Big Heavy tanks!

[berndo]

Knowing your sac can help determine this. Is it essential for recreational diving, no but it adds and extra margin of safety which is always a good thing.

How is it supposed to add safety? Give a specific and realistic example how it does. Half plus x is based an the actual consuption on a giving dive.

 

[Wibble]

Running out of gas because you’d not factored in your actual consumption. Similarly calling the dive early when you’ve loads of gas available for your personal consumption rate.

How about working out the gas in a small 3 litre "pony" bottle if you’re diving to 30m/100ft? SAC is very important then as you may very easily not have sufficient gas for emergencies.

 

[happy-diver]

Man, you'll look at that 18 litre go whoa make a sign of the cross and say no way Jose and that's without lifting it

I will try out a 15 liter and a 18 liter next week, than I will have some kind of opinion. Thank you all!

 

[rsingler]

Since this is the Basic Forum, why don't we parse out why both sides might be right?

Running out of gas because you’d not factored in your actual consumption...
How about working out the gas in a small 3 litre "pony" bottle if you’re diving to 30m/100ft? SAC is very important then as you may very easily not have sufficient gas for emergencies.

Like some other posters don't seem to understand how half-gas works.

Well, exactly! This is the Basic Forum.

Half plus some, turn and swim back in shallower water or turn at 120 bar when you swim back only slighty shallower is perfectly safe and leaves enough gas for an emergency at any point in the dive.

Absolutely true! But there are subtleties in this reply that a newbie might not catch. Like "swim back only slightly shallower". And what if you're on the Spiegel Grove when you have your equipment malfunction? Now SAC x 3-4 atm x stress equals how much gas?

Half plus x is based an the actual consuption on a given dive.

And there's the crux of the issue! An experienced diver knows that once he uses half his/her gas minus an amount for an emergency, it's time to turn the dive. And it works all the time because it factors in actual consumption.
But if you're new and you don't know what "x" to add, or if you can't "go shallower" to reduce your gas use on your first Spiegel Grove dive, how do you plan?
Well, SAC might be one way to game it out until you get more experience. It might be a little scary just jumping and waiting for the needle to hit the half mark plus x.
Both approaches are correct. One might require experience, because of the hidden gotcha of gas consumption during problem solving on a deeper recreational dive.
Why Surface with 500 PSI?

 

[berndo]

Absolutely true! But there are subtleties in this reply that a newbie might not catch. Like "swim back only slightly shallower".

What subtleties? You plan a dive, as in max depth and depth in the way back, it's very much basic scuba dive planing. That's OWD level. Even on a wreck you dive the lower part first and look at the top of the wreck later.

And what if you're on the Spiegel Grove when you have your equipment malfunction? Now SAC x 3-4 atm x stress equals how much gas?

Someone like the OP with a sac rate of 17l is not going to have an 4x increase in breathing rate but whatever you're assuming as 'stress breathing rate', you're just a guess (which makes it pretty ironic that people make assumptions for min gas calculations are calling half-tank a 'magic' number).
In basic level scuba you always have the option to directly ascend to the surface in an emergency. If the dive site doesn't allow you to directly ascent in an emergency it's not basic scuba and arguably shouldn't be done in standard rec gear and more training than just OWD or AOWD.

Well, SAC might be one way to game it out until you get more experience. It might be a little scary just jumping and waiting for the needle to hit the half mark plus x.
Both approaches are correct. One might require experience, because of the hidden gotcha of gas consumption during problem solving on a deeper recreational dive.

I disagree with the notion that knowing your sac rate make it any safer. I never said it's wrong to plan with sac, I think it's pointless information for ndl dives. I also disagree with the notion that you need experience to figure out that you swim back shallower if possible or return earlier. Those are the most basic points of dive planing... it's really as basic as it gets.

The 'knowing sac is safer people' don't give any examples because they can't think of any. That's why I ask for an example. It doesn't check out in real life.

BTW: If you're assuming a person can have so little experience that they don't know that you go deep first and know nothing about dive planing, how would they even know their sac rate reliably?

 

[gordonscuba]

Let's think through a one-way tropical NDL dive (max depth 45 ft, average 30 ft) from shore where the dive plan is to enter at point A, exit at point B further down the coast. Total time in water about 60 mins, EAN32, basically impossible to hit deco. No really appealing exits between A and B, unless you happen to like rock-climbing in surf (haha - no one does). It's a real place in Hawaii; two very easy entry/exit beaches separated by volcanic rock. It's not quite a drift dive but there can be a rather weak littoral current in the direction from A to B.

Bob is the experienced local diver. Bob's SAC rate is 7.25 L / min, measured at home watching TV -- hence not to be confused with the surface RMV of any actual dive. Bob knows that he can easily complete the underwater swim from A to B on a single AL80, starting at 3000 psi at point A, and exiting with ~ 800 psi remaining at point B. Bob also knows that, assuming he can correctly identify the halfway point from underwater landmarks, he should turn the dive if reaching a certain pressure before reaching the halfway point. If he has to turn, the swim back is well within Bob's ability but will require more gas than the swim out. There is a risk of incorrectly identifying the halfway point -- this risk is small for an experienced local like Bob, but not zero. Slow ascent to the surface and swim to the closer beach is always an option, but not a particularly pleasant or appealing one due to boats, surf, sharp rocks, idiots on jet-skis etc.

