Deep vacuums have unique properties. Applying a deep vacuum to hvacr equipment requires leak-proof design not only of the manifold but of all components involved.
Charging and testing hoses are designed to handle pressure. When preparing connecting lines only soft copper tubing, pure rubber or flexible metal hose are absolutely vacuum-tight.
However, even with the advanced technology of today's hoses, permeation through the hose compound still occurs. When checking pressure rise, the atmosphere will permeate to the lower pressure in the hoses and the micron reading will slowly rise.
Another source of leakage is the coupler. The gasket seal in a charging hose requires frequent attention and replacement to keep its leak rate tolerable.
O-ring couplers have a small rubber right that forms around irregularities in the flare fitting. When the flare is screwed down a metal-to-metal seal is created and the o-ring forms around the flare to produce a positive seal.
Source: J/B Industries
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