Ultrasonic Leak Detection

-By Nick Allo and Paul Tashian

The past several years have brought great change to the field of Leak detection.  Thousands of contractors now utilize ultrasonic leak detectors.  But when & how can ultrasonics be used most effectively?

There's no denying it, refrigerant leaks are a headache!  It's just a fact of life.  We have all spent long frustrating hours searching for leaks.  Now pain relief is available, thanks to new technologies that can find leaks more quickly.  Modern technologies, however advanced they  become, will always require skill and intelligence on the part of the technician using them.  A specific leak detection technology can be very effective in certain situations, but useless under other conditions.  It is important that the technician understand the capabilities as well as the limitations of the leak detection options available.  This is the secret to headache relief.

There are many types of leak detectors currently being used in a variety of industries.  These range in price from over a hundred thousand dollars for a mass spectrometer, to just pennies for a few drops of soap solution.  The most popular methods being used in the field of HVAC&R are soap solutions, refrigerant sensing instruments (electronic "sniffers"), fluorescent dyes/lamps, and ultrasonic leak detectors.  All are very effective under the right circumstances.

Manufactures of leak detectors want to sell you their products, and they all make exciting claims that their products are the best, and only method that should be used for finding refrigerant leaks.  While certainly an electronic "sniffer", for example, may offer features that make it better than the competitors brand, it will still have a difficult time locating leaks under certain conditions (I.E.- windy rooftop).  It's just the nature of this technology.  All of the technologies available are going to have situations where they are most effective, and other times when they are not so effective.  When faced with these problems,  the educated technician will reach for a device which operates using a different technology.  One that will be less hindered by the problem effecting the unit which in the past may have been the first choice of detectors.

Ultrasonic Leak Detectors have generated a lot of interest, and have become quite popular in the HVAC/R industry over the past few years.  Offering a unique solution to many of the leak testing problems facing the contractors and service technicians of the 90's.   The main reason for their increasing popularity is their versatility in leak detection, and other applications as well.  Because Ultrasonic Leak Detectors detect the sound of a leak, they will detect any type of gas, even a vacuum leak, and will also be helpful for testing steam traps, ductwork, bearings, and refrigerator/freezer door gaskets.

Ultrasonic Leak Detectors listen for leaks, they do not detect the presence of a gas.  For example, you are probably familiar with the sound air makes as it leaks from a tire.  The sound that your ear can detect in this situation, is only about a third of the actual spectrum of sound that exists.  With very small leaks, most of the sound that is emitted is in the ultrasonic range. This is well above the detection limits of the human ear, but with an ultrasonic leak detector it can be detected and traced to its source.  Vacuum leaks emit ultrasound, and new refrigerants emit sound as easily as the old.  Even nitrogen emits sound when it leaks.  This principle of operation makes it possible to detect leaks in windy, or contaminated gas saturated areas.  Even under vacuum.  The ultrasonic detector eliminates most of the problems associated with other techniques.

The ultrasonic sound of a leak, when detected by the instrument, is translated to a lower frequency range where the user can hear and interpret the sounds through a headset.  The sound of a leak deep within a coil, and far out of sight, can be detected with an ultrasonic instrument, although the closer you place the sensor to the source of the sound, the better chance you will have of detecting it.  This can be compared to going to a concert.  The closer you are to the band or the speakers, the better you will hear.

What about sensitivity? Because ultrasonic leak detectors detect sound and not refrigerant, it is impossible to state their sensitivity in terms of ounces per year.  The detection capabilities of ultrasonic detectors are determined by the amount of turbulent gas flow generated by a particular leak.  For example, a piece of tubing left to freely exhaust into the atmosphere will not generate ultrasound if the volume of gas through it is not enough for turbulence to take place.  Yet the same pressure behind an orifice (leak) size of only .004 of an inch can be heard from many feet away!  Think about what happens when you whistle.  Changing the shape of your lips and tongue, and blowing differently will create different sounds.  Now compare this to a refrigerant leak.  Not much different.  A wide variety of sounds, and sound frequencies can be created by different shaped cracks and holes.  Different pressures will generate a wide variety of sounds and sound frequencies.  In fact, a large leak (.10 in) and a smaller leak (.004 in), side by side, under the same pressure, will create two entirely different  tones.

Although ultrasonic detectors can detect pinhole leaks of less than 3 psi,  a minimum leak rate of .5 oz/yr. can not be traced with ultrasonics.  At a flow rate that small no turbulence exists, however, it would be unlikely you would even be called to service the unit, as it would take approximately 32 years to loose 1 pound of refrigerant!  Also remember, that although many “sniffer” type detectors can detect this leak rate, they were tested to do so in a controlled laboratory environment.  Not in the windy or possibly contaminated area you will be working in.  Under conditions like this, it is common to find a leak faster with an ultrasonic detector, than with a sniffer.  Don't misunderstand, traditional methods are certainly very effective in many applications and should not be replaced, but the Ultrasonic method is much more effective in situations where these other methods are not.

Background noise used to be more of a problem with ultrasonic detectors than it is today.  Early generations of detectors indicated a leak by imitating the sound in the form of a beep or squeal, making it difficult to determine what was being detected.  A good ultrasonic detector will be capable of producing a true reproduction of the leak itself.  The rushing sound of the gas should be clearly identifiable when compared to normal operational sounds.  Do not settle for one that "beeps" or "squeals".  This will make it difficult to discriminate the actual leak sound from any other background noise.

Before using an ultrasonic detector for the first time, be sure you know what to listen for.  Create a "legal" leak using an air gun, or even a room deodorizer, and listen to the sound.  Then listen to a compressor or fan motor at the same time.  With a good ultrasonic detector, you will observe a noticeable difference in the sounds, and will be able to distinguish the unique sound characteristics of each.  The sound of a vacuum leak has a very close resemblance to an external leak.  Gas or fluid flow through restricted areas in pipes may produce a rushing sound similar to a leak, but the most intense signal will be at the point of an orifice or opening.   Most ultrasonic detectors use a sensitivity adjustment which will diminish the less intense sounds, and let you concentrate on the dominant ones.

In closing, when one is faced with a leak detection problem, they need to take a minute and evaluate the system, and the environmental factors that may effect the leak detection process.  Is it windy?  Are there contaminants that could cause false alarms?  How much time is available?  Is the leak within direct line of sight, or is a method needed that can detect the leak from a greater distance.  These are all questions the service technician must consider before choosing a leak detection technique.

There is nothing wrong with less sophisticated methods as long as they work, but one needs to know and understand the capabilities as well as the limitations of the method used.