Recent Advances in Soap Bubble Leak Detection

February 2, 2001
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All systems leak.

A well-soldered system may operate for an extended time — perhaps even 20 years — without needing a charge. But, because no system is perfect, the equipment still has leaks. Those leaks may not be enough to affect performance or even read on a gauge. They can be as small as 1 oz per 20 years or as large as 1 oz per hour.

Every joint, seam, or weld has a “fault” of some kind. Some may be too small to detect even with the best leak-detection instruments. Given enough time under the stress of vibration, temperature, and pressure, the faults grow into detectable leaks.

Some leaks come and go as they plug up and reopen under particular conditions. The escaping gas may not even be leaking at the time of the test.

A leak could be the result of several different faults — solder impurities, a speck of dirt on a gasket or gasket surface, a microscopic scratch, weld fracture, etc. A look under an electron microscope would readily reveal more information.

In measuring the performance of leak-finding devices, their sensitivity — or their ability to measure different levels of leaks — comes into play. Soap bubble testing, for example, would be rated as low sensitivity because only large (above 6 oz per year) leaks in a favorable location can be detected.

The emergence of electronic sensing devices has proven that microscopic leaks not only exist, but in fact can be measured down to 1/10 of what soap bubbles can measure.

The low sensitivity of soap bubbles can lead to problems for the technician. When using soap bubbles to confirm and pin down a small leak site detected by electronics, the leak may be so small that no bubbles form.

So, did the electronic device give a false reading? A great many of the bubble solutions fail the sensitivity test because of their composition — shampoo, bubble bath, or dish washing soap with a variety of additives. However, the recent emergence of several new bubble leak detectors has significantly lowered the detection threshold to the point where they can be used to verify electronically detectable leaks.

Bubble leak detectors are now available in a non-hazardous, non-toxic, non-residue formulae containing no ethylene glycol. They can be used on equipment containing most gases — air, natural gas, all refrigerants, ammonia, S02, etc. They come in different temperature ranges and some are oxygen compatible.

Best of all, leaks down to 1 micron can be easily detected, as some have fluorescent qualities that make the bubbles sparkle under light.

An understanding of the different types of leaks will help technicians locate them.


All of us will have experienced searching for the following leaks at some point in our careers.

  • Static leaks:
  • The equipment will exhibit a leak although off and equalized (i.e., freezer evaporator coils warmed up by defrost). These are probably the most common of all leaks.

  • Pressure leaks:
  • These only show up as pressure builds. Most technicians use pressurized dry air, nitrogen, or helium at 150 psi on the low side and 450 psi on the high side. CO2 and oxygen are not recommended. Pressure should be used when static testing does not work.

  • Expansion leaks:
  • These are associated primarily with the higher heat generated from expansion. High ambient air temperatures, defrost cycle, condenser blockage, etc., are examples.

  • Vibration leaks: These occur primarily during system operation. Unit rotation, motion, valve actuation, refrigerant flow, etc., are associated with vibration leaks.
  • Combination leaks: These require two or more conditions to produce leaks. Vibration along with temperature and pressure on a semi-hermetic compressor discharge manifold could cause enough expansion for seepage to occur.
  • Cumulative leaks: An accumulation of small individual leaks are too small to detect with standard tools. The more fittings, welds, seams, gasket flanges, etc., the greater the potential for cumulative leaks.
  • A quick review of the most commonly used tools would at least include the following:

  • An electronic instrument with high-sensitivity capabilities;
  • An inspection mirror for blind sides;
  • A light source; and
  • A good bubble leak detector.
  • The leaking component is located by screening the system with the electronic gear. The bubble solution pinpoints the leak’s location.

    Publication date: 02/05/2001

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