YD Scuba Diving Forums banner

1 - 16 of 16 Posts

·
Registered
Joined
·
42 Posts
Discussion Starter · #1 ·
Hey guys.

Just upgraded to a shiney new twinset courtesy of those lovely chaps at the dive show and need some advice on weighting.

I'm 6"1' and around the 13st mark. have previously dived single steel 15lt + 3lt pony and used 10kg on my weightbelt with my current drysuit/undersuit combo in scapa.

should have been fairly accurate although never did a 'proper' weightcheck after adding the pony, just took 2kg off belt.

Now diving a steel twin 12lt setup with a 3mm frog backplate. i know i'll need a weightcheck but could do with a couple of ballpark figures to aquire weight blocks etc. also what difference do you guys find you need to alter for fresh water from salt (should the need ever take me) or is it largly negligable?

thanking you verily
 

·
Getting Grumpy, Old and Twisted
Joined
·
3,115 Posts
I use 8Kg in salt and 6Kg in fresh with my Faber twin 12's with 3mm SS back plate and find that is more than enough to keep me negative with 50bar back gas and 20bar 7 lit ali stage.
I am 6' 2" and 17 stone.
Hope that helps.:teeth:
 

·
Registered
Joined
·
11,575 Posts
I'm a similar build and used around 10kg when I dived a 15l/pony.
I now use 4kg in the sea and 2kg in fresh water with twin Fabers and a 3mm Halcyon backplate.

I do run my drysuit quite tight with the auto-dump all the way open bar a couple of clicks.
 

·
Registered
Joined
·
42 Posts
Discussion Starter · #4 ·
Ah, i had a stab in the dark and rekoned around 4kg. i think i may have been a tad overweighted on 10kg 15lt + pony so 4kg should be sufficent. didn't realise fresh water would have made a whole 2kg difference though.

Cheers for the responses
 

·
Kit Junkie
Joined
·
804 Posts
I went from a single 15 (without pony) to twins of varying size. I checked the weight and buoyancy of all of them (without regs). Results for Faber single 15 vs. twin Faber 12's as below:

Twin 12's (with rubber boot) at 232 bar weighs 37.9 kg (6.6kg air), steel weight 31.3 kg.
Buoyancy full = -8.9, 40 bar -3.5, empty -2.3

15l single at 230 bar, full weight is 22.1kg (4.1kg of air), steel weight = 18.0kg.
Buoyancy full = -4.2, 40 bar -0.8, empty -0.1

Thus, the twin 12's were about 2kg less buoyant. Add another kilogram for the extra primary regulator and stuff and you have 3kg less weight to carry.

Weighting also depends on the difference between your harness/bcd (wing and plate?) using the single and what you are using now. E.g. a steel backplate has less buoyancy than an aluminium one.

Rule of thumb take 2kg off your belt for the twins. Add 2kg for salt water from fresh water. Do a test.

IMHO unless you can stay at 2m with 10 bar or less then the extra gas in the twins is just there for ballast and not available to breathe. Better too heavy than too light....

ATB
Ian
 

·
Wreck, reef and muff diver.
Joined
·
118 Posts
also what difference do you guys find you need to alter for fresh water from salt (should the need ever take me) or is it largly negligable?

thanking you verily
Try my Buoyancy calculator here: Utilities & Dive Information

Plug in values for your setup for salt water, calculate and note values. Now select Fresh water and note the difference. I get around half a kilo difference for a steel 12 litre cylinder.
 

·
Registered
Joined
·
2,091 Posts
I changed from a single Faber 15 to twin Euro 12's last week. Everything else remained the same. I did a weight check and removed 4.5 kilos. I add about 2.5 kilos for sea diving
 

·
Wreck Enthusiast
Joined
·
4,161 Posts
Not twinset related but my weighting went from 6kg in the fresh stuff to 11k in the salty stuff using the same set up. Go figure eh?
I have about a 5Kg difference too. Any less in the salty stuff and it doesn't feel "right" and in control.
 

·
Registered
Joined
·
128 Posts
I have about a 5Kg difference too. Any less in the salty stuff and it doesn't feel "right" and in control.
Yeah I found out the hard way too. Put an extra 3kg on the belt thinking that would do it, breathed down about 70bar and found a strange attraction to the surface. Ended up carrying around a bag of rocks in order to make the ascent.
 

·
Team Peanut Butter
Joined
·
2,306 Posts
As a starting point get in the water with no wieght on and 30 bar in your twin set while wearing your suit and undersuit + all the bits of kit that are fixed to you, not the bits you may let go of like laterns and dsmbs. Then have sombody pass you weight until you just have enough wieght to sink when you exhale. This will give you the minimum wieght that you can dive with and control your bouancy. The wieght loss between a full and an empty set of twin 12s is nearly 6 kgs so you do not want to be taking any more lead in than you need.

Or go and do a twin set orientation course.

Graham
 

·
Back to reality, and it's scary.
Joined
·
406 Posts
As a starting point get in the water with no wieght on and 30 bar in your twin set while wearing your suit and undersuit + all the bits of kit that are fixed to you, not the bits you may let go of like laterns and dsmbs. Then have sombody pass you weight until you just have enough wieght to sink when you exhale. This will give you the minimum wieght that you can dive with and control your bouancy. The wieght loss between a full and an empty set of twin 12s is nearly 6 kgs so you do not want to be taking any more lead in than you need.

Or go and do a twin set orientation course.

Graham
I totally agree. I have treated a few divers this year that couldn't do their stops because they were underweighted at the end of the dive. It's critical to get this correct for the end of the dive with residual gas only and not for the start with full tanks. Better to practice in the shallows first and check out the buoyancy in around 5/6 mts.:)
 

·
MIFLEX DIR = "Did It Rupture"??
Joined
·
3,737 Posts
I know its a PITA, but the only way to be sure is to get in the water and try it out under controlled conditions. I expect that you have them filled and so you may well need to waste an air fill to empty them to 30bar or so, but the benefits are precise weighting nad no "Ball park" figures which may just not work when you have the glass ceiling above you!

Only needs doing once and then you wont have to worry again.
 

·
Please delete all my posts
Joined
·
12,604 Posts
I know its a PITA, but the only way to be sure is to get in the water and try it out under controlled conditions. I expect that you have them filled and so you may well need to waste an air fill to empty them to 30bar or so, but the benefits are precise weighting nad no "Ball park" figures which may just not work when you have the glass ceiling above you!

Only needs doing once and then you wont have to worry again.
remembering that 30 bar in twins is the equivalent litres of 60 bar in a single.

I went to 10 bar in the twins for the weight check originally as that gives me the lowest I would ever be at. and yes I wasted a fill to do it properly
 

·
Nigel Hewitt
Joined
·
7,142 Posts
If you know the pressure in the tank you know the weight of the gas. You can do the check on any level of fill and then add that weight to what you need so you would get the same results on empty.

Physics is your friend (and pays my wages).
 
1 - 16 of 16 Posts
Top