Hey Greg, if I may......instead of spending money on tanks, I would suggest investing that money in yourself and take an extreme scuba makeover class. I can almost guarantee you'll increase your comfort in the water, lose a considerable amount of lead off your belt and perfect your trim and buoyancy. All of those things will transfer into a considerable reduction in air consumption. I know it sounds like a sales pitch but it works and it work very well. There's a great shop in Seattle called Frogkick diving.
Frog Kick Diving - FKD
Extreme Scuba Makeover · UTD Scuba Diving
Not that such a class is a bad idea, I agree that it's a great idea. However, I would not give up my big tanks for anything. I went with low pressure 108's instead of steel 100's after doing just a little math, I think they're a far far better tank (for me).
The times I've used 100's it has been rentals and they have been Al100's. AL100's if I recall correctly, have a larger diameter than AL80's. My steel 108's have the same diameter. If so, it will be an issue to deal with on almost every boat ride unless you have your own. Not insurmountable but you have to get used to doing things differently than every other diver if you have 8" tanks.
My point? I think big tanks are a great idea, and I think that @Greg Doyle should do a little tank research before plopping down $500ish dollars. Your dive shop can probably order just about any tank you want. Given the price, get what you want. I put together a little spreadsheet comparing tanks when i was shopping. I have seen a better one that someone else did since then but I have mine, so here it is.
Color legend: Red = >al80 Green = <al80
Those "actual cubuc feet" values account for overfilling. That will only be possible if your local shop is okay with it. Here in Florida, it's not a problem. I get overfills at more than one local shop. I've got them from several shops in the Florida Keys. I've got them from shops near Blue Heron Bridge. Most of the time I don't even have to ask. Your shop may not be willing to do it, so check before you spend money on a tank based on overfull capacities. That said, even with standard fills, a LP108 is a better tank than an HP100 in my opinion. Many shops use "banks" when they do fills. Often the bank pressure is lower than required for a 100% fill of an HP tank so you are stuck with less than a completely full tank. I've never seen a shop that can't put at least 3200 into a scuba tank, though.
When I bought my tanks, I could barely squeeze 30 minutes out of an al80 (east coast fl ~80' dives). Now days I can do much longer. As long as I don't select a boat that has a hard time limit on dives i still love having big tanks. If only my tanks were round, I could play AC/DC every time I got on a dive boat.
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