Diving with Twins 2
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Cate and I got to Stoney Cove for another early start on a Sunday - I'm beginning to think that masochistic tendencies have taken over from any form of residual common sense.
Having made the changes I noted previously I was expecting an easier dive - and that's exactly what I got. The dive was a lot more pleasant than last time but I'm still quite awkward in the water compared to diving with a single and pony - then again 2 dives in a twinset versus 250+ with my trusty BC, I'm not really surprised.
Wearing a 4kg weightbelt gave me better trim in the water but I've still not found my ideal. I've been chatting to friends about configuration and reading up on other peoples views including an excellent posting written by Nigel Hewitt, which dealt with the age old question of using a jacket/wing or drysuit for buoyancy control (jacket/wing of course!) but also covered the weight characteristics of gas which I used in my calculations for adding weight. I've reproduced some of it below but the whole article can be read on the UKRS website here.
So by wearing my 4kg weightbelt + 1kg on each ankle I felt I'd be about right.First get the weight right. That means you need to be neutral on empty so the last part of your ascent is not uncontrolled and abrupt. This means you start the dive with enough weight on you to match all the gas you carry.
Yes. I do mean all. I do count the pony, your reserves and the stages if you have them. You need to be neutral on totally empty. What good is it to heroically rescue your buddy from certain death and then bend him to blazes because, when he breathed the reserves of gas you claimed you carried for him and now you couldn't do the stops you both needed? Lacks finesse that does.
This means that when you step into the water you are overweight by the weight of all the gas you carry. Hence you carry buoyancy to match that weight. Example numbers are:
200bar 10L air 2.4Kgs
230bar 12L air 3.2Kgs
230bar 12L 40% 3.4Kgs (oxygen is heavier than nitrogen)
230bar 20L air 5.4Kgs
300bar 20L 36% 6.8Kgs
300bar 24L 42% 8.3Kgs
I felt that the cylinders themselves were too high on my back, especially as I couldn't reach them well enough to even begin practising shutdown drills... hopefully next time this will change; although in the meantime my trusty buddies are watching my back - literally. One of the contributing reasons for this (I think) was that the right shoulder harness felt quite tight and restricted my normal movement.
I've still got to find ways of securing hoses and to stop my left low pressure inflator from flapping around, in many ways this experience is a sharp reminder of what it was like when I first learnt to dive.
Finally, and completely unrelated to learning to dive again - the zip on my drysuit has broken AGAIN, I'm not a happy bunny as this will cost around £100 for the repair, but more importantly means I'll be in a semi-dry for the next couple of weeks.
More to come next week...