Bob's friend Alice (same age, physical condition etc.) is visiting Hawaii for the first time and wants to do the same dive with Bob. Bob considers Alice's request, and decides that he isn't comfortable if the most likely, expected outcome is that Alice reaches turn pressure before the halfway point, requiring both to swim back -- Bob wants that to be a sort of emergency plan B, not what everyone expects to happen given the risks mentioned above. Moreover, since it's absolutely trivial, they measure Alice's SAC rate while streaming an episode of their favorite show, and find 6 L /min which is significantly less than Bob's 7.25 L / min. Based on this, Bob decides he is comfortable with the risk and green-lights the dive, with each diver carrying an 80cf alu primary and a pony for emergencies, along with all other standard safety equipment such as DSMB etc. I'm curious about the thoughts of the very experienced divers on this thread. Is Bob acting wisely? One answer that I'm happy to accept is: "this is actually a technical dive, not due to depth but due to environment; no recreational diver should attempt this without further training." I bet lots of them do, though.

 

[berndo]

If he has to turn, the swim back is well within Bob's ability but will require more gas than the swim out. There is a risk of incorrectly identifying the halfway point -- this risk is small for an experienced local like Bob, but not zero.

It's a sh!t plan. Swimming against the current should never be part of your plan b. You don't ever want the backup plan to require more gas. Whenever there is current is can get stronger or weaker, so don't rely on weak current. If the current is weak and you're not a 100% sure you can make it easily, make the first dive against the current and go from B to A.
When you're with the current, go shallower if it's looks like the gas wont be enough. If you're worried about boat traffic, either tow a buoy with a dive flag or plan the dive as if you can't ascent (which is not basic level).
BTW: 6 liters is pretty extreme for sac rate, unless you're a hobbit maybe.

 

[gordonscuba]

It's a sh!t plan. Swimming against the current should never be part of your plan b. You don't ever want the backup plan to require more gas. Whenever there is current is can get stronger or weaker, so don't rely on weak current. If the current is weak and you're not a 100% sure you can make it easily, make the first dive against the current and go from B to A.
When you're with the current, go shallower if it's looks like the gas wont be enough. If you're worried about boat traffic, either tow a buoy with a dive flag or plan the dive as if you can't ascent (which is not basic level).
BTW: 6 liters is pretty extreme for sac rate, unless you're a hobbit maybe.

Thank you, that's an excellent point. I agree completely, against the current should not be plan B. Taking that into consideration, we could change the example to zero current. Even in that case, Bob could be forgiven for trying to predict the most likely sequence of events, ie. predicting turn or no-turn. I mean sure, there's tons of ways you can adapt, handle low-gas situations, and survive, but calculating Alice's SAC rate is just super easy and potentially changes the expectation of the most likely sequence of events, so there's very little reason not to do it. I do know one person with a 6L SAC rate... she is a 100lb woman who shops in the children's section and always has more air at the end of every dive than everyone else. I won't tell her the hobbit thing.

 

[berndo]

... potentially changes the expectation of the most likely sequence of events ...

How so?

... so there's very little reason not to do it.

Yes, and there is also little reason to do it. That was my point, it doesn't matter if you know it as it doesn't change any basic ndl dive plan.
Nothing changes knowing the sac rate in your example. If you had said that the divers had double the sac rate, the plan and backup plan wouldn't change.

 

[gordonscuba]

How so?


Yes, and there is also little reason to do it. That was my point, it doesn't matter if you know it as it doesn't change any basic ndl dive plan.
Nothing changes knowing the sac rate in your example. If you had said that the divers had double the sac rate, the plan and backup plan wouldn't change.

In my example, given that Alice SAC < Bob SAC, we formed the expectation that, assuming no equipment problems, the pair would very likely complete the one-way swim from A to B using their AL80s.

Suppose alternatively, that Alice's SAC rate had been found to be significantly more than Bob's. Then we form a different expectation of how the dive would go with the AL80s, i.e. we predict that they will most likely turn around because Alice hits turn pressure, and return to the origin.

I fully accept that if they follow everything they learned in their training, they should be able to keep each other safe from serious harm in either case. I also understand Bob's point of view that this piece of information is free, and it can be used to inform subsequent decisions. For example, sticking with the alternate scenario, where Alice's SAC rate is measured pre-dive, found to be very much on the high side, Bob can then make an informed decision to either choose an easier dive, or he can agree to the dive conditional on Alice bringing more gas supply. (Alice could be recreational-level sidemount certified). If they decide to go ahead with the single AL80s, they do so *expecting* to turn around based on Alice's pressure, which might make Bob a little more attentive to Alice's pressure through the middle portion of the dive.