Rule of Thirds & Shallow Rec diving

[Foxfish]

Dry suit and cold water? Not exactly my idea of a basic level scuba dive. My SSI OW course book says to surface with 35 b or 500 psi which assumes an 18 m limit. We were diving to 25 m during our course and always used the rule of surfacing with 50 b or 725 psi.

If I was to head for the surface at 110 b from 30 m with a 12 L tank I'd surface with about 90 b which is nearly double the required 50 b. I've never heard anyone apart from a few people on this forum pushing the idea that divers should surface with 90 b or 1300 psi. Anyone else think that for a straight forward ascent from 30 m this sounds decidedly whacky.

 

[NAM001]

the 110 bar you cite is not your air. it is there for you to share with your buddy if needed and if needed you would surface with 45 or so and not 90. i start up from 30m with probably 1300 psi depth *10 +300. and when i get to say 50 ft i now have a new figure. which is 800 psi still depth *10 +300. I get to reclaim some air what was at 30m for my buddy. and if i went to 30 ft then bottom would be 600. i get on the boat with 4-500. now other things will change that. 1300 is ok for me with an al80 tank. often I single tank dive with a lp120 or lp95's. all the numbers and methods now change. bottom line is that you need x cu ft to get up and the same for your buddy and say another for stress and gage error. it works very simple if you are doing square dives. 1300 on a al80 is about 33 cuft. 33 cuft on the lp95 is 900 psi and the lp120 is less than 750 psi. And for arguements sake twin lp 149's would be right at 300 psi.

Dry suit and cold water? Not exactly my idea of a basic level scuba dive. My SSI OW course book says to surface with 35 b or 500 psi which assumes an 18 m limit. We were diving to 25 m during our course and always used the rule of surfacing with 50 b or 725 psi.

If I was to head for the surface at 110 b from 30 m with a 12 L tank I'd surface with about 90 b which is nearly double the required 50 b. I've never heard anyone apart from a few people on this forum pushing the idea that divers should surface with 90 b or 1300 psi. Anyone else think that for a straight forward ascent from 30 m this sounds decidedly whacky.

 

[rivers]

Dry suit and cold water? Not exactly my idea of a basic level scuba dive. My SSI OW course book says to surface with 35 b or 500 psi which assumes an 18 m limit. We were diving to 25 m during our course and always used the rule of surfacing with 50 b or 725 psi.

If I was to head for the surface at 110 b from 30 m with a 12 L tank I'd surface with about 90 b which is nearly double the required 50 b. I've never heard anyone apart from a few people on this forum pushing the idea that divers should surface with 90 b or 1300 psi. Anyone else think that for a straight forward ascent from 30 m this sounds decidedly whacky.

Your idea of basic diving is exceptionally narrow. Drysuits and cold water=average UK diving. Not everyone lives near warm water or wants to be a vacation only diver. Most divers in the UK are taught to dive in a drysuit from the start. They seem to manage, it doesn't seem too 'advanced'. I was diving over the weekend with a friend of mine. She has 30 dives. Water temp was 7 degrees and she was in a drysuit. Max depth was 9m in our bimble around an inland site. Not an advanced dive. We even had about 5m vis!

What you are failing to realise with the rock bottom/minimum gas concept is that you leave enough reserve in your cylinder to get you and your buddy back to the surface in case of a OOG/reg failure/etc from the most critical point in the dive, the bottom. If it's a multilevel dive, you've got more available gas once you make it to a shallower depth.

 

[Storker]

Dry suit and cold water? Not exactly my idea of a basic level scuba dive.

If you want to dive at all in Nordic waters, even your basic level dives will be in a dry suit and cold(ish) water unless you limit your diving to late summer and sheltered waters in the southernmost part of the country.

I took my OW cert in a dry suit, that's what people generally do here. My first post-cert dive was in a dry suit. It was as basic level as you can get'em up here. Shore dive, easy entry, no waves, no current, 30 minutes bottom time, max depth 12m. It was late June. Air temp was 12C and bottom temp was 9C, so it wasn't really a cold water dive, though :cool2: I didn't do a proper cold water dive until my dive #20 or 21 (#20: Early December, -7C air temp, 8C bottom temp; #21: early March, 0C air temp, 5C bottom temp).

If I was to head for the surface at 110 b from 30 m with a 12 L tank I'd surface with about 90 b which is nearly double the required 50 b.

That's one of the two reasons that I generally do multilevel dives. My minimum pressure decreases sharply as I go shallower, see my planning example in post # 165.

If I'm at 10m, 60 bar in a 10L, 50 bar in a 12L or 40 bar in a 15L is within rock bottom pressure. Do a multilevel dive with the deepest part in the beginning of the dive, muck around some more in the shallows before surfacing, and you won't have to surface with nearly 100 bar left in your tank.

 

[Storker]

is the Lamont's rule of thumb I posted earlier too inaccurate to use for recreational depths ?

I just checked the math, and it doesn't seem too bad, but it doesn't take into account your personal margins, your tank size or your actual breathing rate.

Also, it's not in Metric, which makes it kind of awkward to use for us non-'murricans

 

[supergaijin]

1. I follow the training I received on my OW and AOW when considering the pressure needed to begin my ascent. I start the ascent so that on a normal dive I surface with 50 b. Why isn't that a gas planning strategy?

What you are failing to realise with the rock bottom/minimum gas concept is that you leave enough reserve in your cylinder to get you and your buddy back to the surface in case of a OOG/reg failure/etc from the most critical point in the dive, the bottom. If it's a multilevel dive, you've got more available gas once you make it to a shallower depth.

