Cited Passage

Second-stage freeze-up is generally caused by moisture in the exhaled breath, which then condenses and freezes on cold metal parts prior to entering the water. First-stage demand regulators are also susceptible to freeze-up for the same reasons when used at temperatures below 39°F (4C). Freeze-up depends on the ambient tem- perature, rate of gas flow, relative pressure drop, the diam- eter and shape of the various passageways inside the demand valve, and the moisture in the breathing gas. Another cold-water diving problem is that the diver’s mask is more likely to fog in cold water. A non-irritating defogging agent applied to the mask before diving, or par- tially flooding the mask and flushing the faceplate will relieve this condition temporarily. In practice, freeze-up problems normally start before entering the water when divers test their demand valves in a sub-zero environment. Ice crystals may form instantly inside the first stage valve and act as sites for further ice formation during the dive. Moisture in the diver’s exhaled breath may produce instant second stage freeze-up. Water is either above or at the point of freezing and, if breathing from the regulator is postponed until under water, the high heat conductance of water will normally keep the demand valve above freezing. NOTE When diving in freezing or near freezing tempera- tures, it is highly recommended to submerge before breathing from or purging the demand valve. Since rate of gas flow is crucial to determining whether the demand valve will freeze, a slow, normal

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