Advice for a rig

[4sak3n]

Hi Tobin

I'd like to ask fro some help with my buoyancy calculations and some advice afterwards if you don't mind.

I dive with a two piece wetsuit whose buoyancy I measured today to be ~ 8 kgs (~18 lbs).

I dive a Faber 12L 232 bar (~ 3410 psi) cylinder and, although I checked the Faber website, I could not find buoyancy characteristics for it. So I am assuming it is about -2 lbs empty and -8 lbs full (numbers garnered from numerous threads on your forum).

My BCD is a jacket style Seaquest so it should be about +4 lbs yes?

I dive with 10 kgs (~ 22 lbs) of lead in seawater.

These numbers don't add up for me. In this setup I am neutral (I did a buoyancy check today) however I have 26 lbs (-22 lead -2 cylinder -2 reg) in negative buoyancy and only 22 in positive (18 wetsuit + 4 BCD). I will assume that my weighting is correct and that the difference comes from estimation errors. For example I did the buoyancy check of my wetsuit in pool water so in seawater it might be closer to 20 lbs. Plus the characteristics of my tank and BCD are also just guesses so I will ignore the difference of 4 lbs.

I am 6' 2". All of this taken into account, what sort of a rig would you suggest for me.

Many thanks for any help you can give.

 

[cool_hardware52]

Hi Tobin

I'd like to ask fro some help with my buoyancy calculations and some advice afterwards if you don't mind.

I dive with a two piece wetsuit whose buoyancy I measured today to be ~ 8 kgs (~18 lbs).

I dive a Faber 12L 232 bar (~ 3410 psi) cylinder and, although I checked the Faber website, I could not find buoyancy characteristics for it. So I am assuming it is about -2 lbs empty and -8 lbs full (numbers garnered from numerous threads on your forum).

My BCD is a jacket style Seaquest so it should be about +4 lbs yes?

I dive with 10 kgs (~ 22 lbs) of lead in seawater.

These numbers don't add up for me. In this setup I am neutral (I did a buoyancy check today) however I have 26 lbs (-22 lead -2 cylinder -2 reg) in negative buoyancy and only 22 in positive (18 wetsuit + 4 BCD). I will assume that my weighting is correct and that the difference comes from estimation errors. For example I did the buoyancy check of my wetsuit in pool water so in seawater it might be closer to 20 lbs. Plus the characteristics of my tank and BCD are also just guesses so I will ignore the difference of 4 lbs.

I am 6' 2". All of this taken into account, what sort of a rig would you suggest for me.

Many thanks for any help you can give.

What do we "know" with the greatest certainty? Looks like the buoyancy of your suit at +18.

I don't know what tank you have. Faber apparently makes "12L 232 Bar" tanks that vary from 12.8 kg empty to 14.4 kg empty. A "12L, 232 Bar" tank holds about 100 cuft of gas, so the empty and full buoyancy will change by about 8lbs, not 6lbs.

If your tank is the 7 inch diameter, 28.2 lbs empty (no valve) tank I am assuming it to be, it should be near neutral when empty, and about -8 when full.


A medium SS back plate and harness is about -6 lbs, and a reg is about -2. I am assuming your tank and valve is about neutral empty and -8 full. That makes your rig -16 with a full bottle and -8 with an empty tank.

If your suit is truly +18 then you will need 8 ~ 10 lbs in a belt.

As far as a wing is concerned you need enough lift to float your rig at the surface if you ditch it (with a full tank) and enough lift to compensate for a fully compressed suit.

Assuming I have the right tank specs, you could use as small as a 20 lbs wing, but I'd suggest a 26. That provides a little extra capacity if you move to a more buoyant suit, or a larger tank.

Here's the faber site, with a little math you can tease out the buoyancy specs, or you can blow down your tank and see if it almost floats by itself.

Tobin

Scubadiving cylinders Faber leader in scuba diving cylinders Faber steel cylinders, diving, scuba, diving cylinders, diving cylinders, diving cylinders, scuba diving cylinders, sub SCUBA scuba DIVING

 

[4sak3n]

Thanks for your reply so far Tobin. Now its Maths Time!

