[Mark Davies]

Originally Posted by pieater

I'm obviously being a bit slow here, can you expand on the concept of having less available air please?

No you're not being slow - my point only makes sense within a context and I didn't really explain it properly.

Your regulators don't keep going until your tanks are empty - they actually stop delivering gas when tank pressure drops to a certain amount. That will vary depending on your regulators but for the sake of argument let's say it is 10 bar.

Now, I'm making the comment in the context of using a half-empty twinset on a second dive rather than a full, second single 12. Most people make the assumption that 110 bar in twins is the same as 220 in a single - it isn't.

The twinset will stop breathing at 10 bar, when you have 24 litres left - giving you 2616 litres of useable air. The single stops at 10 bar when you have 12 litres left - giving 2628 litres.

Of course it is marginal (30 seconds on a safety stop?) and obviously you can factor it in to your diving - but who does? I'm not saying it is critical - just making the point that using a twinset for two dives doesn't always help. I'm just being pedantic really.

Kirstie's experience with the twin 7s raises the point that on a first dive with all that excess air you may be more relaxed and so save a fair bit of air as a result - then it might work - but that's not a given.