couple of questions

[sea-squirt]

Hey
I'm new enough to diving (ow +8 dives) but have had a few problems diving. they probably sound stupid but any help would be great..

first question is, are you supposed to dump all your air out of your bcd when you decide to ascend and swim up, or do you do it gradually?

another question is about air supply. I was at 20m/66ft when I realized we had gone a bit too deep and then noticed i was running low on air (needle in the red caution zone in depth guage) but can't remember exactly what air I had. so signalled to one of my two buddies I wanted to ascend.

My second buddy had swam down deeper (23m) in the meantime but I didn't want to swim down after her to get her so the 3 of us could go up as i know you use more air the deeper you go. so I signalled I was going to ascend alone and for those two to surface together. when I got to the surface after slowly ascending I had 50bar in my tank and they argued that was a normal amount to have and that I was too hasty to suface. But I'm convinced that I had less air when at 20m.Would my guage have shown less air at 20m because I was under 3 bar/ata at the time? considering you have 1/3 of the air volume when at that depth.

I know you should never leave a buddy, and I didn't want to, but made a decision and went with it. Did I do the wrong thing? this is driving me nuts.

 

[speedhound]

Hi Squirt
When ascending (and any other time) you only need as much air in your BC as it takes to maintain neutral buoyancy. If you keep too much air in your BC as you ascend, it expands (Boyles Law) and you accelerate towards the surface. Personally at 20ft I have very little or no air in my BC anyway. As long as you make a controlled ascent (<30ft/min) it makes little difference whether you have an empty BC, or are venting it on the way up.

Depending where you dive the aim is to get back on the boat/shore with 500psi/50bar in your tank. Your tank pressure won't change too much just because of depth - other than breathing off it (more usage at deeper depths). There might be a miniscule pressure increase if the water was warmer the shallower you went (Charles' Law).


Diving with two buddies can be a problem, one wants up, one wants down, the other is in the middle. Did you make any arrangements/plans before you hit the water, especially to cover someone leaving early? I sometimes dive a threesome, one of us is an air-hog, so we know to watch for him leaving early and especially in lakes, will keep stay shallow (20') to watch him make his ascent and leave the water.

As long as everyone knew what was happening (you were leaving early) AND you have enough experience to be safe ascending by yourself, then you shouldn't be so hard on yourself.

If there wasn't a plan in place, or you weren't happy by yourself, then perhaps you need to take action to make plans accordingly before your next dives.
Dive Safe and Have Fun!!

 

[sea-squirt]

thanks for that speedhound. Yeah i don't think I'd be in a rush to dive with three again, I usually only dive with the one buddy, so didn't really think to discuss what we'd do if one of us needed to go up when there was 3 of us. Anyway it's a lesson for the next time I suppose- definately need more planning.

 

[speedhound]

No problemo..threesomes can still be fun though

Oh, and there is no such thing as a dumb question...other than the ones you don't ask

 

[TSandM]

In answer to the first question, you want to vent air from your BC and/or dry suit gradually as you ascend. If you dump all your air at depth, you might have to swim pretty hard to get up. The ideal is to keep your buoyancy very close to neutral all the time. That way you can control your ascent with your breath, and stop it at any point if you need to. It takes time and practice to learn how to control venting to produce a satisfactory ascent rate and to make safety stops.

In answer to the second question, your pressure gauge is reading pressure, not volume. The gas in your tank is contained, and its volume doesn't change as you descend (the tank is not compressible). Therefore, neither the pressure in the tank nor its volume is dependent upon your depth.

You may have misread the gauge, or possibly the red zone on your gauge begins at 50 bar. Does it?

Finally, any diver has the right to thumb a dive at any time, and you need not justify your decision to your buddies. Diving in a team of three can be a great way to do things, but you need some skills and some agreements to do it. You need to agree to stay together -- there is nothing worse than seeing one buddy swim off one way and the other another, and wondering which you should stay with. You need to agree that the first one to reach ascent pressure ends the dive, and all of you ascend together (or, if you are going to do something different from that, it must be discussed and agreed upon ahead of time). One of the things I didn't get from my OW class was much of a sense of a pre-dive discussion, and I'll offer this mnemonic:

S - Sequence. Who is leading? How is everybody going to be positioned? What signals are you going to use? If there are any tasks involved in the dive (sending up a marker buoy, etc.) who is going to do them, and at what point in the dive? This is where you would talk about working as a threesome, and staying together and how you are going to do that.

A - Air. How big are the tanks? How much pressure is in them, and how much gas does that translate into? Who has the least? How much gas can you use on the dive (do you have to get back to someplace, or can you surface at any point)? What's turn pressure for each diver, or the pressure at which you will ascend?

D - Depth. How deep is the proposed dive? What's the max depth you'll set for the dive?

D - Direction. Where are we going? This can include a discussion of site topography and compass headings. All divers should have a good enough idea of where they are to get back to the starting point, and everybody should stay aware of where you are and where you've been, in case the person responsible for navigating gets confused -- it happens!

D - Duration. How long do you expect to be in the water? This is where you might learn that one of your buddies has a high air consumption and doesn't normally manage a dive that long, or that somebody usually gets cold before that point.

D - Deco. What's the strategy for ending the dive? 3 minute safety stop at 15, or some other stop sequence, or simply swimming upslope?

If you go through this list before each dive, you'll minimize misunderstandings and optimize your communications underwater.

The mnemonic is a good idea, because otherwise you forget things. I always forgot to ask who was leading the dive . . . which leads to great confusion in short order once you get underwater!

 

[sea-squirt]

Hi TS and M
I'm not sure about the red zone on the SPG as I only rent the equipment, but I'll look at it the next time.Thanks for the great advice I really appreciate it.

 

[vondo]

What others have said about venting your BC is right, do it gradually as you come up to make sure you neither get too negative or too positive. But, if you have more than a little air in your BC when you near the end of the dive (when your tank is at its lightest) then you are carrying too much weight. So, you can try to lose some. Rental gear can vary a little bit, but you should be able to figure out what you need within a kilo or so. Getting rid of extra weight will improve your diving and the length of your dives too.

 

[jiveturkey]

You need to plan your dive a little better. You should have an agreed upon air pressure at which to end the dive. Also, you shouldn't really be at 20m with 50 bar left in your tank. Generally, as I get lower on air, I gradually move up into shallower water. It increases your margin for safety and extends your dive because you'll be using less gas.