Cressi-Sub ARO Rebreather

Diver Dave's Cressi-Sub ARO-57 Teardown

The Cressi ARO is one of the prototypical 02 rebreathers, little changed from pre WW-II designs. In fact, it's essentially identical to the rebreathers used in the second world war. From a practical perspective, the ARO is functionally interchangeable with the more modern LAR-V, and in fact the mission of divers wearing the LAR-V is little different from the missions accomplished by wartime Italian Navy Divers who rode wet submarines to perform sabotage on British Warships. This particular sample was procured for me by Fabio Bartolucci, who is shown on the main page of this website posing in his ARO in the Mediterranean.

Here she is, sitting on the bench. The ARO is a pendulum rebreather, meaning that the scrubber is positioned between the breathing hose and the counterlung. There is no attempt made to control flow direction, and every breath passes through the scrubber during the exhalation, and back through the scrubber on the inhalation. While there is a small amount of unscrubbed gas inspired on every cycle, the majority of the gas passes through the scrubber. This is a simple and effective scheme.

This example was provided with the well known Cressi-Sub full face mask, which has been used for many years by open circuit divers as well as rebreather divers.

Here's the business end of the Cressi mask. The mouthpiece is passed through the flange on the mask, and is then gripped by the divers teeth in the usual manner.

The DSV is a work of art. Brass, with a nickle finish, it's almost art deco in form. The slot shown is the water-purge slot, so that the diver can clear the water from the DSV prior to opening it to the loop.

Buckles and straps are a simple neck-loop of nylon webbing, and a waist strap, also of nylon. Hardware is of good quality, and is marked with the "Cressi Sub" logo.

There is no automatic addition of 02, the counterlung being kept filled by periodic manual additions controlled by the diver. With excess counterlung volume available, the volume of 02 carried in the counterlung can be used for buoyancy adjustment. Addition of 02 is normally done when the diver starts becoming negatively buoyant, not when the counterlung is collapsed and not able to provide a full breath.

The addition valve is finely machined from brass stock, and then nickle plated. It functions as a pressure reducer and flow control valve. Inlet and outlet connections are visible here. There is no intermediate pressure reduction, the valve controls full cylinder pressure and allows it to be fed at a controlled rate into the counterlung.

Detail of the manual control valve: To add 02, the diver pushes the lever shown under my thumb.

The right side of the rig shows the 02 addition line as well as the simple straps that hold the cylinder in place on the bottom of the rig.

The cylinder is 2 liters, and is filled to 200 BAR. The on/off knob is substantial, with a rubber periphery for easy gripping with cold hands, and is marked with the Cressi-Sub logo.

The inlet filler fitting is, surprisingly enough, the same as that found on my Russian IDA series of 02 bottles. Obviously this was (is?) a standard metric 02 fitting of some sort.

The counterlung is shown with the hose plenum and hose assembly removed. Position of the scrubber in the interior of the counterlung is easily seen.

The scrubber is filled through the filler port located at the top of the rig.

The bottom of the plenum chamber that connects the hose assembly to the scrubber is furnished with a "Snot Locker" to catch all of the little dribbles that we humans (unfortunately) pass towards the exhale side of our rebreathers. This trap is (obviously) drained after each dive.

Detail of the hose plenum, with breathing hose on the right and "snot locker" on the left. The T-nut is tightened to attach the assembly to the scrubber.

The scrubber assembly is removed from the counterlung by removing the filler cap, unscrewing the retaining nut and washer, and then removing the scrubber (carefully) through the main counterlung opening. There is no need for removal of the scrubber for filling, and removal would have only been done for detail cleaning and drying of the counterlung itself.

The scrubber is a well made cannister, formed from brazed sheet brass and nickle plated for corrosion resistance. Behind the large perforations is a fine screen to contain the scrubber media. This scrubber resides in the interior of the counterlung.

This is the entire functional portion of the loop of the rebreather. It has been removed from the counterlung for study. The simple design is evident, the DSV, single hose, and scrubber can being clearly visible.

Front of the molded counterlung, showing the Cressi-Sub logo.

Model designation is on the counterlung, "Autorespiratore Ad Ossigeno 57B", better known as the ARO-57.

The counterlung is a finely molded rubber bellows, vulcanized in one piece. This is the body of the rebreather, to which all other components are mounted. This one is in remarkably good condition, showing only the barest signs of age. I'll store it carefully a plastic bag and will dust it with talcum powder to preserve the rubber from ozone deterioration.

