34 replies to this topic

[Racer184]

cold exposure
diver below average physical fitness level
diver fatigue
diver age
diver dehydration
stress
obesity
and previous history of DCI

Hmmm... no matter how many times I work the math, that fourth item on your list keeps working against me ---- and it gets worse every year. Twenty years ago, it wasn't much of a factor, now it is. What am I doing wrong?

[peterbj7]

500psi is a nice round number but there's nothing sacrosanct about it. In the UK the norm for back on the boat is 50bar, which is around 750psi. That allows for the fact that diving conditions there are tougher than most warm water recreational divers see. I imagine that in northern US or Canadian waters the same higher safety margin would apply.

The reserve is largely adhered to for two reasons - (1) the pressure gauge may not be accurate, so having a decent indicated margin is prudent, and (2) there may be rough water up top and perhaps a lengthy wait for pickup. You need to be able to breathe from your regulator without any fear of it running out. Actually there's a third - there's a good chance you'll be sitting in the water for some time (waiting while other divers are picked up) in engine exhaust fumes. In those seas there's no question of shutting down the engine. It's a LOT more healthy if you have a regulator to breathe from.

Edited by peterbj7, 23 August 2010 - 10:24 AM.

[Landlocked Dive Nut]

cold exposure
diver below average physical fitness level
diver fatigue
diver age
diver dehydration
stress
obesity
and previous history of DCI

Hmmm... no matter how many times I work the math, that fourth item on your list keeps working against me ---- and it gets worse every year. Twenty years ago, it wasn't much of a factor, now it is. What am I doing wrong?

Not a thing, my friend! Fortunately that number keeps getting bigger....'cuz the alternative means you're done diving in this physical realm.

You can negate some (but not all) of the age factor by staying in good health and physical condition....

....and diving just a bit more conservatively than you did when you were 20!

[PerroneFord]

Two things.

1. I see people are disagreeing with this rather simplistic rule and dismissing it because it doesn't take personal SAC rates into effect. Fair enough. But the danger in this is not that it doesn't take SAC rate into effect, it's that it leaves out the CRUCIAL element in gas planning. And that is, that gas planning isn't about what Diver A's SAC rate is, or Diver B's SAC rate. It's about the COMBINED SAC rate of the divers. Assuming that we are following standard practices of actually having a dive buddy. If you're at 100ft on the Spiegel Grove, and your buddy's LP hose bursts and drains the tank, will YOU have enough gas to get BOTH of you to the surface safely without having to do a rapid ascent, and while still being able to perform a normal safety stop. Once you take people through the real math of what is required to do this, going to 100ft on an AL80 generally seems like a bad idea. I've done it, and would do it again depending on the dive buddy, but I wouldn't do it with a stranger.

2. If you want something basic that works and takes breathing rates into effect, use the half depth / half gas rule of thumb. Determine your max depth for the dive, and check your starting pressure. Endeavor to be at half your max depth, when either of your tanks is at half pressure. And half again of that, when the tanks are at 1/4. For instance. On a dive to 100ft with 3000psi in the tanks, you want to ascend to 50ft when the person with the least pressure is at 1500. And you want to be at 25ft when the person with the least pressure has 750. This way, you can ignore matching cylinders, ignore SAC rate differences, etc. For basic recreational diving, this will get you to the surface safely, with time for a safety stop, and enough gas to inflate on the surface and breath the tanks back to the boat. Assuming all has gone well. If your depth is shallower, you'll get more time at max depth and more time mid-water. If the dive is deep, you won't have a lot of time deep, but you'll get off the bottom with enough reserve to keep both you and your dive buddy safe.

[drbill]

While it may be a possible rule of thumb for some divers, it is very much dependent on a diver's SAC Rate as Rick pointed out. It also depends on the type of ocean conditions one is diving in. Here in California a tank may last me 67-75% of the time the same tank would last in tropical waters. Even given that, I find the maximum depth I'll dive here in our waters on an 80 is 150 ft and with a 120 it is 200 ft. I do carry a redundanbt air supply but have only needed it on one of my dives.