15 replies to this topic

[Capn Jack]

Now that I have your attention, I want to hear about OUT of air/gas... as in not getting to that point.

Based on the type of diving you do, what rule to you follow to make sure you don't get the blues? (blue lips, blue fingernails etc.)

There are several turn rules I've heard/used:

Rule of thirds - turn back when you've burned 1/3 of your air (1/3 to make the dive, 1/3 to return and 1/3 as your safety margin)
"500# at the ladder" (not my favorite - or WW's)
Rock Bottom calculation

What's your preference? Do you have a rule you like? Please share with us!!!

[drbill]

Easy... I dive an HP120 tank. The only times I've ever been OOA were with my intermediate pressure 80. Two of those times were when I knew I was running out but was filming something unusual in shallow water (< 20 ft). The other time was when my debris tube clogged and there was no air coming out of the freshly filled tank at all (none to the second stage, octo or SPG). I also carry a pony just in case (the only time I've had to use it was when the debris tube on my primary tank failed... but that was the one day I didn't strap the pony to my main tank). Haven't done that since.

It still astounds me that folks can run OOA at all. Don't they teach gas management these days. Freshy certified divers should have their eyes glued to the SPG (well, at least view it frequently). More seasoned divers should have a good feel for their air consumption diving different profiles.

One way to train for this is to use your dives to test your gas management capabilities. The way I do this is predefine a maximum depth and total dive time (often the same such as 60 ft for 60 min so my poor memory only has to handle one number). See if you can manage your gas so you come up with a predefined psi (say 500). You manage it by varying your depth of course.

When I dive with my HP120, I usually try to get two dives out of each fill (about 60 cu ft each dive). This makes the above game even more interesting as I set two different depth/time combinations (or try to match them on each dive... say 60 ft for 50 min total time). Keep in mind that the total time is not "bottom" time (time at max depth), but total dive time. Occasionally I'll miscalculate, or locate something really interesting to film, and have to spend the last 10 minutes at 15 ft to meet my goal. Fortunately there's plenty to see at that depth!

[PerroneFord]

Now that I have your attention, I want to hear about OUT of air/gas... as in not getting to that point.

Based on the type of diving you do, what rule to you follow to make sure you don't get the blues? (blue lips, blue fingernails etc.)

There are several turn rules I've heard/used:

Rule of thirds - turn back when you've burned 1/3 of your air (1/3 to make the dive, 1/3 to return and 1/3 as your safety margin)
"500# at the ladder" (not my favorite - or WW's)
Rock Bottom calculation

What's your preference? Do you have a rule you like? Please share with us!!!

Based on my rather simple diving, I use some combination of thirds or rock bottom. However, rock-bottom calcs generally require understanding your own, as well as your buddies SAC rates and this goes beyond most basic diving.

My profiles generally look like this:

1. Enter water shallow, bubble check, orientate to surroundings, allow breathing rate to drop.
2. Drop to depth, fullfill deep dive portion.
3. Turn on thirds (halves if in less than 80ft of water with no overhead)
4. Spend second third on ascent to shallow water, cruising around enjoying the scenery.
5. Move to within safe distance of the exit point (20-50ft) and do shallow decompression until at ~400psi.
6. Surface, debrief, take off fins, breathe surface air, relax, then exit water.

I find this profile differs from many other's I've talked to in that my bottom time is a relatively short period of time relative to the total dive length. Many recreational divers that I've spoken with tend to try to maximize bottom time and will spend very little time shallow either early or late in the dive. I believe this increases the risk of OOA and it's potential to do harm.

The oft repeated get back on the boat with 500psi is not a gas management plan, it's a goal. If conditions warrant, I will be back on the surface with 1000psi or more. In calm circumstances (freshwater springs) I may be getting on the ladder with less than 200psi. It's all dependent on how you manage your gas.

