No-Flow Safety Factor / Margin

[LFMarm]

Many thanks @inquis for asking the question. I realized that I was using very different safety factors depending on flow.

I built a simple spreadsheet to calculate the safety factor for a given turnaround pressure (column C) or calculate the turnaround pressure for a desired safety factor (column D). Input cells are in yellow, output in orange. You can use either metric or imperial units as long as you stay consistent throughout.

 

[rddvet]

I teach my students that thirds is reserved for 3 man teams so in the event of an OOG you can switch donors midway through the exit. For a 2 man team, penetration gas is based on thirds minus 200 psi (or thirds minus 15 bar). That produces a large safety factor.

Starting PSI: 3600
Penetration gas (thirds-200): 1000
Gas at turn: 2600
Exiting gas (thirds-200): 1000
Gas at exit ("emergency gas"): 1600

That's 600 psi more than you theoretically need to get a 2 person team out from max pen. It should provide enough extra gas to handle things like additional stress, etc. You can also go more conservative if you decide.

Thirds.

Two simultaneous major failures at max penetration isn’t really something I concern myself with too much.

Your risk tolerance might be different.

I use both of these choices, and both are good choices. It really depends on who I'm diving with, where I'm diving, conditions of the day, how I feel, and where we're at in the cave when I start getting into the are of what Ken is describing which I just refer to as "conservative thirds." Diving with my regular buddy in Peacock, I'm good with thirds. Diving with someone I don't know well, conservative thirds (even in a high flow cave).
More often than not its a call based on where we are in the cave. When I see I'm 200 before I hit thirds and that last 200psi isn't going to get me anywhere I really want to see or I'm just going to be staring at the same cave I've seen 200 times, I'm probably calling it at "conservative thirds" because that last little bit of gas isn't going to have a big impact on my enjoyment of the dive.
All the math hurts my head. Its pretty simple. 1/3s was made for a team of 3. So if you are feeling conservative, add 200-300. Feeling good and want to extend the dive (even in a team of 2), go to 1/3s. I think if we go back and look at past accidents, someone turning it at 1/3s wasn't what killed them. Its usually something else or they broke 1/3s.

 

[halocline]

For me it’s really dependent on specific conditions, team experience, familiarity, etc. Most of my dives these days are stage (1/2-200, 1/4s) but without a stage, I generally start the conversation with the assumption of thirds and add conservatism as I feel will be appropriate for the dive. I’m talking about in the predive planning, not adjusting on the fly.

When I think about the issues that are actually a credible threat in real life, an immediate and total loss of gas at maximum penetration is not one of those diving SM. Making a wrong navigation decision or getting hung up somewhere in the dive due to a problem, or losing a stage, those are credible threats and I tend to worry about those sorts of things more.

We all want to feel secure in our planning and enjoy the dive. Anxiety avoidance is important to me, especially with a new teammate. So if adding gas to a turn pressure achieves that, great. But I wouldn’t set a hard/fast rule about all dives.