The Biomarine Mk15 Rebreather

The Mk15 rebreather has been set up specifically for diving in deep, cold water. It has been fitted with two external cylinders to complement the two onboard gas spheres, and includes a gas changeover switch to swap between Trimix and Nitrox operation on the fly. The left hand cylinder is filled with air and is used for the diluent in Nitrox mode, as well as inflation of the primary wings. The right hand cylinder is filled with Argon and supplies the drysuit and the secondary wings.

Here is the arrangement of the offboard diluent supply from the left hand cylinder. The Poseiden 1st stage connects, via a short hose, to the Willcox Orifice. This port includes a stainless steel bulkhead connector fitted with a brass swivel block and 1/4 inch BSP thread. The internal side of the bulkhead is hard plumbed to the second input of the Whitey gas switch which can be seen on the side of the unit just above the diluent manual add valve. This area looks congested, but operation of each control is quite easy.

Using the two aluminium 2.7 litre cylinders, the unit is still quite light at around 45Kg gassed up and ready to go, but the balance is thrown out a bit by the weight of the cylinder valves and 1st stage regulators being so low down. This tends to trim the diver to an upright position rather than the preferred horizontal position. To rectify this, the unit incorporates an extra weight in the top to bring the trim back to normal. This was accomplished by using both the standard Mk15 internal weight as well as one from a Mk15.5