Air Question

[txapacheguy]

What is the hand sign for "I am bored" Is it different from "I am your wife and I am bored"

About the same as the signal to end any other dive, but with the addition of that unmistakable glaring look that most of us are all too familiar with...

 

[Charlie99]

Doesn't that mean from 30ft, you head back up at 600

SPGs aren't always accurate. 500psi on the gauge might not really be 500psi. The typical intermediate pressure is around 140psi, so if your tank goes below 140psi (plus another 15psi for 33' depth) your regulator performance will really fall off. The amount of air you need to safely get both you and your buddy to the surface while sharing air is both dependent upon depth and also for how long you hang around sorting stuff out after his reg fails.

Put it all together and you need a fixed allowance of a few hundred psi independent of depth. (300, 500, 800psi .... reasonable arguments can be made for each)

So at 120' I might be happy with just 1200psi with a known buddy that is calm under pressure, or I might want 1500 psi minimum.

What has more variability is what I'm willing to have at 30'. When I'm solo and it's benign conditions, I might go below 500 before heading up, since I do a 3 minute safety stop using less than 100psi out of an AL80. More likely, if I'm with a buddy, I won't let my tank get below 800 or so.

I posted a rule of thumb that is a good guide for starting. Even the fully calculated in detail rock bottom numbers are precise calculations that are based upon crude, variable, unreliable assumptions.

The important thing to understand is the concept that you are carrying your buddy's spare gas, and he has yours. And you need to have enough at any point in the dive to be able to safely abort it while both you and your buddy are breathing off of one tank.

Dive smart. Dive safely.

 

[TMHeimer]

Not sure that this is really advice that should be in the new divers forum. Whilst we all know that air does not stop being dispensed when one hits 500 PSI anything under 500 should be purely for contingency and should never be part of a dive plan. Perhaps when the OP is more experienced he can possibly experiment with hanging out at 20 feet enjoying the drift at 400 or 450 PSI just because he doesnt feel like getting back on the boat but as part of a dive plan that 500 PSI mark should be sacred

Yes, you're right. I shouldn't have posted that advice on the New Divers forum. And there is always the possiblity your SPG isn't quite accurate. Experimenting with this in 10 feet of water would let you know--something I probably did years ago. There is a big difference between a benign lake dive and a 70 foot boat dive, but that is something the OP would probably figure out by himself. Having said that, I still personally would not exit Vortex Spring with more that 200 PSI unless I was bored (which can happen there). Would you still exit with 500?

 

[markr]

...Note that these are "ascent pressures" that are only about you being able to safely get back to the surface. You also need to figure out the "turnaround" pressure, which has to do with when you start heading back towards the boat.

Of the two, ascent pressure and turnaround pressure, agreeing on the ascent pressure ahead of time with my dive partner always seemed more important to me. Surfacing twenty feet from the dive ladder gets you style points which ultimately count for very little. Getting safely back to the surface lets you dive tomorrow,

 

[Charlie99]

Of the two, ascent pressure and turnaround pressure, agreeing on the ascent pressure ahead of time with my dive partner always seemed more important to me. Surfacing twenty feet from the dive ladder gets you style points which ultimately count for very little. Getting safely back to the surface lets you dive tomorrow,

To extend this thought a bit, I always distinguish between being able to safely COMPLETE a dive in the intended manner, and being able to ABORT the dive in reasonable safety. The turnpoint pressure is needed to complete a dive as planned. The ascent pressure or rockbottom pressure is needed to be able to safely abort a dive.

I'll trust a DM to lead me to the most interesting stuff and I'm willing to depend upon the DM to make it so we complete the dive in the intended manner. But I will never do a dive where I cannot by myself safely ABORT the dive.

The term "trust me" dive is often used on Scubaboard. I'll trust someone else to help me do a dive that I might not be able to do otherwise, but I will never knowingly put myself in a position where I need someone's help to safely abort a dive.