This may have been covered by your instructor during OW/AOW training however it is not part of a typical PADI-like syllabus. This is one big failing point of this training IMO- these recreational training courses give the tools and training to descend to 4 and even 5ata- but not the tools and training to ascend with a buddy in an (inevitable) emergency.

I'd be interested in learning what training you received in your OW/AOW courses so that you know that you can ascend from 4ata with an out of air buddy. What is the minimum gas you need for both divers? How does one start to guess at something like that?

 

[Foxfish]

Your idea of basic diving is exceptionally narrow. Drysuits and cold water=average UK diving. Not everyone lives near warm water or wants to be a vacation only diver. Most divers in the UK are taught to dive in a drysuit from the start. They seem to manage, it doesn't seem too 'advanced'. I was diving over the weekend with a friend of mine. She has 30 dives. Water temp was 7 degrees and she was in a drysuit. Max depth was 9m in our bimble around an inland site. Not an advanced dive. We even had about 5m vis!

What you are failing to realise with the rock bottom/minimum gas concept is that you leave enough reserve in your cylinder to get you and your buddy back to the surface in case of a OOG/reg failure/etc from the most critical point in the dive, the bottom. If it's a multilevel dive, you've got more available gas once you make it to a shallower depth.

Why do you think divers are required to surface with 50 b?

 

[runez]

I note that you've calculated that you need 110 b (1600 psi) to ascend safely from 30 m (100 ft) with an al80 (about 11 L). That seems very conservative to me. Maybe you have a very high air consumption rate?

The 110 bar calculation is based on several, very conservative assumptions.

It's supposed to allow two divers to adopt a safe (and slow) ascent profile from depth while distressed, hence the high SCR. 110bar is used because most SPGs display pressure in 10bar increments.

Many of the assumptions will not hold true in most scenarios, e.g. different sized tanks, different ascent rates, different SCRs.....



That said, if a diver with a normal SCR of 17L/min starts to surface at 110bar.. he'll find himself on the surface with 80bar! That's a lot of unused gas, especially if you paid extra for a Nitrox fill! In practice, if you're diving a reef, you could just go up to a shallower depth and then the Min gas requirement changes.

IMO, I think the concept is useful, but many recreational divers will find it too conservative..

 

[Doppler]

Dry suit and cold water? Not exactly my idea of a basic level scuba dive.

I think others have pointed out that your views are limited in their scope. On the east coast of Canada, for example, I know a shop that starts OW punters off in drysuits in the pool.

If I was to head for the surface at 110 b from 30 m with a 12 L tank I'd surface with about 90 b which is nearly double the required 50 b. I've never heard anyone apart from a few people on this forum pushing the idea that divers should surface with 90 b or 1300 psi. Anyone else think that for a straight forward ascent from 30 m this sounds decidedly whacky.

Depends how are you calculate your gas needs?

Let's make this simple and put it into a context that sport divers might adopt.

We can use an average SAC of 14 l/min (which is low for most divers but whatever). Travel from 30 m to the safety stop at three or four metres should take a minimum of three minutes, more like five to seven for a careful diver (and we are promoting careful diving where the liklihood of "unearned" DCS is managed... yes?). So let's use five minutes. Mean depth for those five minutes is 16.5 m which nets a ambient pressure of 2.65 bar. The consumption will therefore be 185.5 litres... WITHOUT any dive factor. So, let's say that 185 l becomes a more realistic 200 litres. The diver executes a safety stop because a dive beyond 18 m certainly warrants one. So let's say three minutes at three metres... Gas needs = 55 litres. Our diver would need a few additional litres to surface after completing his/her stop. Make that 55 litres 60.

Total needs for a single diver doing what works = 260 litres. In a 12 l cylinder, 260 litres equals about 21 bar.

So, if the goal is to surface with 50 b, our diver has to leave the bottom with about 71-75 bar.

THESE NUMBERS ASSUME no trouble during ascent: A low SAC rate: A low Dive Factor (workload, stress, mild temperature, buoyancy control etc... and frankly, for a sport diver, these may be safe or unsafe assumptions. I do not know who the divers are. I would say that if I were doing this dive with any of my mates, the numbers in my example would be golden. Couple of reasons.

1) I don't dive with muppets so an OOA is not likely
2) if someone has a gas emergency, the surface with 50 bar is out the window... that gas may get used on the way up

HOWEVER, what if the punters in our example are charlie foxtrots? Leaving the bottom with 75 bar might mean they will be on seeds and stems when they surface.

Your take-home messages from all the crap contained in this thread is surely that gas management is not so complex that it can be ignored; and secondly and perhaps most importantly THERE ARE LOTS OF VARIABLES. One can choose to take that into account or ignore it. Seems most of the folks responding to your post choose the former.

 

[D_B]

I just checked the math, and it doesn't seem too bad, but it doesn't take into account your personal margins, your tank size or your actual breathing rate.

Also, it's not in Metric, which makes it kind of awkward to use for us non-'murricans


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Thank you for the charts
.. the/my link to Lamont's Rock Bottom gas planning is not working for me now so I cant check, but that is for a HP100 tank and, if I remember correctly, a RMV of 1 cubic feet per minute for each diver with 1 minute at depth, and a direct ascent
Edit: I see that no reserve if a 3 minute stop ... I'll be changing it to add enough for one with a reserve