Rock Bottom Pressure and Turn-Around Pressure

[DevonDiver]

You can plan a dive without something as convoluted as Rock Bottom. It's so dirt simple. Take out the 500psi your supposed to come back with add a little for you safety stop (say 300psi)

So, at what pressure do I need to leave the bottom to ensure that I arrive at my safety stop with 300psi and a 500psi reserve?

PADI students are taught to plan dives by accounting for time and depth. Consideration of air consumption is woefully neglected.

I'd estimate that most of the recreational divers I encounter will do nothing more than 'guesstimate' their gas use on a dive - that guestimation becomes better with experience. Novice divers don't have much of a clue. More experienced divers will have a clue, but can be rapidly thrown into error if anything unforeseen occurs (causing accelerated respiration) - again, because they have no conceptual understanding of gas consumption, they often fail to appreciate how cause-effect are linked with regards their air time.

When asked, many divers won't even have considered their gas usage in respect of dive planning. Their gas state is a purely reactive issue - monitoring their gauges and if they notice they are getting low, they'll make the decision to ascend. Because no strict limit is pre-planned, that means it's much easier for them to deliberately or negligently over-stay beyond a safe minimum.

All of which increase the likelihood of low/out-of-air emergencies.

and then divide [the remainder] by 2; to get your turn around..

Did I edit that right?

Maintain a reserve of 500psi.
Allot 300psi for the safety stop.
Divide the remaining (consumable) gas by half to establish a mid-point in the bottom-time?

3000 - 500 - 300 (= 2200psi) / 2 = 1100 psi until the midpoint.
Turn point is at (1100 + 500 + 300) 1900 psi (on a 3000psi tank).

Although those calculations still neglect any air consumption that occurs between leaving the bottom and reaching the safety stop.

 

[TSandM]

What Peter does with his students is discuss the CONCEPT of rock bottom, give a quick example, and then hand out a sheet that has the numbers on it for commonly used tanks at 60 feet and 100 feet. At least the student comes away with the IDEA that it isn't just YOU you have to plan to get to the surface, and the concept that diving deeper than the cubic feet in your tank is dubious.

 

[Ste Wart]

So, at what pressure do I need to leave the bottom to ensure that I arrive at my safety stop with 300psi and a 500psi reserve?

The halfway point. I already mentioned.

PADI students are taught to plan dives by accounting for time and depth. Consideration of air consumption is woefully neglected.

Give them a basic outline to run dives on. Not something that requires a calculator.

I'd estimate that most of the recreational divers I encounter will do nothing more than 'guesstimate' their gas use on a dive - that guestimation becomes better with experience. Novice divers don't have much of a clue. More experienced divers will have a clue, but can be rapidly thrown into error if anything unforeseen occurs (causing accelerated respiration) - again, because they have no conceptual understanding of gas consumption, they often fail to appreciate how cause-effect are linked with regards their air time.

Again doesn't matter. Plan your dive, dive your plan. A simple plan never fails. If you hit your turn point early because of high respiration... you turn. Simple.

When asked, many divers won't even have considered their gas usage in respect of dive planning. Their gas state is a purely reactive issue - monitoring their gauges and if they notice they are getting low, they'll make the decision to ascend. Because no strict limit is pre-planned, that means it's much easier for them to deliberately or negligently over-stay beyond a safe minimum.

Then why on earth are we talking about Rock Bottom calculations?

Did I edit that right?

Maintain a reserve of 500psi.
Allot 300psi for the safety stop.
Divide the remaining (consumable) gas by half to establish a mid-point in the bottom-time?

3000 - 500 - 300 (= 2200psi) / 2 = 1100 psi until the midpoint.
Turn point is at (1100 + 500 + 300) 1900 psi (on a 3000psi tank).

Although those calculations still neglect any air consumption that occurs between leaving the bottom and reaching the safety stop.

Nope all there.

Lets do BAR, I hate PSI.

