(Author Update) This old thread discusses the fallacy of "dive planning" which consists merely of "Return to the boat with 500 psi."
It gives you a tool to do deeper dives safely as you increase your diving experience after getting your OW certificate...Rock Bottom Pressure.
Every boat trip, the crew will brief the site before splashing, and finish with the warning, "Be back on the boat with no less than 500 psi in your tank!" Some boats are more dogmatic about it than others, and the consequences of surfacing with less will usually depend upon whether you also screwed up in some other way to give you less than their required minimum.
Why 500 psi? Well, an absolutely empty tank might also mean that it could have been contaminated with salt water during your OOA ascent, prompting the need for a new visual inspection before putting the tank back in service. But that's not really what concerns the boat crew.
The bigger reason, of course, is that the crew wants you back safely, and mandating 500 psi means that for an average diver, on an average dive, to average depths, you can get into a little trouble at depth, get yourself out of trouble, ascend (skipping your not-mandatory safety stop) and reach the surface with the 150 psi or so your regulators need to function properly.
How long can you take at depth (eating into your reserve) before things end badly?
Well, that all depends on your SAC/RMV. There are lots of threads on ScubaBoard discussing SAC and RMV if you use the Search function, and you will receive some training on it in your Advanced Open Water course, if not in your initial Open Water class.
But basically, Surface Air Consumption (SAC) is how many psi per minute of air you breathe during a relaxed scuba swim, at the surface. If you recall your Open Water Theory, at 33ft the ambient pressure is doubled, the air dispensed by the second stage is twice as "thick", and you will therefore go through twice as many psi per minute with the same volume of air breathed. At 99 feet, you will go through four times as much gas as at the surface.
If you think about it, a diver with a tiny tank will have a faster psi drop than a diver with a giant tank, for the same volume of gas breathed. Therefore SAC numbers are tank-dependent, so most divers convert the SAC psi from a test dive to liters per minute, or cubic feet per minute, as that value will be constant from tank to tank. That conversion is known as RMV, or Respiratory Minute Volume. You really want to know what your RMV is as you start diving a little deeper.
Why? Because ascending from 45 feet using an AL80 tank and doing a safety stop might require 200 psi for a new diver, so leaving the bottom with 700 psi will get you back on the boat with your required minimum. But finishing your dive by getting caught in some fishing line at 80 feet, getting a little stressed and taking 2 minutes to find your cutting tool and free yourself, and only then starting a normal ascent might require 875 psi even if you skipped your safety stop! If you used your usual 700psi as your "leave the bottom minimum", you wouldn't even make the surface with any air left, much less have 500 psi in the tank.
So how do you plan your dive to surface safely, and only after that arrive with 500 psi in the tank so the crew isn't mad at you?
First, measure your SAC.
Next, calculate your Rock Bottom Pressure. Rock Bottom is the pressure corresponding to the amount of gas you need to ascend and surface, with or without solving a problem, and with or without an OOA buddy. It varies with the depth of your dive, and the length of time needed to solve your problem.
This 7-year-old thread by @scubadada has exceeded 50,000 views, and is still generating contributions!
Average Gas Consumption
In it, you can use others' experience to guess what your SAC might be, until you can get around to measuring it yourself.
Then, using your SAC, you calculate a Rock Bottom Pressure at which you must leave the bottom and ascend. If not, you are now going into the reserve calculated for an emergency. If you are instead using that reserve in the middle of an emergency, you can't exceed ANY of the parameters you set, or you will not have enough gas to surface safely.
Attached are two simple spreadsheets (one Imperial, one metric [thank you, @stepfen!]) which will calculate your Rock Bottom Pressure, after entering
a) your "normal" RMV;
b) whether or not you are supposing a temporary stress-induced increase in your gas consumption (2x, 3x, 4x or more?);
c) whether there is a problem which requires "x" minutes at max depth to solve before you can start your ascent;
d) how little true tank pressure you really need to have upon surfacing (default is 150 psi);
e) whether the problem was with your buddy, who got stuck, went OOA or had an equipment malfunction, and is now using your gas via your octo to ascend.
They make some conservative assumptions, like stressed RMV/SAC gradually returning to normal as you approach the surface, and both divers being stressed to the same degree during an air share. But if anything, that may overestimate the gas pressure you need to start with, and will therefore still keep you safe.
After playing with this tool, you'll quickly find that the pressure you need to safely deal with a problem and ascend even without a safety stop is often more that the 700 psi limit you may have become accustomed to using as "the end of the fun." The starting values in the spreadsheet below are for an average new diver spending 60 seconds to solve a scary problem at 80' before starting up.
Making a safe ascent, but skipping the safety stop still requires more than 700 psi to safely arrive at the surface with 150 psi left in your tank for one diver! There's not enough for an OOA buddy; there's not enough for a problem that takes more than 60 seconds; there's not enough if you're at 90 feet.
It's food for thought, when you contemplate the shift from 35' reef dives to your first deeper foray with your buddy. Play with the numbers. Write your Rock Bottom on your slate, every dive! Always leave that max depth when you hit that pressure, or know that the little emergency you contemplated can no longer be dealt with successfully. It's why the glamorous Tech diving community always returns with a full 1/3 of their gas unused, and new cave divers return with 2/3 unused! Even little problems at depth can use up lots of precious air.
EDIT: Here are the conventions for the spreadsheets. They're locked with the password "scuba", so you can change formulas if you choose. The white cells are user adjustable without unlocking.
The deep ascent rate is user adjustable: below 60 feet/18 meters, the spreadsheet allows you to enter an increased ascent rate if you believe that in an emergency, 60fpm/20mpm may be safe enough. It drops to the standard 30fpm/10mpm above 60 feet/20m. My thanks to the usual offenders for their suggestions for improving the toy.
