If a U-boat is under water for a prolonged period of time the air will become foul and there will be a buildup of carbon dioxide. To breath this air a U-boat crewman had 2 different devises to use one was the KALIPATRONEN which is a big metal box with a carbon dioxide absorbing material inside that strapped to the mans chest and on top a rubber hose with a rubber face mask that attached to his head with 2 straps. This could only be used when breathing contaminated air and not for escape from a sinking boat,by mid war this was phased out and a central KALIPATRONEN system was installed on the bulheads of the boat.
The second and most popular device was the DRÄGER TAUCHRETTER which is a rebreather and escape device. This was a rubber/canvas life vest that contained a tin of a carbon dioxide absorber to breath air from inside the boat and an air flask to either breath from underwater when escaping a sinking boat or with a flip of a lever inflate the life vest.