As we all know, one of the fundamental tenets of cave diving is to always have at least 2x the gas needed to exit. This obviously allows one to air-share in the event of one member's catastrophic loss of gas and is the basis for the Rule of Thirds: 1 in + 2 out = 3 total, assuming no flow. By design, your safety factor at the turn (call it S) is 2. (Or if you prefer to think of just the reserve, excluding what you need to exit, that would be S-1.) My understanding is that Thirds was actually devised assuming a team of 3, but for air-sharing in a team of 2, you'd just barely make it... in theory.
I think everyone would agree this is fairly aggressive without the luxury of flow, so what do you do in that case? A common theme is to enter slow & casual and beeline for the exit if SHTF, but is that your only margin over Thirds? Some will increase the turn pressure by X psi on top of the casual entry (e.g., Thirds+100 psi would turn at 2500 psi for a 3600 psi start). Diving 4ths is another easy-to-compute option. Diving in a team of 3 would help, but I'm not sure that's always possible. Do you leverage the fact that the safety factor / margin is actually variable (the minimum is at the turn pressure but grows with proximity to the exit)? I'm curious what safety factor or rule you typically use in no-flow caves like Peacock or many places in Mexico. To keep it simple, let's assume exiting from the entry point and ignore any adjustments for tight passages, new teammates, separation/solo, etc.
For those who are interested, the safety factor including flow can be computed relatively simply using the ratio of entry to exit speed (call it V). For example, a flow of 10 fpm (not a stretch, as that's only 2 inches per second) and a no-flow swim speed of 40 fpm, that ratio V = (40-10) / (40+10) or V = 0.6. Denoting the penetration rule by P (i.e., 3 when diving Thirds), the safety factor is:
For Thirds in that modest flow (P=3 & V=0.6), the safety factor is 3.3. If you lose half your gas (either a total sidemount failure or donating to a teammate whose manifolded doubles died), it will be ~1.7. That means a 70% buffer against SAC increase or speed decrease (for whatever reason). Everyone knows you gain margin exiting with flow, but on the flip side without flow, you'd need to turn before Fourths (4.3 actually) to achieve this same buffer.
Does any of this make you want to bump up your no-flow conservatism, especially after considering it could be cut in half in the event of a failure? Is this a bit immaterial since you & buddy dive sidemount and Thirds? (In that case, there's a 50% margin for the team, assuming a single gas failure and no separation.) I'd love to hear what other thoughts you might have.
Sidenote 1:
Hopefully it's clear the above is for an open circuit dive. However for the CCR divers, an OC-equivalent safety factor for comparison would be 1 more than your BO strategy at normal SAC and flow-enhanced exit speed. If you plan on 1.5x the gas needed to exit including flow (giving a 50% margin for consumption increase or delays), S = 1+1.5 = 2.5. If you ignore flow in your BO calculations, but you do exit with flow, you'd bump up the BO factor by a factor of 1+Flow/Kick. E.g., if BO is 1.5x at a 40 fpm kicking speed, but there's a 10 fpm flow aiding your exit, the bonus factor would be 1.25, and the OC-equivalent safety factor would be S = 1 + 1.5*1.25 = 2.9. Feel free to cast a vote based on the closest OC-equivalent safety factor.
I think that on average, CCR divers tend to run larger safety factors than OC divers. Granted, the possibility of a hypercapnic event is a really good reason, but I've found myself wanting to bump up my OC conservatism after diving my Fathom for the last year & a half.
Sidenote 2:
The above may also give some insight into the commonly cited Rule of Sixths when OC diving a siphon. In that case, the speed ratio V will be greater than 1. For this minor 10 fpm siphon flow and the same 40 fpm kick speed, V = 1.67, resulting in a safety factor of 3. After an assumed failure, that would give a 50% margin against consumption or exit difficulties, which is perhaps reasonable. However, even Sixths would be inappropriate for a siphon flow of 17 fpm (and that 40 fpm kick speed). (That's a safety factor of 2 or equivalently zero margin after a failure.) At a minimum, I'd take a minute to hover and make sure that I hadn't been blown more than a couple of wingspans. Perhaps repeat a few times during the penetration as well.
