I was just reading a post by TS&M on another thread and I wanted to be sure I understand her example of gas management scenarios. I haven't even started training yet but I like to learn before I'm taught so that I can be sure to retain as much information and ask questions while I'm learning. Sooooo if you'll indulge me please...
Easy so far.
That all makes perfect sense - no questions so far.
Quick examples:
Makes Sense - got it.
Makes sense - got it.
Where did the 900 psi come from? If I understand the above it should be 40/3.5 =11.4 yielding 1200 psi to be conservative. Or is it that 60 fsw=20 cu ft, 100 fsw=40 cu ft so 80 fsw=>30 cu ft and 9x3.5= 31.5? I'm figuring that this is it but it wasn't specifically explained that way. Please don't get me wrong, I LOVE TS&M's posts, they are always very intelligent and well written, so I'm not flaming her, just trying to make sure I understand correctly.
So if I'm right above then I understand this but I have a question:
Does the 60 fsw=20 cu ft, 100 fsw=40 cu ft take into account the fact that being a new diver I'll be sucking a lot of air, and if both divers are new, and we're sharing air due to a problem we'll be using a TON of air? In other words, do I have to be even more conservative if it's a situation where longer underwater travel is required.
Thanks much.
...gas management at the simple recreational dive level basically breaks down into three steps.
First, determine your "rock bottom" or minimum gas. That's the reserve you need to carry to get you and your buddy to the surface from the deepest part of the dive. A reasonable rule of thumb is 20 cu ft for 60 fsw, and 40 from 100fsw. Knowing the volume, you can figure what proportion of your tank it is -- for example, 40 cu ft is half of an Al80 which is normally filled to 3000 psi, so it's 1500 psi. If you know the tank factor for your tank, you can figure it that way.
So -- you take out the rock bottom, and figure out how much gas you have left. That's your "usable" gas.
Easy so far.
Now you have to decide how you are going to portion out that gas. If you are diving off a charter boat that will come and get you wherever you end up, you can use it all! Your rock bottom is plenty to get you to the surface, even if you have to share with a buddy. So you head up when you hit rock bottom.
If you are shore diving, most of the time, you would prefer to get back to where you started underwater, rather than do a long surface swim. But you CAN do the long surface swim if you have to. So you divide your usable gas in half. Whatever number you come up with, you add that to your rock bottom, and that's your turn pressure (you turned when you had enough gas to get back to the shore, and to ascend if you needed to).
If you are in a position where you MUST get back to your starting point, then you have to consider that you might have to bring your buddy back there on your gas, AND do the ascent on your gas, so you have to divide that usable gas into thirds. There aren't many situations where open water divers are faced with these constraints, but diving wrecks in high current off an anchored boat is one of them.
Don't make it too complicated in your head. It's really simple. Put away a safety reserve that you won't use. Then decide whether the dive is one way, two way, or "absolutely have to get back to the upline". That tells you how to divide the rest.
That all makes perfect sense - no questions so far.
Quick examples:
I take it that tank factors are something that I'll look up somewhere and that larger or higher pressure tanks hold more gas so they have higher tank numbers.Diving off a Bandito charter at Sunrise. Max expected depth is 60 fsw. You're diving an LP95, with a tank factor of 3.5 (rounded off for easy math).
You need a safety reserve of 20 cu ft, which is roughly 600 psi (6 x 3.5 = 21). So you put 600 psi away. The boat will come and get you wherever you surface, so you can use your tank down to 600 psi, so you have 2000 psi usable (assuming you got the 2640 fill the tank is rated to). You have no turn pressure. You ascend at 600 psi, or when you get cold, whichever comes first.
Makes Sense - got it.
Diving at Edmonds. Max expected depth is probably 33 fsw, and rock bottom is never less than 500 psi, so that's what it will be for this dive. This is a halves dive, because you'd really LIKE to swim back to the shore, although if you have to surface, you can. So you take that 2640 you have, remove the 500 psi reserve, and that gives you 2140 -- rounding down to 2000, you have 1000 to use going out, and 1000 to use coming back. So add the 1000 you need to come back to the 500 you aren't going to touch outside of an emergency, and you have a turn pressure of 1500 psi.
Makes sense - got it.
Diving off the Dash on the Possession Point ferry. Max depth is 80 fsw, and the boat is anchored, and surface currents can be fierce. So if you have a problem at depth, you would like to share gas to the upline and ascend there if you possibly can. So, using the 100 fsw value of 40 cu ft, you need to set aside 900 psi for rock bottom.
Where did the 900 psi come from? If I understand the above it should be 40/3.5 =11.4 yielding 1200 psi to be conservative. Or is it that 60 fsw=20 cu ft, 100 fsw=40 cu ft so 80 fsw=>30 cu ft and 9x3.5= 31.5? I'm figuring that this is it but it wasn't specifically explained that way. Please don't get me wrong, I LOVE TS&M's posts, they are always very intelligent and well written, so I'm not flaming her, just trying to make sure I understand correctly.
That leaves you with 1700 psi usable. Now you want to divide that into thirds, and you have to make a decision. 1700 isn't divisible by 3, so you either have to call it 1500 or 1800 to make the math easy. 1800 reduces your safety cushion; 1500 is very conservative. What you do may depend on how much you trust your buddy, how much experience you have, or how worried you are about drifting in the open water until the boat finds you Say we want to be conservative and careful -- we'd call our outbound leg 500 psi, and turn at 2100.
So if I'm right above then I understand this but I have a question:
Does the 60 fsw=20 cu ft, 100 fsw=40 cu ft take into account the fact that being a new diver I'll be sucking a lot of air, and if both divers are new, and we're sharing air due to a problem we'll be using a TON of air? In other words, do I have to be even more conservative if it's a situation where longer underwater travel is required.
Thanks much.