Lets do the standard brief on the boat. "Be back with 50 BAR" (which is a lot more than 500psi)

So we have 150 bar to play with. Lets give, 20 Bar to a safety stop. so 130.

Divide by 2 gets us 65. So we have a turn point of 135 (assuming a 200 BAR Fill)

So we descend to depth, follow along until 135 BAR. Now we turn. Note that time has included the descent and part of the bottom time.

We now return to for the safety stop. Should take us 65 BAR. In reality it will take us less.
Why?

Firstly, as the tank hits the water it will cool, pressure will drop slightly.
Secondly, We always breathe a little faster in the first point of the dive, until we find our rhythm.

Working on those principles we now arrive back at 5m ready for our safety stop with 70 BAR +.

With the math. 20 BAR in a standard 80 (11.3 litre) tank. Gives us 226 litres of gas. Divide by 3 to get per min, then by atmosphere to get the worst case scenario SAC. (226/3/1.5) = 50.2 SAC.

I really don't try to injure my students, whatever people on the board may seem to think. I give them simple plans they can use straight after the course.

 

[Peter Guy]

Wart here you go again. This is not Basic Scuba and the OP wasn't asking about basics. The OP had a misunderstanding that you unfortunately attempted to reinforce. Rock Bottom is a great concept for a diver, any diver, to understand. Turn pressure is a concept for a diver, any diver, to understand.

BTW, it is not always the case that tank pressure goes down on water entry. You are making invalid assumptions again.

 

[lv2dive]

BTW, it is not always the case that tank pressure goes down on water entry. You are making invalid assumptions again.

I was just thinking the same thing... Specifically, that I was very grateful this weekend when I had 3100 psi AFTER I got in the water, when I only had 2900 on the surface. 1000 psi for penetration suited me better than 900!!

 

[lowviz]

...//... Rock Bottom is a great concept for a diver, any diver, to understand. Turn pressure is a concept for a diver, any diver, to understand.
...//...

You and I haven't had the best history, but , I can only agree...

 

[SCUBA482]

Subscribed & agreed Peter Guy...knowledge is power & if the OP is asking he is ready to know...

 

[Thalassamania]

Ask PADI.

I'm not taking flak for the PADI curriculum.

Sorry, if you chose to use it, you take flack for it.

I teach my students to plan dives easily.

Rock bottom is completely useless for the the newbie dive.

Why?

No OW courses teach SAC/RMV. Hence why in the opening salvos of this thread we had comments asking what SAC even was. Most people don't know it.

You can plan a dive without something as convoluted as Rock Bottom. It's so dirt simple. Take out the 500psi your supposed to come back with add a little for you safety stop (say 300psi) and then divide by 2; to get your turn around.. Easy done. And that will leave enough gas for you and your buddy for safety stop. My OW students can understand this.


The first time you have a dyslexic student, or a 10 year old student, you'll understand why this method is far more efficient

"No OW courses teach SAC/RMV." - Really ... is that a fact?

 

[Pullmyfinger]

Sorry, if you chose to use it, you take flack for it.

"No OW courses teach SAC/RMV." - Really ... is that a fact?

Truth.

 

[Pullmyfinger]

I teach my students to plan dives easily.

Rock bottom is completely useless for the the newbie dive.

Why?

No OW courses teach SAC/RMV. Hence why in the opening salvos of this thread we had comments asking what SAC even was. Most people don't know it.

You can plan a dive without something as convoluted as Rock Bottom. It's so dirt simple. Take out the 500psi your supposed to come back with add a little for you safety stop (say 300psi) and then divide by 2; to get your turn around.. Easy done. And that will leave enough gas for you and your buddy for safety stop. My OW students can understand this.


The first time you have a dyslexic student, or a 10 year old student, you'll understand why this method is far more efficient

I know a former YMCA (now SEI) instructor that taught SAC/RMV during open water.
Not everyone is reading from the same playbook.

Not bashing you.......I'm just of the opinion that these things ARE basic concepts.

If you could change the OW curriculum, wouldn't you include this?

-Mitch