Twin set first dive

[diving_moose]

Hi,
Managed to go out today to a local quarry only 6 mtrs deep but ideal for trying my new wing and twin set. Managed to get weight sorted but found that i was always swimming with my head aiming slightly downwards in a arrow type formation and i am putting this down to my tanks been to high up my back and making me top heavy, does this sound about right. I think i need to lower them slightly on the transpac. Also when reading my gauage at the end of the dive it is showing 50bar for arguments sake, now after the dive closing the manifold and putting the gauge onto each cylinder in turn it showed them both as having 50 bar as i would expect. Now this is were i get confused ( not hard to do ) Now back to the end of my dive again and as stated the manifold is open and the pressure gauge showing 50 bar as is the vytec transmitter, now having done my little test on land i now assume that yes they are showing 50 bar but in fact there is 50 in each cylinder. Now i will cut to the point, do the gauges read the pressure and decrease as the total left decreases or do i double what is remaining. Just wondering how it works and how the gauge reads the total left. Sorry for been long winded.
Cheers Lee

[Mark Chase]

If the gauge shows 50 bar that’s 50 bar in each cylinder (as long as the manifold is open). That equates to 100 bar in a single 12ltr. I often use 25bar as reserve when multi diving the set and ending on a shallow dive. It sounds wrong but its the same as leaving 50 bar in a 12.

The height of the tanks on the back is determined by your ability to reach the valves. If you lower the tanks make sure you can still reach the isolator and post valves. If you cant then either work on your technique with an instructor or trusted mentor, look at your harness set up or finally jack them back up and use tail or V weights low down on the set to alter your trim in the water.

ATB

Mark Chase

[Finless]

I think of my twin 10s as a single 20 ltr cylinder when diving with the manifold open. 50 bar = 1000 ltrs of gas as opposed to 1200 ltrs on twin 12s or 500 ltrs of gas on a single 10 or 600 ltrs on a single 12.

Obviously you have more gas to play with using the twins cylinders - just be aware that SPGs are not reputed to that reliable at recording lower pressures. Whether that is true or not or just one of those myths that linger on from old equipment I don't know.

I suppose I ought to know - what is the answer more learned ones?

[nigelH]

Originally Posted by Finless

- just be aware that SPGs are not reputed to that reliable at recording lower pressures.

My YBOD oxygen gauge reads 10 bar on empty. This was brought home to me when a certain instructor was oxygen flushing it post dive and it stopped. That meant I had entered the water for a 60m/90min dive with not 100bar (pushing it) but 90bar. <gulp>

Also the two SPGs on my 300bar twinset read differently. And the difference changed as they went down maxing out at about 15bar at about 180ish.

An SPG is a rough guess - nothing more. Take one to bits one day and laugh at it.

nigelH