• Welcome to ScubaBoard


  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

A Case for Spare Air

Discussion in 'Advanced Scuba Discussions' started by certainmisuse, Mar 28, 2019.

  1. 2airishuman

    2airishuman Solo Diver

    # of Dives: 100 - 199
    Location: Greater Minnesota
    2,402
    1,612
    113
    Plan your dive, dive your plan.

    Sometimes I bring two cylinders on a dive. Usually they're the same size, so I maybe have a pair of 72 cf cylinders, or a pair of 100 cf or 120 cf cylinders. Ordinarily when I do this I have a gas plan that involves using gas from each cylinder and some sort of minimum turn pressure. Sometimes the cylinders are part of a twinset, but not always.

    Sometimes the cylinders are different sizes. So I might have, for example, a 120 and a 40. Usually when I do that my gas plan is to start on the 40, stay on it until I reach some minimum reserve pressure -- say 1000 PSI depending on the depth -- and then switch to the 120 for the remainder of the dive.

    I suppose I could start on the 120 and then switch to the 40. It doesn't make much difference from a standpoint of safety or the amount of useful gas. However, it would be necessary to maintain a suitable reserve in the 120, because the gas in the 40 is not sharable since it only has one regulator.

    I don't think there's anything wrong with planning dives in those ways.

    In practice the reserves most divers/DMs plan to with an AL80 are not sufficient to get themselves and an OOA diver to the surface (e.g. 500-700 PSI). Therefore it would be unwise to further reduce these reserves when diving with a pony. (In most cases the low reserves don't matter because most recreational dives involve a group with widely varying RMVs; many include a DM; so there ends up being enough gas to go around). That said, if you actually perform a rock-bottom calculation and plan your dive around the result, I believe it is within the bounds of prudence to plan one diver on the pony for the ascent, which reduces the reserve requirement somewhat.
     
    Coztick, markmud and Bob DBF like this.
  2. 2airishuman

    2airishuman Solo Diver

    # of Dives: 100 - 199
    Location: Greater Minnesota
    2,402
    1,612
    113
    It would be wise to get some instruction.
     
    markmud likes this.
  3. BRT

    BRT Giant Squid

    11,221
    7,240
    113
    Problem with a bigger tank is that if the air stops coming out of the tank, it doesn't matter how much is left in it. I watched this happen once and vaguely know one very bent diver who had this happen.
     
    Coztick, markmud and 2airishuman like this.
  4. BRT

    BRT Giant Squid

    11,221
    7,240
    113
    But it doesn't work that way. There are a lot more than 8 breaths if you are heading for the surface. If I go OOA I probably won't be able to get the tank out in the time it takes me to go up 20 feet.
     
  5. certainmisuse

    certainmisuse Nassau Grouper

    # of Dives: 100 - 199
    Location: Atlanta GA
    152
    15
    18
    Care to elaborate? I believe that goes for most of us. Regardless, plan to, thanks. My past instruction is adequate in general for the diving I plan to do and I'm not going to debate it, but it can only help my re-entry and regular instruction is default for me.
     
  6. 2airishuman

    2airishuman Solo Diver

    # of Dives: 100 - 199
    Location: Greater Minnesota
    2,402
    1,612
    113
    It is widely considered wise to start with some instruction if you have not been diving for over two years. Some authorities say, one year.

    Moreover, while everyone has to make their own decisions on whether they are ready to dive solo, few would consider it wise immediately after a prolonged furlough.
     
    markmud likes this.
  7. ReefHound

    ReefHound PADI Pro

    # of Dives: 1,000 - 2,499
    Location: Houston, TX
    5,488
    1,563
    113
    Costwise, I'd say a standard 6cf can be cheaper. The issue is that unless you bring a transfill whip you have to deal with getting someone to fill it. Sometimes no problem, sometimes a problem. But 6cf is 6cf, if it's enough in a standard pony it's enough in a container called "Spare Air". Also, a lot of divers don't want to configure it all. Someone oughta sell a configured package of 6cf bottle, first stage, second stage, button or 6" spg, with carrying handle and bands and bolt snaps, ready to go.
     
    markmud likes this.
  8. certainmisuse

    certainmisuse Nassau Grouper

    # of Dives: 100 - 199
    Location: Atlanta GA
    152
    15
    18
    I am in agreement.
     
  9. Compressor

    Compressor ScubaBoard Supporter Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: NYS
    2,935
    1,398
    113
    Thanks to those who submitted some evidence regarding the utility of spare air.
    Without being judgmental, I’ll continue using my present startegy for dealing with emergencies (not in order of importance);

    1. Dive with a reliable buddy who knows and is willing to share air.
    2. Dive with redundant system who affords sufficient air to get to the surface without having to think how many breaths I have left.
    3. Dive conservatively when it comes to air supply.
    4. Appreciate that being underwater requires situational awareness with respect to the environment around you.

    I am not concerned about cost when my life or my buddies well being are concerned. Being able to enjoy the underwater world is priceless and something not everyone can accomplish.
     
    markmud, RyanT and certainmisuse like this.
  10. caruso

    caruso Banned

    # of Dives: 200 - 499
    Location: Long Island, NY
    1,662
    1,198
    113
    No, I plan my dive so I typically end up with more gas than a typical AL80 diver, with around 300 psi in my main tank and a full pony which is the equivalent of about 750 psi in an AL80. Add it up and you get over 1000 psi at the end of each dive.

    I don't believe that it's risky to carry a redundant gas supply for an entire dive and end the dive with more gas than just about everyone else on the boat.

    I'd rather have the redundancy of a second tank and regulator throughout the dive.
     
    Coztick and markmud like this.

Share This Page