weighting

[Rick Murchison]

sheck33 once bubbled...

i am assuming you mean an aluminium 80 for a tank?
this is very generalized. It depends on what BC you are using and what tank and exposure protection is used.
There is a big difference in bouyancy between a simple back inflate wing like Halcyon's and say a Zeagle ranger BC.
A regulator isnt even in the equation. Also someone's weight is NOT a very relevant factor. The MAIN things that are bouyant are the exposure protection, BC (if wearing something bulky like a Zeagle) and the difference full/empty tank one has to account for.
And of course 500 psi only equals 13 CF when using a 80 CF tank with a WP of 3000 psi

Weight has everything to do with it. Everything. You must displace your weight in water in order to sink. As salt water weighs 102.5% as much as fresh water, and since you'll displace the same volume of water whether fresh or salt, you'll displace 102.5% more salt water by weight than you did fresh, and must compensate by adding weight - once you have your weighting for a particular set of gear figured out in the pool, you add 2½ pounds per hundred pounds total weight to get what's right for salt water. Every time. In this case Robz said he was wearing only a skin, so he's only having to sink his body, BC & tank - later he said it was an AL 80 - no exposure suit.
But you're making things entirely too complicated for yourself... just take each piece of gear to the pool and see how much weight it takes to sink it, or, using a balance, how much weight it takes to hold it up. Make sure the lead you're using for these calculations is submerged when you do the measuring. Measure the various tanks at "end of dive" pressure (typically 500 psi, but whatever suits you). I measure my tanks with regulator attached, as those two are unlikely to be used separately. Include your body on the list - with a nearly full breath. Write the weights down and keep the weighting list in your logbook. You only have to do this once for each piece of gear you use. (more than once for your body if you gain or lose a bunch of weight) Now, when you get ready for a dive, just add up the total weight needed (algebraicly) to sink you and what you're wearing, and if it's salt water, add 2½ pounds per hundred pounds total weight (your body weight + all your gear + your fresh water weights) and you'll have perfect weighting every time. Every time.
Rick

 

[Rick Murchison]

Butch103 once bubbled...



Did that just roll off your tongue, or did you have to think about it...... You seem to have the formula ingrained somewhere in your brain.......I have read your weighting explanation before and I am still baffled........

see above post...
The "formula" to go from fresh to salt is to add 2½ pounds per 100 pounds total weight. (this is an average and a "useable" number. The actual difference is something like 102.45% average, and salinity - and therefore specific gravity - of sea water varies from sea to sea, ocean to ocean and season to season - but 102.5% will get you close enough)
Rick

 

[MikeS]

Rick Murchison once bubbled...

see above post...
The "formula" to go from fresh to salt is to add 2½ pounds per 100 pounds total weight. (

Rick,

If I understand correctly; when properly weighted for fresh water, all I have to do is get on a scale and measure the total dry weight of the equipment and myself. To calculate the increased weight requirements for salt water divide by 100 and multiple by 2.5 or simply multiply the total dry weight by 0.025.

Practical Application

Please let me know if I am applying the principles correctly for this application. I have established weighting (just below surface, near empty tank) in the quarry, fresh water. I’m getting ready to go to NC, salt water, and need to adjust weighting. The only equipment configuration change is going from a 7 mil wetsuit (fw) to a 2.5 mil wetsuit (sw).

I need to find:
The difference in dry weight between the two wetsuits.
The difference in buoyancy (weight required to sink) between the two wetsuits.
The total dry weight of myself and equipment, as configured for the quarry.

Calculation

Total dry weight
- difference in dry weight between the two wetsuits
= revised dry weight
* 0.025
= Increase in weight due to increased buoyancy of salt water
- difference in buoyancy between the two wetsuits.
= Required weight increase

Thanks,
Mike

 

[Rick Murchison]

MikeS once bubbled...
Calculation

Total dry weight
- difference in dry weight between the two wetsuits
= revised dry weight
* 0.025
= Increase in weight due to increased buoyancy of salt water
- difference in buoyancy between the two wetsuits.
= Required weight increase (*** or decrease ***)

Thanks,
Mike

That'll do it.
In your logbook, write down those "sinking" weights for each wetsuit for future reference.
Rick

 

[MikeS]

Rick,

A few more questions at the risk of trying your patience:

Rather than using a balanced lever to measure negative buoyancy can I get the same results by using a lift bag? For example, to determine the buoyancy of a near empty cylinder, attach a lift bag and add enough air to make the combination neutral. Then remove the cylinder and the amount of weight needed to make the lift bag neutral again is the negative buoyancy of the cylinder.