[Walter]

That depends on the type of diving you are doing. It helps when you know how quickly you use your air. It's also a great idea to monitor your buddy's air as well. Someone who uses air more quickly will need to start an ascent sooner than someone who doesn't use as much. The rule of thirds is good for overheads, but overkill for most dives. Most folks plan to be back on the boat with 500 PSI in an AL 80. That's approximately 13 cu ft (actually a tad less) of air. Using 500 PSI with other tanks is an easy plan, but a tad silly. The important concept is to have enough air for an unexpected emergency, so I usually plan to end my dive with about 15 cu ft of air regardless of the size tank I'm using. Sometimes that's more than 500 PSI, sometimes less. Most people should start an ascent from a moderately deep dive (60-100 ft) with at least 1000 PSI (more if you use lots of air). If you're making a shallow dive right under the boat, you can start up with 700 - 800 PSI. It's really a matter of common sense. You need more air to make multiple and/or longer stops on you ascent.

[normblitch]

Good Points ALL...

One potential issue often overlooked is 1st stage performance as Tank pressure aproaches Internediate pressure...better know THIS before you "plan" on breathing lower than 200#...

Regarding the third rule...I started Caving when some folks were still diving 1/2 + 500 in high-flow systems...this was finally abandoned by ALL when it was realized (from studies of Fatalities) how extreme conditions can adversley affect SACs...and I am growing increasingly wary even of Thirds based on this...Remember, there is NO OC Reg that can't be over-breathed by a Panic-ed Diver...cattle boats often "offer" long waits in high seas on the tag line, with THOSE moments rarely being relaxed...although I will NOT lie and say it never happened to me, I DO insist on a 500# at the ladder, and usually do as Walter sez, min 1000# turn on Dives > 60'...

nhb

That depends on the type of diving you are doing. It helps when you know how quickly you use your air. It's also a great idea to monitor your buddy's air as well. Someone who uses air more quickly will need to start an ascent sooner than someone who doesn't use as much. The rule of thirds is good for overheads, but overkill for most dives. Most folks plan to be back on the boat with 500 PSI in an AL 80. That's approximately 13 cu ft (actually a tad less) of air. Using 500 PSI with other tanks is an easy plan, but a tad silly. The important concept is to have enough air for an unexpected emergency, so I usually plan to end my dive with about 15 cu ft of air regardless of the size tank I'm using. Sometimes that's more than 500 PSI, sometimes less. Most people should start an ascent from a moderately deep dive (60-100 ft) with at least 1000 PSI (more if you use lots of air). If you're making a shallow dive right under the boat, you can start up with 700 - 800 PSI. It's really a matter of common sense. You need more air to make multiple and/or longer stops on you ascent.

Another good point...

Having used an LP95 for almost a year now, a HEARTY AMEN for steelies!

Normally, I'm entering my 1st Dive with 120 cubic...after a third, and I STILL have 80 left...two fulfilling Cavern/Intro Cave dives, & I haven't had to lose my prime parking spot for a fill, and NO embarrassing Tank changes!

And the buoyancy of a steelie is wonderful as well...carrying lead adds NOTHING to a dive; if you have to wear weight, wear BREATHABLE weight!!



Norm

Easy... I dive an HP120 tank. The only times I've ever been OOA were with my intermediate pressure 80. Two of those times were when I knew I was running out but was filming something unusual in shallow water (< 20 ft). The other time was when my debris tube clogged and there was no air coming out of the freshly filled tank at all (none to the second stage, octo or SPG). I also carry a pony just in case (the only time I've had to use it was when the debris tube on my primary tank failed... but that was the one day I didn't strap the pony to my main tank). Haven't done that since.

It still astounds me that folks can run OOA at all. Don't they teach gas management these days. Freshy certified divers should have their eyes glued to the SPG (well, at least view it frequently). More seasoned divers should have a good feel for their air consumption diving different profiles.

One way to train for this is to use your dives to test your gas management capabilities. The way I do this is predefine a maximum depth and total dive time (often the same such as 60 ft for 60 min so my poor memory only has to handle one number). See if you can manage your gas so you come up with a predefined psi (say 500). You manage it by varying your depth of course.