I think everyone would agree this is fairly aggressive without the luxury of flow, so what do you do in that case? A common theme is to enter slow & casual and beeline for the exit if SHTF, but is that your only margin over Thirds? Some will increase the turn pressure by X psi on top of the casual entry (e.g., Thirds+100 psi would turn at 2500 psi for a 3600 psi start). Diving 4ths is another easy-to-compute option. Diving in a team of 3 would help, but I'm not sure that's always possible. Do you leverage the fact that the safety factor / margin is actually variable (the minimum is at the turn pressure but grows with proximity to the exit)? I'm curious what safety factor or rule you typically use in no-flow caves like Peacock or many places in Mexico. To keep it simple, let's assume exiting from the entry point and ignore any adjustments for tight passages, new teammates, separation/solo, etc.
For those who are interested, the safety factor including flow can be computed relatively simply using the ratio of entry to exit speed (call it V). For example, a flow of 10 fpm (not a stretch, as that's only 2 inches per second) and a no-flow swim speed of 40 fpm, that ratio V = (40-10) / (40+10) or V = 0.6. Denoting the penetration rule by P (i.e., 3 when diving Thirds), the safety factor is:
S = (P - 1) / V
For Thirds in that modest flow (P=3 & V=0.6), the safety factor is 3.3. If you lose half your gas (either a total sidemount failure or donating to a teammate whose manifolded doubles died), it will be ~1.7. That means a 70% buffer against SAC increase or speed decrease (for whatever reason). Everyone knows you gain margin exiting with flow, but on the flip side without flow, you'd need to turn before Fourths (4.3 actually) to achieve this same buffer.
Does any of this make you want to bump up your no-flow conservatism, especially after considering it could be cut in half in the event of a failure? Is this a bit immaterial since you & buddy dive sidemount and Thirds? (In that case, there's a 50% margin for the team, assuming a single gas failure and no separation.) I'd love to hear what other thoughts you might have.
Sidenote 1:
Hopefully it's clear the above is for an open circuit dive. However for the CCR divers, an OC-equivalent safety factor for comparison would be 1 more than your BO strategy at normal SAC and flow-enhanced exit speed. If you plan on 1.5x the gas needed to exit including flow (giving a 50% margin for consumption increase or delays), S = 1+1.5 = 2.5. If you ignore flow in your BO calculations, but you do exit with flow, you'd bump up the BO factor by a factor of 1+Flow/Kick. E.g., if BO is 1.5x at a 40 fpm kicking speed, but there's a 10 fpm flow aiding your exit, the bonus factor would be 1.25, and the OC-equivalent safety factor would be S = 1 + 1.5*1.25 = 2.9. Feel free to cast a vote based on the closest OC-equivalent safety factor.
I think that on average, CCR divers tend to run larger safety factors than OC divers. Granted, the possibility of a hypercapnic event is a really good reason, but I've found myself wanting to bump up my OC conservatism after diving my Fathom for the last year & a half.
Sidenote 2:
The above may also give some insight into the commonly cited Rule of Sixths when OC diving a siphon. In that case, the speed ratio V will be greater than 1. For this minor 10 fpm siphon flow and the same 40 fpm kick speed, V = 1.67, resulting in a safety factor of 3. After an assumed failure, that would give a 50% margin against consumption or exit difficulties, which is perhaps reasonable. However, even Sixths would be inappropriate for a siphon flow of 17 fpm (and that 40 fpm kick speed). (That's a safety factor of 2 or equivalently zero margin after a failure.) At a minimum, I'd take a minute to hover and make sure that I hadn't been blown more than a couple of wingspans. Perhaps repeat a few times during the penetration as well.
