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1910.430(h) Masks and Helmets
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A non-return valve at the attachment point between helmet or mask and hose which shall close readily and positively; and
Surface-supplied air masks and helmets shall have a minimum ventilation rate capability of 4.5 acfm at any depth at which they are operated or the capability of maintaining the diver's inspired carbon dioxide partial pressure below 0.02 ATA when the diver is producing carbon dioxide at the rate of 1.6 standard liters per minute.