Question number two has to do with the weight adjustments for a stage slung pony bottle. The bottle in question is 30 cf aluminum with DS4/TX40.

I should use the method above to determine the amount of negative buoyancy for an full and empty cylinder.

I should not remove any weight to compensate for the negative buoyancy of the pony as I may hand it off and still want to make a safety stop. The extra energy required is the penalty for caring the pony.

Since the pony hangs from the left side, it throws off trim. It appears I have three choices for adjusting trim.
1. Do nothing, which will cause a tendency to roll to the left when the pony is full and attached.
2. Shift half the weight of the pony from the left weight pocket to the right pocket. This will realign trim till air is used from the pony or it is handed off, at which time there will be a tendency to roll to the right.
3. Shift a third of the weight of the pony, from the left to the right. There will always be some imbalance but it would never be extreme.

Any thought are appreciated.

Mike

 

[sheck33]

Rick Murchison once bubbled...

Weight has everything to do with it. Everything. You must displace your weight in water in order to sink. As salt water weighs 102.5% as much as fresh water, and since you'll displace the same volume of water whether fresh or salt, you'll displace 102.5% more salt water by weight than you did fresh, and must compensate by adding weight - once you have your weighting for a particular set of gear figured out in the pool, you add 2½ pounds per hundred pounds total weight to get what's right for salt water. Every time. In this case Robz said he was wearing only a skin, so he's only having to sink his body, BC & tank - later he said it was an AL 80 - no exposure suit.
But you're making things entirely too complicated for yourself... just take each piece of gear to the pool and see how much weight it takes to sink it, or, using a balance, how much weight it takes to hold it up. Make sure the lead you're using for these calculations is submerged when you do the measuring. Measure the various tanks at "end of dive" pressure (typically 500 psi, but whatever suits you). I measure my tanks with regulator attached, as those two are unlikely to be used separately. Include your body on the list - with a nearly full breath. Write the weights down and keep the weighting list in your logbook. You only have to do this once for each piece of gear you use. (more than once for your body if you gain or lose a bunch of weight) Now, when you get ready for a dive, just add up the total weight needed (algebraicly) to sink you and what you're wearing, and if it's salt water, add 2½ pounds per hundred pounds total weight (your body weight + all your gear + your fresh water weights) and you'll have perfect weighting every time. Every time.
Rick

''Weight has everything to do with it. Everything. You must displace your weight in water in order to sink''

No, it does not. In order for an object to sink the actual weight is of very little importance. When i submerge an object it will displace water, the weight of the displaced water then is the upward force that has to be overcome to actually sink it. What is the determining factor is the VOLUME of the object one tries to sink. For any human body without gear or any exposure protection very little to no weight is needed to sink because the average density of the body is very close to the density of water so a body submerged in water is already neutral, one can stay afloat by completely inhaling and sink by exhaling. If weight is important how is is that a 5 lbs lead weight sinks and a 200 lbs wood log floats??? This rule of so much percent of ones body weight + something to calculate how much weight one needs is nonsense, the reason bigger or fatter people need more weight is NOT because the weigh more, it is because they displace more water. This stupid rule is cause for a lot of openwater students to have too much weight in their class. One can figure out ones weighing without having to try and see how much weight it takes to sink your divegear.
The bouyancy characteristics of tanks are known, my cannister light is 2# negative, my BP 6# negative and i am sure the bouyancy characteristics of undergarments are known too.

note by the way that 'Weight' and 'Mass' are not the same thing and strictly speaking weight is not even expressed in pounds, the sentence 'my weight is 200 lb' is meaningless.

physics indeed

 

[Rick Murchison]

MikeS once bubbled...
Rather than using a balanced lever to measure negative buoyancy can I get the same results by using a lift bag? For example, to determine the buoyancy of a near empty cylinder, attach a lift bag and add enough air to make the combination neutral. Then remove the cylinder and the amount of weight needed to make the lift bag neutral again is the negative buoyancy of the cylinder.

