Turn pressures

[NAM001]

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?

Look into ROCK BOTTOM calculations.

 

[Kevrumbo]

Recreational -Minimum Gas Reserve (MGR) Planning,
"CAT" vs old "Rock Bottom":

Using the new GUE acronym "CAT" formula [Consumption x Avg ATA x Time] at 100'/30m depth within NDL in open water for example:

Emergency Contingency plan requiring gas sharing at 100'/30m, with 1min to problem-solve prior to dive abort & ascend; (Ascent rate 10fpm, same as 3 meters per min, to surface takes -->10min):

So Total Time to Ascend is 10 plus 1 equals 11min.

Therefore given a novice diver Surface Consumption Rate (SCR) of 0.75 cfm, same as 20 liters per min,
CAT = 2 divers x (0.75 x 2.5 ATA x 11min) = 41.25 cuft or 1200 liters in metric.

For a team each diving a single Aluminium 11L/bar AL80 Cylinder in this particular example, notice this is at or more than half-tank consumed, so for conservatism, just signal a nominal and normal ascent when a teammate's SPG reading encroaches on the MGR 100 bar pressure mark (same as 1500 psi in US Imperial units).

An example of a very conservative safe & sane ascent strategy for a novice team in a gas sharing contingency (after 1 minute at operational depth to problem solve): a 30 second hold/30 seconds to ascend for every delta 10'/3m from operational depth -for an ascent rate of 10 fpm (3mpm).

This above is a somewhat simpler calculation & tactically an easier consistent ascent to perform, than UTD's Rock Bottom example below. . .

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By comparison for the 100'/30m example,
UTD recreational Rock Bottom procedure still retains the fast maximum recommended ascent strategy of 30 fpm/9mpm rate from operational depth to 50% of max operational depth, THEN the slow 10 fpm/3mpm ascent rate from there to the surface, and using the higher SCR of 1 cfm (30 liters per min):

Rock Bottom = 2 divers x (1 x 2.5 ATA x 8min) = 40 cf or 1200 liters in metric.

(See 14:30 mark in video below):

 

[KenGordon]

I understand how to plan a dive properly to have enough gas to get a diver and an OOG buddy to the surface by doing the maths.

I am after whether there is a short cut like ‘rule of thirds’ or ‘be on the boat with 50 bar’ which is ‘turn at 100’ that I was not previously aware of.

 

[boulderjohn]

I am after whether there is a short cut like ‘rule of thirds’ or ‘be on the boat with 50 bar’ which is ‘turn at 100’ that I was not previously aware of.

It's called the rule of halves.

It is commonly taught that there are three such rules, and they go like this.

1. All usable: You can ascend where you are when the time comes, so you only need to reserve the gas needed for you and a buddy to ascend properly.

2. Rule of Halves: You are traveling away from your starting point and would like to return to your starting point, but if you don't make it, the worst consequence is a surface swim.

3. Rule of thirds: You are traveling away from your starting point and MUST return to it.

Those rules are general categories, and you will adjust according to conditions For example, if swimming with the current at the beginning of a rule of thirds situation, you had better turn before the thirds.

As others have pointed out before, the rule of halves is extremely common in sites like Bonaire and Curacao. You start the dive at maximum depth swimming into a mild current. You may ascend a little during the first half. You turn the dive at the halfway point and gradually ascend as you swim back with the current. Depending upon the rate at which you ascend during such a dive, you should be back at your starting point with 60-70 bar, giving you enough time to piddle around at the top of the reef before final ascent.

 

[1atm]

I understand how to plan a dive properly to have enough gas to get a diver and an OOG buddy to the surface by doing the maths.

I am after whether there is a short cut like ‘rule of thirds’ or ‘be on the boat with 50 bar’ which is ‘turn at 100’ that I was not previously aware of.

Well, in real life you wouldn’t do the above calculations for every dive. You would have done them during your training, and have remembered (or written in your wetnotes, logbook, etc) a few common examples. E.g. rock bottom on a single 12L tank is 100bar at 30m, 80bar at 20m, etc usually with a limit to never use below 40bar due to SPG inaccuracy. If you have a dive depth in between you would round up for simplicity and conservatism, so that may be where the 100bar you referenced came from.

 

[Ayisha]

Back in the early 2000's, our LDS owner told us to turn around on deep dives in Tobermory at 1500 psi, which I believe is about 100 bar. When I did the Arabia, which claimed a dozen divers lives back then, he told me to be back at the line when either of us reached 1800 psi. I followed that advice for many years, as I'm sure others did. After that, I started using Rock Bottom, and then Minimum Gas and Minimum Deco on the Fly. CAT was after my time and I didn't learn it.

"Turn around at half the tank" is at least better than "guess what pressure to turn around at to be back on the boat with 500 psi/50 bar", though it doesn’t take into consideration at least a minute to solve a problem plus a reserve.

 

[Storker]

Rock bottom calculation could easily have 100 bar as turn pressure for a dive profile.

Can you give an example? I am wondering if this is a language thing. By ‘turn’ do you mean pressure at which to leave the bottom? Or actually turn back?

My standard tank is 10L 300 bar. That means some 2700 surface liters. If I do a proper min gas calculation, assuming some 15-20 surface liters as my RMV, my min gas pressure at 30m is some 100-120 bar. That means that at that pressure I'd better get shallower unless I want to be in some serious crap if the bovine manure decides to make close contact with the rotating ventilation device.

It's pretty simple math.

 

[KenGordon]

2. Rule of Halves: You are traveling away from your starting point and would like to return to your starting point, but if you don't make it, the worst consequence is a surface swim.

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?

 

[DogDiver]

Turn pressure is real easy to figure out here in cave country. It’s stamped right on your cylinders. Usually at 2400 psi. Of course we fill them to 3600 psi to 3900 psi. Lol.

 

[boulderjohn]

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?

In those cases, divers are usually planning to be near the surface at the end of their dives. In fact, it is a navigation technique, as I will explain. If you turn at half pressure and gradually ascend, you will be very near the surface at 60-70 bar, so there is no need to plan for more reserve. If, on the other hand, you are planning to get back to a point from which you will make a more serious ascent, you will want to plan a reserve, subtract that from the total, and plan your halves on the rest.

I was in Curacao and did a series of dives with a woman's dive group from Cincinnati. I was the only male on the boat. The first few dives were done with a DM, and then we did a dive without one. I knew the boat was moored on the top of the reef at 15 feet, so all I had to do as make sure I was at 15 feet by the time we got to that vicinity, and I was sure to find the boat. I assumed everyone did it that way.

As we were heading back with the current, I was hanging back behind everyone else. They were not getting shallower enough for my taste, and I assumed they were using a different navigation technique. (On your descent you take note of your compass heading and a prominent landmark at some depth along the way. You come back at that depth, find the landmark, and follow the reverse azimuth to your starting point.) Since I had not done things that way, I was uncomfortable following them, so I started ascending. After a while, I noticed they were ascending, too. I got to 15 feet about 100 feet from the boat. 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."