How much air to surface with?

[gopbroek]

I use a MK25 and it will draw the tank down to zero differential pressure, which when done at a SS level will still provide a few PSI positive pressure in the tank on the surface to prevent moisture ingress. I have done this a couple times just to test it and see how the 2nd stage reacts. I learned that the draw on the second starts feeling noticeable different at the IP point of the 1^st stage and draws increasingly harder as the pressure continues to drop, which I feel is good knowledge to have. I have also periodically measured my IP pressure with an independent gauge, to see if it is starting to creep (indicating service time) and know it to be currently steady at 130 PSIG. Armed with these two pieces of info, periodically, I will hang out on an extended SS and draw the tank down till I can feel the difference when the IP set point is reached and then cross check the readout on my A/I computer and backup SPG. The A/I is spot on and the SPG reads just at zero, which I feel is good info to know.

 

[AdivingBel]

In the setup on my Atomic Coblat, it says reserve 300 psi. I have looked din the manual and can't seem to find an answer. Does that mean when the Cobalt reads zero psi I have 300 psi left?

I think it would mean when calculating dive time left, it assumes you want to surface with 300 PSI and calculates the "time" accordingly. I would hope the gauge portion would continue to read as accurately as possible.

 

[NAM001]

I think you are right. I think i ran mine at 300 till i went low on air because of sensor errror. Then i sent the xmitter in for sensor replacement and used 500 form that point. Those xmitter errors is why i went to an spg. That and one time i set up my gear the night before shut the air off, but did not vent the reg enough times and the psi built up and the xmitter ran all night till the battery died.

I think it would mean when calculating dive time left, it assumes you want to surface with 300 PSI and calculates the "time" accordingly. I would hope the gauge portion would continue to read as accurately as possible.

 

[dmaziuk]

My 1st and only real life CESA was from 70FSW no bends. I exceeded the then recommended 60' per min ascent rate but I didn't drown. No matter how long a drowned diver is in a chamber they are still dead.

A diver either has too much N dissolved in the blood to ascend non-stop or not, SS are optional. No one ever got bent by blowing off a SS on an NDL dive that I know of.

Oh, I get all that. What I meant was, based on my cursory skimming of googlepidea I'd think you would have your 4 min compartment close to saturated after ~15+ min at 80ft. Which is where you should be if you breathed a whole Al80 dry at that depth. I also understand that coming up fully saturated from 20-mumble feet (whatever 1.58 adds up to) is theoretically perfectly safe. That's where my understanding ends, I've no idea how that translates to CESA'ing up from 80' with only the fast tissue compartment 89% saturated. Some day I'll read Deco for Divers and may be able to figure out what should happen there in theory. (Of course what might happen to a real diver IRL is a different story, as they say in my profession "in theory, theory and practice are the same".)

 

[Michael Guerrero]

I don't think anyone has ever actually asked me how much is still in my tank. But I think the boats I've dived on are not uptight like that. I figure as long as I'm still breathing off it I'm OK. After all, I paid for the air in it.

 

[Seamutt]

For what it's worth I'm in the 500 camp. I recall several instances where more air in the tank was useful.
The first was a rough day at the ladder and the person in front of me took a fall into the ladder while trying to get on it and sheared off the inflator hose and knocked off of and away from the ladder. I grabbed them as they headed down and had plenty of air to fully inflate hold both of us up and air to breathe while they got rid of excess ballast and generaly got their act together.

The next was a "technical' diver who was going back in as I was coming out, He was yelling a a bunch of kids playing in the area and steped off in deep water, bobbing just below the surface, no fins, no reg in mouth as he approached the water, no inflator hose connected, no drysuit connected and tanks off. dog paddled over to him and maxed out my BC gave him a reg and pulled him ashore. Having plenty of air kept this as a funny story instead of a hair raising event, at least from my perspective. He got back in his car and left, not even a "thanks"... go figure.

Item three: was a guy who connected with the ladder with his head in very rough conditions, he surfaced in good shape with air in tank but did not need BC to float at that minute. Hit on the head changed all than and the first person to him hit the inflator and was able then to flip him on his back and tow him in.

Then there was the guy having the heartattack..... enough of that you get my drift.

My point being, its not all about breathing specifically its the entire risk management concept.

 

[Storker]

How much to surface with is one thing. Minimum pressure at a given depth is another. After doing a few min gas calculations, I know roughly my min pressure at different depths (e.g. if I'm at 30m with a 10L, I want my SPG to show at least 110 bar, preferably 130). If I'm below that, I'd better have started on the shallower section of my dive. I try to reach my SS with at least some 50 bar left (40 if I'm on a 15L).


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[dumpsterDiver]

For what it's worth I'm in the 500 camp. I recall several instances where more air in the tank was useful.
The first was a rough day at the ladder and the person in front of me took a fall into the ladder while trying to get on it and sheared off the inflator hose and knocked off of and away from the ladder. I grabbed them as they headed down and had plenty of air to fully inflate hold both of us up and air to breathe while they got rid of excess ballast and generaly got their act together.

The next was a "technical' diver who was going back in as I was coming out, He was yelling a a bunch of kids playing in the area and steped off in deep water, bobbing just below the surface, no fins, no reg in mouth as he approached the water, no inflator hose connected, no drysuit connected and tanks off. dog paddled over to him and maxed out my BC gave him a reg and pulled him ashore. Having plenty of air kept this as a funny story instead of a hair raising event, at least from my perspective. He got back in his car and left, not even a "thanks"... go figure.

Item three: was a guy who connected with the ladder with his head in very rough conditions, he surfaced in good shape with air in tank but did not need BC to float at that minute. Hit on the head changed all than and the first person to him hit the inflator and was able then to flip him on his back and tow him in.

Then there was the guy having the heartattack..... enough of that you get my drift.

My point being, its not all about breathing specifically its the entire risk management concept.

I think your stories also validate the benefit of having some excess capacity with regard to lift in your BC... it can be very useful when trying to help others..

 

[stevensamler]

I think your stories also validate the benefit of having some excess capacity with regard to lift in your BC... it can be very useful when trying to help others..

Redundancy , you can never have enough of it and never dive without it.

 

[stuartv]

I think your stories also validate the benefit of having some excess capacity with regard to lift in your BC... it can be very useful when trying to help others..

Redundancy , you can never have enough of it and never dive without it.

Are y'all advocating that everyone should dive with redundant buoyancy? So, for wetsuit divers, they should all use dual bladder BCs/wings?