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?
Because you are taking a huge risk if you resurface with less gas in a single 80 cuft tank.
Do you fully realize how little reserve gas 500 psi is in an 80 cuft tank, would something go Seriously wrong?
When I resurface I will have 725 to 1015 psi in BOTH OF MY TANKS (more than three times the gas you are talking about).
I have done dives where there was ample gas when resurfacing, and I have done dives where there was around 0 psi at depth. Now, try to guess which ones were more pleasant. You only live once, understand?
We could calculate the actual PSI value based on max depth and your (1.5x) and your buddys (3x) RMV during panic, but those details do not change the fact that 500 psi is a very very small amount of gas.
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.
Yeah. It is the near death experience that concerns the crew.
The bigger reason, of course, is that the crew wants you back safely,
Correct
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,
A LITTLE,
not much
reach the surface with the 150 psi or so
and that is NOT a lot.
your regulators need to function properly.
It is beneficial, yes
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.
SAC/RMV depends on your breathing rate, not necessarily on your gas exchange NEEDS. The mental aspect is important.
Your consumption of oxygen is one thing.
Your tolerance of carbon dioxide is another thing.
It is complicated.
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.
Correct
as that value will be constant from tank to tank.
RMV, yes.
That conversion is known as RMV,
I have never been interested in psi/minute values. When 1/3 is used, I will turn around.
or Respiratory Minute Volume. You really want to know what your RMV is as you start diving a little deeper.
Yes, it is really helpfull in planning.
Why? Because ascending from 45 feet using an AL80 tank and doing a safety stop might require 200 psi for a new diver,
it is also possible to ascend from 60 feet with no air whatsoever, and it is not all that bad (I have done it), but it will require a mental calm - certainly not possible 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.
I did not do the calculations, but this seems correct. A minor problem already could waste A LOT of gas.
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?
Someone being mad is irrelevant. Being safe is.
It's not about the crew being mad. It's about you and your buddy surviving. Not all do.
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.
No. It is "with an OOA buddy with leg cramps and near panic" which would mean 1,5x (you) and 2,5x (him) RMV. Reality tends to be complicated.
It varies with the depth of your dive, and the length of time needed to solve your problem.
It does.
This 5-year-old thread by
@scubadada is approaching 20,000 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.
Rock Bottom is a nice concept. I prefer though to make sure I will return alive. There are so many things affecting this in addition to depth and time.
And, I’d like to see the DM’s face when I was diving a Trieste II double hose regulator from the 1970s with my doubles.
