The Russian IDA71 military and naval rebreather is an oxygen rebreather intended for use by naval and military divers. As supplied it is in a plain backpack harness with no buoyancy aid. Its casing is metal, not plastic. It has a small optional diluent cylinder which can be clipped on its outside to convert it to nitrox mode. It contains one oxygen cylinder and two absorbent canisters. In the bottom of its casing is an empty space which is intended for an underwater communications set.
Here, "up", "back", etc refer to a man wearing the set standing on land.
It gets thinner towards the lower end, to avoid the frogman being slowed by hydrodynamic drag as he swims. (One rebreather maker tested a rebreather with a big plain rectangular box backpack and found that its big flat stern end cut his swimming speed by half a knot.)
On the front of the harness is a projecting metal plate. This is said to be to clip a limpet mine to. The front of the harness is a thick tough rubber "apron" which would protect much of the frogman's belly from blows from the front.
The loop of each breathing tube can be strapped down to the shoulder to keep it under control to stop it from catching on things or being easily grabbed from behind.
On each side of its casing is a small clip to fasten a parachute to.
It can run as an ordinary diving rebreather. Or it can be run with one of its two absorbent canisters filled with potassium superoxide, which gives off oxygen as it absorbs carbon dioxide: 4KO2 + 2CO2 = 2K2CO3 + 3O2; in this mode the oxygen cylinder is a bailout.[1] This mode gives the set more duration underwater, but is dangerous and not to be risked by civilians because of the explosively hot reaction that happens if water gets on the potassium superoxide; whereas ordinary modern diver's rebreather absorbents have been designed to avoid producing a caustic solution (commonly called "cocktail") if they get wet. Tests at the United States Navy Experimental Diving Unit in Panama City, Florida showed that the IDA71 could give significantly longer dive time with superoxide in one of the canisters than without.
The article about rebreathers in the Russian Wikipedia says that this second canister was filled with manganese peroxide. If this means manganese dioxide, then absorbing carbon dioxide would not make it release as much oxygen as the diver consumed to make the carbon dioxide.
For many years the IDA71 and similar have been a standard Russian frogman's and naval work diver's breathing set. The "71" in its name may be the year that it was designed, like with the numbers in the names of the AK series of Russian rifles. The name IDA comes from Russian: Изолирующий дыхательный аппарат (translit. izoliruyushchiy dykhatel'nyy apparat, literally Insulating/Isolating Breathing Apparatus). Other name is Individual Breathing Apparatus (Russian: Индивидуальний дыхательный аппарат, translit. Individualniy Dykhatelniy Apparat).
In its original Russian mode as an oxygen rebreather, its dive duration is said to be 4 hours. Filling both canisters with soda lime and putting a second oxygen cylinder in the empty space at the bottom, might increase its dive duration to 8 hours.
Quantities of IDA71's have found their way out of the ex-USSR to Europe and America, where sport divers often put them into wings-type BCD's and convert them into KISS or automatic mode.
