Code of Federal Regulations Title 46. Shipping § 46.197.322 Surface-supplied helmets and masks

(1) A nonreturn valve at the attachment point between helmet or mask and umbilical that closes readily and positively;

(2) An exhaust valve;  and

(3) A two-way voice communication system between the diver and the dive location or bell.

(1) Ventilate at least 4.5 ACFM at any depth at which it is operated;  or

(2) Be able to maintain 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.