By Brian Shreve – TDI Instructor Trainer
Running out of breathing gas
The only true emergency in diving is running out of breathing gas underwater. Fortunately, it is one that can be addressed through proper planning. In sport diving, a casual approach to gas planning is often taken. We frequently hear advice such as “Make sure to get back on the boat with 500psi, or 50 bar” or “Turn around when you get to half your starting pressure.” With a multilevel guided dive, these approaches usually suffice for that type of diving.
In technical diving, a more rigorous approach to gas planning is needed. Due to overhead environments such as mines, caves or wrecks, or due to the soft overhead of mandatory decompression stops, the casual approach used in sport diving simply won’t work. Instead, technical divers plan out their dive including the anticipated gas use for each team member before even entering the water. To do this, each team member needs to know their gas consumption at the surface. This is frequently referred to as Surface Air Consumption Rate, or SAC rate for short. That can be used in combination with the planned depth and time to arrive at the anticipated gas use for the dive for each diver.
Planning for failure
Sport diving allows a diver to return to the surface in a low-on-gas situation. The same cannot be said for technical diving. As a result, technical divers plan for reasonably foreseeable failures to make sure that each diver has a contingency plan, and this includes planning gas supplies for the dive.
Common failures can include:
- Gas loss due to equipment failure
- Loss of staged gasses
- Inability to access gas in a cylinder due to regulator or valve malfunction/damage
- Failure to adequately monitor gas supply
Any one of the above issues must be addressed by the diver and/or the dive team while underwater. When planning for these types of issues, the team must determine the minimum gas needed for the dive if something goes wrong.
Minimum Gas Planning
There are two common approaches to determine minimum gas supplies needed for a given dive. Each has its proper application but can also result in insufficient gas when applied incorrectly. Let’s take a look at each:
Rule of Thirds
- Simple approach, used most often in cave diving
- 1/3 of starting gas supply (pressure) used for ingress
- Should leave 1/3 for egress and 1/3 for reserve
- Requires gas matching if dissimilar tanks are used in the team
- Works best for teams of three and spring caves
- Conservative approach for open water technical diving
- Not conservative enough for teams of two, and for low/no flow or siphon caves
- Can be used in sport diving, sometimes applied in open water technical diving with a team of two
- More math involved than Rule of Thirds
- Team must decide on a minimum reserve pressure to retain – usually at least 500psi or 50 bar to account for errors in SPGs at the low end of the scale
- Team calculates Rock Bottom – the gas required for two divers to ascend from the maximum depth of the dive to the first gas switch (or to the surface for sport dives) from a single gas supply
- If calculated Rock Bottom is lower than the agreed-upon minimum reserve pressure, then the minimum reserve pressure is used.
- Rock Bottom changes with depth and time, so must be calculated for each dive
- If divers are using dissimilar tanks, must be calculated for each diver individually
- May results in insufficient reserve gas supply if a direct ascent to a breathable gas or the surface is not possible
There’s an App for That
Not too long ago, all the gas planning calculations had to be done by hand. Fortunately, today we have easy access to dive planning applications for laptops, tablets and smartphones that make gas planning much simpler. You simply input data and the app takes care of the rest.
Data needed includes:
- SAC rate (sometimes referred to as Respiratory Minute Volume or RMV) in either cubic feet per minute or liters per minute at the surface.
- Depth and time of planned dive
- Gasses available for use on the dive including decompression gasses
Many apps are able to calculate multi-level profiles as well as square profiles and may also offer lost gas and contingency planning for next deeper depth or longer time as preset options. By using the app, each diver can determine the gas supply needed for the dive, and then add in the contingency gas. Most importantly, they can answer the question “Do we have enough?”