SEFA

The SEFA is a make of backpack industrial breathing set formerly made by Sabre Safety. It is an oxygen rebreather. "SEFA" is an acronym for "Selected Elevated Flow Apparatus".

It is in a shiny backpack stainless steel sheet casing with rounded corners and edges, 16 inches wide, 21½ inches high, 6½ inches front to back. It has two wide corrugated breathing tubes leading to a fullface mask which has an inner orinasal mask and a front panel for talking through. Its breathing tubes are 22 inches long, and face forward as they come off the backpack casing.

Its full duration on a filling is 2 hours. It does not have a demand valve or electronic parts, and in theory this would be fewer parts to suffer from failures. It is not designed for scuba diving, but can be used for short shallow submersion such as going through short flooded sections of underground passages. Its casing, to keep grit and stones out of its working, is completely sealed, except for a large vent panel covered with metal mesh, and holes for the oxygen cylinder's on/off valve and the cylinder pressure gauge. Its oxygen flow can be set to 5 or 10 liters/minute. Its intended absorbent is a special make called SefaSorb, which is mostly calcium hydroxide.

As usual, the absorbent makes the breathing gas in circuit hot as it absorbs carbon dioxide. This would be welcome while scuba diving in cold water, but in warm air in a deep mine would be unwelcome. To try to get rid of that heat, the breathing gas is passed through a damp chamber, so the damp will absorb some heat as it evaporates, as in some air conditioning units. It is not very effective, in normal use the breathing air temperature of the SEFA will rapidly exceed 45 °C, and more under heavy work. Being made almost entirely from metal, the SEFA does dissipate its thermal load relatively easily.

...
Wikipedia