When we plan how much bailout gas we need there are a number of different strategies, ‘bailout philosophies’, we can employ. It is also perfectly fine to combine strategies depending on the nature of the dive, how many divers there are on the team, how many support divers there are to help etc. Here are some of the options. The pros and cons of each and how to plan the set up will be looked at in a further article.
Individual bailout. This one is fairly obvious, each diver carries all of their own bailout gas. They each enough gas to get them through the ascent and decompression from the furthest and deepest point of the dive. On some dives this could mean carrying a lot of gas!
Staged bailout. Using this technique, some or most of the gas requirements are placed in the water in advance of the dive. This could be on a decompression stations or shot-line or placed in the cave etc. We can combine this with individual bailout plans or a team bailout philosophy.
Drop cylinders/support divers. By using a prearranged time or a pre-planned signal to the surface support divers can deliver gas to the dive team. For example on a deeper dive you can signal the surface cover at a pre-planned depth and support divers can descend and take away all the deep trimix bailout cylinders which are no longer needed and exchange them for shallower decompression gasses. This means you don’t have to carry absolutely everything throughout the entire dive regardless of how you have calculated your gas volumes. This system can also be used to cover emergencies. A separate emergency signal can be used to alert the support dives and have them commit to deliver gas in advance of any pre-planned depths or times if something has gone wrong and one of the team has had to bailout.
Team bailout. Using this philosophy, the gas requirements are split between the team in accordance with the guidelines below. It is very easy to incorporate team bailout into staging cylinders or the use of support divers.
First to put it in its simplest terms, calculate the gas needed to get one diver safely through the exit/ascent and decompression to the surface. Multiply this volume by 50% (the ‘enough gas for one and a half divers rule) and that is your gas volume requirement. This is the standard ‘1 ½ diver rule’
This is then divided amongst the team with the following provisos:-
Firstly, every diver in the team must have an OC bailout gas suitable for the deepest part of the dive. It needs to be a life sustaining volume of gas, not just the right mix.
Secondly, (and this is optional, not every dive team would adopt this part) no matter how many people you have in your team, you divide the gas as if there were only two team members. For example, in a team of three, each of the three potential pairings would have enough gas for one and a half divers between them.
The gas mixes can be decided upon based on likely OC bailout decompression, but whenever we can we base all our gas mix choices on the inert gas ratios in the rebreather to keep any transitions from one gas to another (if on OC bailout) as smooth as possible and avoid any inert gas spikes.
The subject of using inert gas ratios to pick ideal bailout gasses is on the list for future blog topics.