Turn pressures

[AdivingBel]

As we were heading back to the marina, one of the women asked me how I had known how to find the boat. I told them, and then I asked, "How were you doing it?" She said, "We were following you."

Been there...

 

[yle]

Sounds like a fairly typical warm water, rec wall dive. 3000 psi AL80. Dive to the wall. Drop to your max depth (70/80 ft.). Cruise along the wall until 1500 psi. Turn and ascend to 30/40 ft. or top of the wall. Work your way back to the boat. Paddle around under the boat. Surface with about 500 psi. 50/60 min. dive. Also works in Bonaire & Curacao with it's ubiquitous "swim to the drop," but usually with a longer dive time given overall shallower average depth and shallow swim out/in given it's a shore dive. Pretty standard in the Caribbean.

Exactly. Standard dive briefing from the guide "let me know when you reach half a tank and we'll start making our way back."

 

[stevensamler]

I have had several people, in two contexts, suggest that a turn or turnaround pressure of a 100 bar is a thing.

Does anyone have practical examples of when that would be the case in simple single tank diving?

First establish the mgr for the dive profile then you'll know what's usable then apply the gas strategies. All usable , rule of halves or thirds and you might add for the environment and teammate experience.

 

[RainPilot]

In this case how is the turn pressure (being when the divers start to retrace) calculated? Is is it half of the available gas for the dive, ie start gas minus ascent reserve? And if so, to what degree do people assume this is half a cylinder?

Rock bottom gas: This is the gas that I must start ascending when I reach it. This is calculated to take 2 divers from the bottom, after 1 minute problem solving, to the surface with a safety stop or any required stops included. If the actual SAC rates get out of hand, the safety stop gas can be used while aborting the actual stop.

Turn pressure: Total gas minus rock bottom equals usable gas. Usable gas divided by 1, 2 or 3 as alluded to by John earlier. All usable is when there is no need or plan to return to a point (eg drift dive). Half usable is when you want to return to a point but it is not essential (return to ascent line on OW dive where a blue water ascent is not especially hazardous). Third usable when a return is compulsory.

Sample calculation:

40m dive (5 ATA), nominal RMV of 20L/min (0.7 CF)

1 min at the bottom is 100 litres
Ascend at 10m min (use average depth) so 4 min at 3 ATA is 240 L
3 minutes at 5 metres is 90 L

Total of 430 L per diver so 860 L for the buddy pair. In an AL80 that will equate to just under 80 bar. If one uses 30 LPM for the panicked diver scenario, then the total goes up to 1075L or 100 bar (half a tank). Thats the number I use for students doing their first deep dives or in any situation where I am unsure of the temperament of the other half of the team.

Usable gas for a 80 Bar rock bottom will be 120 bar, thus on a drift dive I start ascent at 80 bar. On a "nice to get back but not essential" dive I will turn at half usable so after consuming 60 bar, or a SPG reading of 140. On a "have to get back" dive I will turn after consuming 1/3 usable, or 40 bar used (160 SPG reading). Once I get back to the ascent point, I may spend a little time there until reaching rock bottom if that is desirable (swimming around the ascent line on the deck of the wreck for example).

 

[KenGordon]

There seem to have been two sets of replies, some like Rainpilot’s above are discussing optional returns with a turn pressure of start - usable/2. Others, like yle, are talking about dive practice in some locations.

Is it the case that the practice is a specific case of the rainpilot’s general calculation or is it a completely different thing? Is it a taught thing or just a guide thing (50 out, 50 hanging about at the interesting features, 50 back, 50 to ascend sort of plan)?

 

[Storker]

Usable gas for a 80 Bar rock bottom will be 120 bar, thus on a drift dive I start ascent at 80 bar. On a "nice to get back but not essential" dive I will turn at half usable so after consuming 60 bar, or a SPG reading of 140. On a "have to get back" dive I will turn after consuming 1/3 usable, or 40 bar used (160 SPG reading). Once I get back to the ascent point, I may spend a little time there until reaching rock bottom if that is desirable (swimming around the ascent line on the deck of the wreck for example).

And of course the picture may be different for a multilevel dive, such as a shore dive over a sloping bottom, or a wall dive.

My rock bottom pressure for 30m is some 100-120 bar depending on how large a margin I use for increased breathing rate due to stress. At 10m, my rock bottom pressure is pretty close to the classic be-at-the-safety-stop-pressure of 50 bar. So a typical wall dive would go like this: descend to some 25-30m, swim along the wall at depth until I reach some 120 bar or half of my planned bottom time, whatever comes first. Then ascend to 10-15m and swim back. Back on my safety stop just below the boat with some 50-ish bar left, having seen new scenery during the whole dive.

 

[boulderjohn]

Is it a taught thing or just a guide thing (50 out, 50 hanging about at the interesting features, 50 back, 50 to ascend sort of plan)?

It is a taught thing.

I learned it somewhere in my training that I cannot recall. I have encountered a number of people who have used that same terminology that I used above, with the same explanations.

About 7 years ago I submitted a distinctive specialty in dive planning to PADI for their approval. When I did that, they had to look at my curriculum and approve of every detail. In that section of the course (all usable, halves, and thirds), they paid special attention, to a degree and to an effect that I will describe later. They saw that explanation, they commented on it, and they approved it. That means it is in at least one PADI-approved distinctive specialty class.

As to the effect our discussion had, they initially did not like my inclusion of the rule of thirds in the course because they felt it was not appropriate to recreational diving and was more appropriate in a technical diving course. I gave specific examples of when it was useful for recreational diving, and they agreed. Two years later PADI produced a new OW diving class, and the rule of thirds was included both in the course curriculum and on the final exam.

 

[Mike1967]

Two years later PADI produced a new OW diving class, and the rule of thirds was included both in the course curriculum and on the final exam.

What class is that?

Edit: Nvm, I've just been looking at your website. How is there PADI courses that are created by you but aren't mentioned anywhere in PADI information?

 

[Sevenrider860]

Two years later PADI produced a new OW diving class, and the rule of thirds was included both in the course curriculum and on the final exam.

What question on the final exam (A or B) references the rule of thirds?

 

[boulderjohn]

What class is that?

Edit: Nvm, I've just been looking at your website. How is there PADI courses that are created by you but aren't mentioned anywhere in PADI information?

They are called Distinctive Specialties. They are created by individual instructors and approved by PADI. There must be thousands of them around the world today.