Minimal Loop Volume
I recently received an email from a diver who was having a bit of a struggle with buoyancy on his new rebreather. An experienced rebreather diver, he was diving a back mounted counterlung for the first time and wondered whether this was a part of his frustration. My advice to him included two things. The first step was to work on achieving comfortable minimum loop volume, which entails exhaling additional volume until your deepest inhalation just begins to trickle the ADV. At this volume, any molecules of oxygen that are used up are replaced by the solenoid valve (or manually in some rebreathers).
When your loop volume is too high, a small rise in buoyancy causes the OPV to vent and dump gas. When that happens, the solenoid also fires and then the diver tends to drop. Adjusting buoyancy, the diver adds diluent and a vicious cycle of additions and venting occurs causing sloppy buoyancy and gas overuse. With the minimal volume in the loop, a small rise will simply cause a gas expansion without venting. Any PO2 changes can be managed without significant gas exchange and loss.
The second piece of advice I gave the diver was to assure that their OPV was not too sensitive. Many OPVs are adjustable and if yours vents too easily, you will lose gas easily and have to rebuild the PO2 and volume again. Tightening the OPV slightly may help. Some OPVs are vented through tubes behind the diver. These tubes should be balanced and positioned as close to the diver’s centroid (center of the lungs) as possible. If it is affixed to the back of the rebreather too high above the diver’s back, it may vent too much. Try repositioning it closer to the back.
