The O2ptima CM uses a special loop overpressure valve (OPV) located on the exhalation (right) counterlung. A specific spring is used to optimize the cracking pressure for rebreather counterlung use. Do not replace with a standard drysuit exhaust valve as they can have a much higher cracking pressure, which can lead to lung overexpansion injuries.
This valve will normally be operated in the open position (turned fully counter-clockwise). Divers with a large tidal volume may need to close the OPV slightly in order to maintain proper loop volume. The valve can also be manually opened by pressing on the valve. It is screwed into a welded flange on the counterlung inner bladder.
During ascent, the gas in the loop will expand, increasing buoyancy and slightly increasing the breathing effort. Even though the maximum volume in the O2ptima CM’s breathing loop is relatively small, it is important to set the OPV properly so that buoyancy shifts will be kept to a minimum without any diver action.
The OPV is also used for de-watering the loop. This procedure will be covered in your O2ptima CM class.