During normal service operations a leak test does not entail a prior evacuation of the system unless the system is contaminated or the refrigerant has been completely discharged. It is essential however, that a minimum of 30 psig (pounds per square inch gauge) or 2 bar exists in the system when testing for leaks. If no leaks are found then the system should be tested again at operating pressures.
A pressure leak test is more satisfactory and will be dealt with later.
Methods of testing
The most common and inexpensive test is the bubble test. A water/soap solution is simply brushed around a joint or component which is suspected of leaking, or sprayed on with an aerosol. It is recommended that proprietary rather than made-up solutions are used, as they are more viscous and the bubbles are stronger and longer lasting. Ordinary soap bubbles are weaker and are normally short lived.
The disadvantage of this method is that a large leak can blow through the solution and then no bubbles will appear, although in most such cases the leak will be audible.
This consists of a small burner assembly mounted on top of a container of gas, for example propane. The burner comprises a hand valve and a venturi or mixing chamber with an attachment for the exploring tube. Above the orifice of the burner there is a copper ring, a strip or a tube through which the flame passes when the torch is ignited.
When the torch is lit the air will be drawn into the venturi via the exploring tube, and the flame will burn slightly blue or colourless. When a trace of halogen refrigerant (Rll, R12, R22, R500, R502 etc.) mixes with the air, the flame will immediately change colour as the refrigerant vapour contacts the hot copper ring or tube. The colour will range from green for a small leak to dark blue or purple for a large leak. When the refrigerant bums off, a toxic atmosphere will be created. To test for leaks the exploring tube is passed around the suspected area slowly, and for effectiveness the plant should be stopped.
Electronic leak detector
This is the most sensitive type of leak detector, and many designs are available. Some respond to an ion source, and others to a change of temperature (thermistor); the dielectric type is based on the conductivity of different gases. These instruments are dry cell battery operated. When used the sensor or sensing tip should be inspected for cleanliness; tips should always be kept free from dirt and lint. Filters should be changed regularly, because a contaminated filter will cause the instrument to respond as if a leak was detected.
Normally the instruments will respond to atmospheric air by giving out an audio signal (bleep) at approximately one bleep per second. When the halogenated refrigerant contacts the sensor the signal will accelerate, depending upon the degree of vapour leaking: a large leak could produce a continuous signal or oscillation.
One disadvantage is that because of the sensitivity of the instrument it will respond to minute volumes of refrigerant vapour, and sometimes it will prevent the actual pin-pointing of a leak. It is also responsive to expanded foam insulants, thereby making somewhat difficult the detection of leaks on pipework passing through coldroom walls and parts of domestic appliance systems.
When using a detector the amount of air movement must be reduced to a minimum, i.e. all fans should be switched off and draughts excluded. The sensing tip should be applied below a joint because refrigerant vapour is heavier than air, and then moved slowly around the area.
This is a chemical solution used to detect leaks on water cooled systems charged with ammonia, which has an affinity for water. The solution is added to the recirculated water. If ammonia is present in the water, the solution will react to the nitrates contained therein to change the colour of the water to brown.
This takes the form of a small candle or taper which, when ignited, will smoulder and give off sulphur fumes. It is also used to detect leaks on pipework and components in ammonia systems. When it is passed around a joint suspected of leaking, the ammonia fumes will mix with the sulphur fumes to produce a white vapour.
Both ammonia and sulphur fumes are toxic. Thus adequate ventilation must be provided, and precautions must be taken not to inhale the fumes. Ammonia leaks are easily detected by its pronounced odour at 3 to 5 parts per million. At approximately 15 ppm the vapour is very toxic, and at 30 ppm a suitable respirator will be required. Ammonia is lethal at 5000 ppm, and the maximum exposure time to atmospheres of 50 ppm is 5 minutes. Ammonia becomes flammable at 150000 to 270000 ppm.