*ducks into phone booth to change into tights with a big 'M' on the front and a cape*

I hope you'll excuse me if I work in metric and then convert the final answer into pounds?

The cylinder is, according to its markings, a 12,2 L, 232 bar cylinder which weighs 13,6 kg when empty. And no, it is not listed on Faber's website.

Rough calculations:

Ignoring the thickness of the walls, it displaces at least 12,2 litres of water. Thus it has 12,2 * 1.03 (1 litre seawater = 1,03 kgs) = 12,566 kgs of positive buoyancy. With its 13,6 kgs of negative buoyancy this comes to 12,566 - 13,6 = -1,034 kgs or - 2,27 lbs of buoyancy.

Of course, this ignores the volume added by the steel walls.

At least - 1 kg or -2 lbs of buoyancy empty.

Not-so-rough-but still-rough calculations:

The cylinder has a circumference of ~ 0,65 m and has ~ 0,33 m of 'straight' wall. Now I don't know how much volume the shoulder or curved portions of the tank add but I'll assume that each (the bottom and the top) is one half of a sphere with diameter = diameter of tank. This assumption could easily be incorrect.

c = 2 * pi * r
r = c / (2 * pi)
= 0,65 / (2 * pi)
= 0,103 m

So the volume of the cylinder is:

pi * radius squared * height + 4/3 * pi * radius cubed

(pi * (0,103) ^ 2 * 0,33 ) + ( 4/3 * pi * (0,103) ^ 3)

= 0,0110 + 0,0046
= 0,0156 cubic metres (1 cubic metre = 1000 litres)
= 15,6 litres

So the cylinder displaces 15,6 * 1,03 = 16,06 kgs of water and is - 13,3 kgs negative. Thus overall using these (admittedly rough) calculations it is at most 2,77 kgs or 6,09 lbs positive.

Clearly this is a VERY rough answer. I would guess that my assumption that the curved shoulders add up to a complete sphere was incorrect and this is where the error was introduced.

Obviously the true buoyancy is somewhere between the two ( -2 lbs and + 6 lbs) so your very well educated guess of neutral when empty is born out.

Sorry for the long winded calculations but I was just itching to use at least some part of the stuff I had to learn in my DM course. Maybe I am just an incurable showoff I could always have bled it down and gotten into the pool to test but I don't have easy access to a pool.

Once again, thanks for any help you can give. I apologise that you had to wade through all of that maths but I did want to show that I am taking this seriously and thinking it through instead of just demanding answers from you. It would be rude of me to demand answers served up on a silver platter if I am not willing to work a little for them as well.

 

[4sak3n]

Now, assuming that my tank is neutral when empty, -8 lbs when full and my suit is 20 lbs positive in seawater:

Should I lose all buoyancy in my suit I need ~ 20 lbs of lift to compensate. This isn't taking into account my weight belt though. Does that figure into the calculations?

My rig will be (-8) + (-2) + (-6) = -16 lbs when full and (0) + (-2) + (-6) = -8 when empty. So I need at most 16 lbs of lift to float it at the surface.

I would dive with ~ 14 lbs of lead on my weight belt.

And you would suggest a 26 lbs wing for this correct?

 

[cool_hardware52]

Thanks for your reply so far Tobin. Now its Maths Time

I applaud your rigorous approach to the problem at hand.

There is however an easier, more accurate method.

If you know the empty weight of your cylinder, the material it's made from, and internal volume, you can calculate the buoyancy.

If your 13.6 kg figure includes the valve, you probably won't find it on the Faber site.

What volume does the tank displace? The internal volume + the volume of the tank material.

We know the internal volume, 12 liters. We also know the density of steel. 7.8 grams / per cubic centimeter. I'll assume your valve is about .6 kg 13.6 -.6 leaves 13 kg of steel in the tank itself.

13000 grams / 7.8 grams/ cc = 1667 cc of steel. Total displaced volume of your tank is 12000cc(internal volume) + 1667 cc of steel = 13667cc.

The specific gravity of water is 1 meaning 1 cc or water weighs 1 gram.