Sure.. seems a little more complicated than need be, but that'd work - what you're going ot find, by the way, is that even with a regulator attached an AL 80 will float with 500psi, and take about a pound to sink it.

Question number two has to do with the weight adjustments for a stage slung pony bottle. The bottle in question is 30 cf aluminum with DS4/TX40.

I think you'll find the imbalance hardly noticeable - the swing from full to empty on a 30 is only 2½ pounds, and it's just slightly positive (with reg) when empty. If you find the 2 pounds on the side too objectionable you can always sling it centered on your belly - may sound funny but it's actually easy and comfy (I used to sling a steel '72 stage that way). I personally make no weight changes for pony stage or deco bottles, and I add only one pound each for AL 80 stages.

 

[Rick Murchison]

sheck33 once bubbled...



''Weight has everything to do with it. Everything. You must displace your weight in water in order to sink''

No, it does not. In order for an object to sink the actual weight is of very little importance. When i submerge an object it will displace water, the weight of the displaced water then is the upward force that has to be overcome to actually sink it. What is the determining factor is the VOLUME of the object one tries to sink. For any human body without gear or any exposure protection very little to no weight is needed to sink because the average density of the body is very close to the density of water so a body submerged in water is already neutral, one can stay afloat by completely inhaling and sink by exhaling. If weight is important how is is that a 5 lbs lead weight sinks and a 200 lbs wood log floats??? This rule of so much percent of ones body weight + something to calculate how much weight one needs is nonsense, the reason bigger or fatter people need more weight is NOT because the weigh more, it is because they displace more water. This stupid rule is cause for a lot of openwater students to have too much weight in their class. One can figure out ones weighing without having to try and see how much weight it takes to sink your divegear.
The bouyancy characteristics of tanks are known, my cannister light is 2# negative, my BP 6# negative and i am sure the bouyancy characteristics of undergarments are known too.

note by the way that 'Weight' and 'Mass' are not the same thing and strictly speaking weight is not even expressed in pounds, the sentence 'my weight is 200 lb' is meaningless.

physics indeed

Ok, let's try again...
How do you compute the volume of an irregularly shaped object - like, for example, a human body or an exposure suit - or a log, for that matter?
I'll use a scale, and a pool, and some weights - you use any method you choose.
Rick

 

[MikeS]

Although Omar has revoked my Bill Nye impersonation license I’m going to take a stab at this anyway.

Fresh to Salt Water

The 102.5% is the ratio of the weight of fresh water versus salt water displaced for a given volume. The volume of water you and your equipment displace does not change as you go from fresh water to salt water. What changes it the weight of the water displaced and hence the amount of weight required to compensate. There is some error involved because adding the weights increases your volume slightly but for the purpose of this exercise and based on using lead for weight the impact is negligible.

Mike

 

[-=>Larry<=-]

sheck33 once bubbled...



''Weight has everything to do with it. Everything. You must displace your weight in water in order to sink''

No, it does not. In order for an object to sink the actual weight is of very little importance. .......
note by the way that 'Weight' and 'Mass' are not the same thing and strictly speaking weight is not even expressed in pounds, the sentence 'my weight is 200 lb' is meaningless.

physics indeed

Sheck - Check your CRC again! In the wonderful English system of units - the Pound has a definition as BOTH a mass and a Force! The force is, of course, in common usage assuming standard 32ft/sec2 gravity - but it is easy on us lazy folk that forget to type out "Poundals" - the correct unit.

That said - Weight and displacement define it all as does avg. density and the volume - - keep all the units straight and it works in any system.

By the Way - RobZ - Congradulations on nailing it! - I'm 6' & 225 POUNDS - and I take 8 lbs in salt water with a skin & Al80 !!