That's about all there is to this little rig. Comparing it to the more modern LAR-V shows that this rig will do essentially the same job, with a simpler design. The major difference is, of course, the method of operation. The main thing for a diver to remember when using a pendulum rebreather is to take full breaths, and not to maintain a shallow breathing cycle. If this is accomplished, the efficiency of the scrubber in this type of system is quite good, the scrubber media granules being presented dirty gas on both sides of the grains. In addition, the dwell-time of the gas in the scrubber is increased as the main portion of inspired gas is that gas that has been sitting in contact with the scrubber media for a few seconds. All in all this is an interpreting engineering solution that is worthy of additional study.

Return to Rebreather Teardown Main Page

		Here she is, sitting on the bench. The ARO is a pendulum rebreather, meaning that the scrubber is positioned between the breathing hose and the counterlung. There is no attempt made to control flow direction, and every breath passes through the scrubber during the exhalation, and back through the scrubber on the inhalation. While there is a small amount of unscrubbed gas inspired on every cycle, the majority of the gas passes through the scrubber. This is a simple and effective scheme.
		This example was provided with the well known Cressi-Sub full face mask, which has been used for many years by open circuit divers as well as rebreather divers.
		Here's the business end of the Cressi mask. The mouthpiece is passed through the flange on the mask, and is then gripped by the divers teeth in the usual manner.
		The DSV is a work of art. Brass, with a nickle finish, it's almost art deco in form. The slot shown is the water-purge slot, so that the diver can clear the water from the DSV prior to opening it to the loop.
		Buckles and straps are a simple neck-loop of nylon webbing, and a waist strap, also of nylon. Hardware is of good quality, and is marked with the "Cressi Sub" logo.
		There is no automatic addition of 02, the counterlung being kept filled by periodic manual additions controlled by the diver. With excess counterlung volume available, the volume of 02 carried in the counterlung can be used for buoyancy adjustment. Addition of 02 is normally done when the diver starts becoming negatively buoyant, not when the counterlung is collapsed and not able to provide a full breath.
		The addition valve is finely machined from brass stock, and then nickle plated. It functions as a pressure reducer and flow control valve. Inlet and outlet connections are visible here. There is no intermediate pressure reduction, the valve controls full cylinder pressure and allows it to be fed at a controlled rate into the counterlung.
		Detail of the manual control valve: To add 02, the diver pushes the lever shown under my thumb.
		The right side of the rig shows the 02 addition line as well as the simple straps that hold the cylinder in place on the bottom of the rig.
		The cylinder is 2 liters, and is filled to 200 BAR. The on/off knob is substantial, with a rubber periphery for easy gripping with cold hands, and is marked with the Cressi-Sub logo.
		The inlet filler fitting is, surprisingly enough, the same as that found on my Russian IDA series of 02 bottles. Obviously this was (is?) a standard metric 02 fitting of some sort.
		The counterlung is shown with the hose plenum and hose assembly removed. Position of the scrubber in the interior of the counterlung is easily seen.
		The scrubber is filled through the filler port located at the top of the rig.
		The bottom of the plenum chamber that connects the hose assembly to the scrubber is furnished with a "Snot Locker" to catch all of the little dribbles that we humans (unfortunately) pass towards the exhale side of our rebreathers. This trap is (obviously) drained after each dive.
		Detail of the hose plenum, with breathing hose on the right and "snot locker" on the left. The T-nut is tightened to attach the assembly to the scrubber.
		The scrubber assembly is removed from the counterlung by removing the filler cap, unscrewing the retaining nut and washer, and then removing the scrubber (carefully) through the main counterlung opening. There is no need for removal of the scrubber for filling, and removal would have only been done for detail cleaning and drying of the counterlung itself.
		The scrubber is a well made cannister, formed from brazed sheet brass and nickle plated for corrosion resistance. Behind the large perforations is a fine screen to contain the scrubber media. This scrubber resides in the interior of the counterlung.
		This is the entire functional portion of the loop of the rebreather. It has been removed from the counterlung for study. The simple design is evident, the DSV, single hose, and scrubber can being clearly visible.
		Front of the molded counterlung, showing the Cressi-Sub logo.
		Model designation is on the counterlung, "Autorespiratore Ad Ossigeno 57B", better known as the ARO-57.
		The counterlung is a finely molded rubber bellows, vulcanized in one piece. This is the body of the rebreather, to which all other components are mounted. This one is in remarkably good condition, showing only the barest signs of age. I'll store it carefully a plastic bag and will dust it with talcum powder to preserve the rubber from ozone deterioration.