When I dive with my HP120, I usually try to get two dives out of each fill (about 60 cu ft each dive). This makes the above game even more interesting as I set two different depth/time combinations (or try to match them on each dive... say 60 ft for 50 min total time). Keep in mind that the total time is not "bottom" time (time at max depth), but total dive time. Occasionally I'll miscalculate, or locate something really interesting to film, and have to spend the last 10 minutes at 15 ft to meet my goal. Fortunately there's plenty to see at that depth!

[Dive_Girl]

Rule of thirds is quick and easy and our shop DOES introduce, teach , and use this method starting at Open Water training.

[Latitude Adjustment]

I always run out of bottom time before I run out of air but here's a twist on coming up with 500psi on an open water or drift dive. What if the boats not there, do you have a snorkel? Because if you keep breathing off that tank that Dive/SubAlert isn't going to work when you need it most.
Norm, I've seen divers back up on the tag line when cross currents make only one tag line/side line/ladder useable and they start wishing they'd come up with more air or packed a snorkel.

[normblitch]

ANOTHER excellent Point!

(you must have been on some of the same Trips as Me!)

And for whoever mentioned 5'+ seas in another thread as being emminently Divable, I would challange them to hang on the clothes-line with nuggets flailing and STILL breath off a snorkel for a (seemingly) indefinite time.

The wait for the ladder is SO less stressful in a bad sitch (and therefore better to avoid DCS) on a reg than a plastic tube...

Norm

I always run out of bottom time before I run out of air but here's a twist on coming up with 500psi on an open water or drift dive. What if the boats not there, do you have a snorkel? Because if you keep breathing off that tank that Dive/SubAlert isn't going to work when you need it most.
Norm, I've seen divers back up on the tag line when cross currents make only one tag line/side line/ladder useable and they start wishing they'd come up with more air or packed a snorkel.

[ScubaDadMiami]

I go with different rules based on the dive plan and the team.

Thirds (or even more conservative) is typical for technical deep wreck penetration.

For easy, recreational dives, I'll determine a minimum amount of gas that I want for a reserve, and then I will use 1/2 of available gas and then leave an additional 200 to account for "what ifs" along the way.

On a live drift with a boat, I will go with all useable gas minus a small reserve.

[Walter]

One potential issue often overlooked is 1st stage performance as Tank pressure aproaches Internediate pressure...better know THIS before you "plan" on breathing lower than 200#...

If your reg won't breathe a tank empty without issues you need a different reg.

[normblitch]

Walter,

Point taken; HOWEVER, how many Divers know this about their (or Rental) gear?? (until it is too late)

Norm

One potential issue often overlooked is 1st stage performance as Tank pressure aproaches Internediate pressure...better know THIS before you "plan" on breathing lower than 200#...

If your reg won't breathe a tank empty without issues you need a different reg.

[Walter]

Norm,

What regulators won't breathe a tank down to empty? I don't know of any.

[finGrabber]

Aren't there some regulators that breathe different once you hit 500 psi or so?

I do know that mine doesn't breathe any different at 3000 psi than it does at 50 psi

[scubafanatic]

Aren't there some regulators that breathe different once you hit 500 psi or so?

I do know that mine doesn't breathe any different at 3000 psi than it does at 50 psi

........well FG, generally speaking, modern regs behave as you suggest....you are probably thinking of the handful of very low-end unbalanced 1st stage regs whose breathing resistance climbs at low tank gas pressures......... these are commonly used in rental 'fleets' and as deco regs.

Karl

[finGrabber]

Aren't there some regulators that breathe different once you hit 500 psi or so?

I do know that mine doesn't breathe any different at 3000 psi than it does at 50 psi

........well FG, generally speaking, modern regs behave as you suggest....you are probably thinking of the handful of very low-end unbalanced 1st stage regs whose breathing resistance climbs at low tank gas pressures......... these are commonly used in rental 'fleets' and as deco regs.

Karl

Karl,

thanks for the info...I had heard that even mid-line regs have the breathing resistance issues...