Siebel Gorman Proto Breath Apparatus used by Birmingham Fire Brigade until it was superseded by Compressed Air Sets in the 70s. Picture taken at Aston Fire Station on the right my ex comrades Fm Clive James and Fm Mick Rooney. The duration of Proto could be extended by closing and opening the oxygen cylinder I believe the entrapped procedure was extended to 24 hrs by a wearer lying down and cracking open the oxygen now and again with the Potosorb absorbing the exhaled Carbon Dioxide.