The specific gravity of your empty tank will be 13000grams / 13667 cc = .95 That means empty, with out a valve you cylinder will be just slightly positive. Add back in the valve (.6 kg) and your tank will be very close to neutral.

More fun with numbers.


Tobin

 

[cool_hardware52]

Now, assuming that my tank is neutral when empty, -8 lbs when full and my suit is 20 lbs positive in seawater:

Should I lose all buoyancy in my suit I need ~ 20 lbs of lift to compensate. This isn't taking into account my weight belt though. Does that figure into the calculations?

My rig will be (-8) + (-2) + (-6) = -16 lbs when full and (0) + (-2) + (-6) = -8 when empty. So I need at most 16 lbs of lift to float it at the surface.

I would dive with ~ 14 lbs of lead on my weight belt.

And you would suggest a 26 lbs wing for this correct?

You pretty much nailed it.

If we assume that you start the dive weighted so you are neutral, i.e. eyelevel with no gas in your wing and a full bottle, the only thing that will loose buoyancy is your suit. and it cannot loose more than the buoyancy it starts with.

With a suit that's + 20 and a rig that's -15 to -16 with a full tank, I'd predict that you will need about 4-6 lbs in a belt.

Remember at the end of the dive you will be lighter by the gas consumed, typically about 6 lbs, but your exposure suit will compress and loose quite a bit of buoyancy from the surface to 5 meters. For divers wearing a thick exposure suit, this loss of buoyancy will typically offset the weight of the gas consumed from small to medium capacity single cylinders.

With a suit that's +20 and a rig that's at most - 16, a Torus 26 is a good choice.

Tobin

 

[4sak3n]

The tank was stamped with 13,6 kgs on the shoulder. I would assume that this would be the weight of the empty cylinder only since tank manufacturers don't sell them with the valve. The tank is also stamped with 12,2 litres on the shoulder, not 12. Whoops ...

I only dive in seawater so work with the specific gravity of water being 1,03. Of course when estimating its great to work with fresh water at 1 and then add in a "fudge factor" or 1-2 kgs lbs.

And I hadn't though of using the density of steel. Doh!

Wheeeee ... fun.

Thanks so much for all your help Tobin. :thumbs_up:

P.S. Putting my order in soon, probably on Monday. Super excited! :fruit:

 

[4sak3n]

Its here, its here, its finally here!!!! :multi:

My package, sent all the way across that big wet, blue thing, finally arrived today>

Needless to say I immediately tore into it and had some great fun putting it together.

Sadly I don't have time today to play with my nice new shiny rig because I have to get to our [Underwater] Club for tonight's meeting and then go to my mother's birthday party. Plus I can't dive it until this weekend! :furious:

Anyway, I just want to give many thanks to Tobin for assisting me with my purchase and, more importantly, being the opposite of what you expect from most salesmen because he actually let me know when I was buying extra stuff which I didn't need!

So thank you Tobin. I know you receive messages like these so often (because you deserve them!!!) but this meant a lot to me so I am extremely grateful.

Will post an update next week after I've given it a few dives!

P.S. Dya think I'm somewhat excited or something? :luxhello:

 

[4sak3n]

Whoops! Got so excited I forgot to ask my question.

The plastic 'nipples' (as someone oh so eloquently called them on another thread) or locators which you use to position the wing on the plate ... how do they work? Does one push the ridge along them all the way through the hole in the plate? Or just ensure that the thinner bit fits into the hole?

I ask because I have yet to be bale to push the ridge through the hole and am left wondering whether I am trying to do something which isn't right.

Hope this adequately explains what I am trying to ask.

P.S. Seriously ... overflowing with excitement. I had to prevent myself from cackling out loud as I carried my packaged rig to my car from the post office. I think I let one or two hysterical giggles out by accident.

 

[cool_hardware52]

Its here, its here, its finally here!!!! :multi:

My package, sent all the way across that big wet, blue thing, finally arrived today

Glad to hear the goods arrived. One of the things I enjoy about dive business is the chance to talk with divers all around the world.

Let me know if you have any questions after you get it wet.

Tobin