It gives you a tool to do deeper dives safely as you increase your diving experience after getting your OW certificate...Rock Bottom Pressure.
Every boat trip, the crew will brief the site before splashing, and finish with the warning, "Be back on the boat with no less than 500 psi in your tank!" Some boats are more dogmatic about it than others, and the consequences of surfacing with less will usually depend upon whether you also screwed up in some other way to give you less than their required minimum.
Why 500 psi? Well, an absolutely empty tank might also mean that it could have been contaminated with salt water during your OOA ascent, prompting the need for a new visual inspection before putting the tank back in service. But that's not really what concerns the boat crew.
The bigger reason, of course, is that the crew wants you back safely, and mandating 500 psi means that for an average diver, on an average dive, to average depths, you can get into a little trouble at depth, get yourself out of trouble, ascend (skipping your not-mandatory safety stop) and reach the surface with the 150 psi or so your regulators need to function properly.
How long can you take at depth (eating into your reserve) before things end badly?
Well, that all depends on your SAC/RMV. There are lots of threads on ScubaBoard discussing SAC and RMV if you use the Search function, and you will receive some training on it in your Advanced Open Water course, if not in your initial Open Water class.
But basically, Surface Air Consumption (SAC) is how many psi per minute of air you breathe during a relaxed scuba swim, at the surface. If you recall your Open Water Theory, at 33ft the ambient pressure is doubled, the air dispensed by the second stage is twice as "thick", and you will therefore go through twice as many psi per minute with the same volume of air breathed. At 99 feet, you will go through four times as much gas as at the surface.
If you think about it, a diver with a tiny tank will have a faster psi drop than a diver with a giant tank, for the same volume of gas breathed. Therefore SAC numbers are tank-dependent, so most divers convert the SAC psi from a test dive to liters per minute, or cubic feet per minute, as that value will be constant from tank to tank. That conversion is known as RMV, or Respiratory Minute Volume. You really want to know what your RMV is as you start diving a little deeper.
Why? Because ascending from 45 feet using an AL80 tank and doing a safety stop might require 200 psi for a new diver, so leaving the bottom with 700 psi will get you back on the boat with your required minimum. But finishing your dive by getting caught in some fishing line at 80 feet, getting a little stressed and taking 2 minutes to find your cutting tool and free yourself, and only then starting a normal ascent might require 875 psi even if you skipped your safety stop! If you used your usual 700psi as your "leave the bottom minimum", you wouldn't even make the surface with any air left, much less have 500 psi in the tank.
So how do you plan your dive to surface safely, and only after that arrive with 500 psi in the tank so the crew isn't mad at you?
First, measure your SAC.
Next, calculate your Rock Bottom Pressure. Rock Bottom is the pressure corresponding to the amount of gas you need to ascend and surface, with or without solving a problem, and with or without an OOA buddy. It varies with the depth of your dive, and the length of time needed to solve your problem.
This 7-year-old thread by @scubadada has exceeded 50,000 views, and is still generating contributions!
Average Gas Consumption
In it, you can use others' experience to guess what your SAC might be, until you can get around to measuring it yourself.
Then, using your SAC, you calculate a Rock Bottom Pressure at which you must leave the bottom and ascend. If not, you are now going into the reserve calculated for an emergency. If you are instead using that reserve in the middle of an emergency, you can't exceed ANY of the parameters you set, or you will not have enough gas to surface safely.
Attached are two simple spreadsheets (one Imperial, one metric [thank you, @stepfen!]) which will calculate your Rock Bottom Pressure, after entering
a) your "normal" RMV;
b) whether or not you are supposing a temporary stress-induced increase in your gas consumption (2x, 3x, 4x or more?);
c) whether there is a problem which requires "x" minutes at max depth to solve before you can start your ascent;
d) how little true tank pressure you really need to have upon surfacing (default is 150 psi);
e) whether the problem was with your buddy, who got stuck, went OOA or had an equipment malfunction, and is now using your gas via your octo to ascend.
They make some conservative assumptions, like stressed RMV/SAC gradually returning to normal as you approach the surface, and both divers being stressed to the same degree during an air share. But if anything, that may overestimate the gas pressure you need to start with, and will therefore still keep you safe.
After playing with this tool, you'll quickly find that the pressure you need to safely deal with a problem and ascend even without a safety stop is often more that the 700 psi limit you may have become accustomed to using as "the end of the fun." The starting values in the spreadsheet below are for an average new diver spending 60 seconds to solve a scary problem at 80' before starting up.
Making a safe ascent, but skipping the safety stop still requires more than 700 psi to safely arrive at the surface with 150 psi left in your tank for one diver! There's not enough for an OOA buddy; there's not enough for a problem that takes more than 60 seconds; there's not enough if you're at 90 feet.
It's food for thought, when you contemplate the shift from 35' reef dives to your first deeper foray with your buddy. Play with the numbers. Write your Rock Bottom on your slate, every dive! Always leave that max depth when you hit that pressure, or know that the little emergency you contemplated can no longer be dealt with successfully. It's why the glamorous Tech diving community always returns with a full 1/3 of their gas unused, and new cave divers return with 2/3 unused! Even little problems at depth can use up lots of precious air.
EDIT: Here are the conventions for the spreadsheets. They're locked with the password "scuba", so you can change formulas if you choose. The white cells are user adjustable without unlocking.
The deep ascent rate is user adjustable: below 60 feet/18 meters, the spreadsheet allows you to enter an increased ascent rate if you believe that in an emergency, 60fpm/20mpm may be safe enough. It drops to the standard 30fpm/10mpm above 60 feet/20m. My thanks to the usual offenders for their suggestions for improving the toy.
