device, such as a transistor, to produce two new frequencies, corresponding to the sum and the difference of the input frequencies. In the days of the thermionic valve, this was the fundamental technical discovery that made mass broadcast radio with clear speech and music reproduction possible. The difference frequency resulting from heterodyning is the key to producing realistic audible sounds from ultrasonic sound. Because the detector unit is tuned to listen to the ultrasonic range, it is unaffected by everyday sounds such as wind, voices, and traffic, all of which can in normal circumstances reduce the technician's ability to listen for a potential problem, so what you hear is just a faithful reproduction at audible frequency of the ultrasonic sound that the detector locates.

The USX system

The latest, and one of the most sophisticated, ultrasonic leak detectors of this type is the Primalec USX1, which is surprisingly affordable at under 300 Euros complete with specially selected headphones of very high quality. With the USX kit, small leakages of gases (or air) from pressurised systems, or of air into a system that should hold a vacuum, become clearly audible. Move the probe around and you soon find where the leak is happening.

You also get the bonus that the USX kit is just as capable of locating early signs of mechanical wear or lubrication failure in bearings, gearboxes or other rotating mechanical equipment, and electrical discharge caused by failing insulation. In air-conditioning compressors, or in engines and gearboxes, bearing failures or wear on shafts emit ultrasonic sound as evidence of a fault developing long before the problem gets bad enough to emit audible sound. The Primalec USX is sensitive enough to convert this inaudible ultrasonic sound to audible sound. By listening to the earphones as you position the probe at vulnerable points you will quickly isolate exactly where there is a problem, in plenty of time to do something about it without a major breakdown.

Insulation failure in electrical circuitry often causes arcing or electrical discharge, particularly in humid atmospheres. Sometimes such discharges are bad enough to see, if the site of the discharge is in bad light or darkness. But in most cases the discharge is small, the light is bright, and there is no audible sound from it. However, there is always ultrasonic sound emitted by such discharges, and the USX enables you to hear and trace the problem or problems. Insulation failures can cause fire or damage, so having the right ultrasonic sound detection available can save a fortune – even lives. The new USX ultrasonic technology is available now from Primalec, - Email to This email address is being protected from spam bots, you need Javascript enabled to view it or phone +44 01474 854417 for more information.