totterfield once bubbled...
Hi
i have just started diving and i was just wondering when i come to getting a pony cylinder and a 15 Lt tank for longer divers or what ever, would it be better to buy an 18 litre tank instead of the pony and the 15 lt tank?? is there a massive advantage in having a pony except as it being a good back up???? it would be alot cheaper to buy the 18 Litre. cna any one tell me what you think is best and what sort of set ups you have??? cheers Tom
Generally speaking a pony is a good idea for an independent "bailout" but not such a good idea for extending your bottom time. I'm not such a big fan of them but YYMV.
Apart from the question of air supply I think you need to think about the weight and balance.
I use a 15 litre tank but it's fairly big and heavy and after the fact I'm not sure it was the right choice. The european 15l tanks weigh in at about 18kg empty, which means 20-21 kg full. If you dive in a dry suit you'll have a big 30kg of kit on before you're done. I chose this tank because in addition to being larger the extra weight of the tank is weight you don't need on your weightbelt. Probably not bad reasoning but there are "lighter" options, which I'll get to in a minute.
I think an 18 litre tank (+/- 23-24kg full) would be problematic in terms of weight but also in terms of stability with a normal BCD. I find the 15 already pushes the stability envelope and you'll most likely have a lot of "roll" with 18litre tank. So.....if you choose for an 18 litre tank then you should think about a bp/wing configuration for the stability. And assuming you go the bp/wing route then since you'll have 18litres and 24kg already you're probably better off with a double 10 to get 20litres at 25-26kg (full). That's a nice stable set that you can use in the N.Sea if you choose to go this direction with your training but a bit of an overkill for everyday "puddlestomping". In other words, I think the 18L tanks aren't worth it given the options.
A twin 7 is about 3 kg lighter than a 15litre and you get almost as much air. You can also mount them on a wide range of BCD's. If I were to buy new tanks tomorrow I'd seriously consider this option. A twin 7 is lots of air for most of your diving and it has the added benefit that you can put an isolator between them and redundant 1st stages (you can do this with a 15 too, I put a Y valve on mine but it's not a nice as having an isolator). If you're going to do much diving on the N. Sea then you'll find the double 7 a little on the small side. Many of the N. Sea crowd use double 10's or double 12's. And most of these are set up for Nitrox.
Other popular sizes are the 12litre and 10litre tanks. These are progressively smaller and lighter. The 10litre tanks weigh about 11kg empty (13-14 full), which is about as light as it's going to get. A 10 litre tank is very stable with any modern BCD you care to use which makes it a nice size to start out with. The downside is that your divetime will be a little shorter but the comfort and stability might be worth it if you don't want to hump around a lot of weight.
I hope I haven't confused you. Can you use this?
R..
