weighting- freshwater vs. seawater

[sandanbob]

Thank you for the replies. I did over simplify, and I think I know where the error was in the process. The diver and gear would displace a specific volume, at a given total weight, and that would be used (if going that route) to determine the weight needed to counteract the difference.

However, my initial calculation was only looking at the portion that would have been above the surface without the extra weight, as the rest would have been submerged, whatever portion that would be.

When looking at the diver (and gear) as a whole, then the difference in densities of the two types of water make the larger difference as mentioned by other posters.

Does that sound right?

 

[boulderjohn]

The dive buddy calculator, like pretty much all online calculators I have ever tried, tells me I need more than double the weight I actually need.

 

[OrcasC205]

The dive buddy calculator, like pretty much all online calculators I have ever tried, tells me I need more than double the weight I actually need.

You must be pretty dense -- no pun or slight intended.

 

[happy-diver]

No pun now that is funny and considering you have directed it at one of the denizens of the forum
with backroom sympathies
courageous too

Anyone thinking that half a pound is of any consequence at all
unless you are at the top of the buoyancy game and diving in
still water will get a shock if exposed to the oceans elements

 

[CT-Rich]

The dive buddy calculator, like pretty much all online calculators I have ever tried, tells me I need more than double the weight I actually need.

Some of us are softer and doughyer than others, I suppose. YMMV.

 

[boulderjohn]

Some of us are softer and doughyer than others, I suppose. YMMV.

I am softer and doughyer than I would like to be.

I think a key factor not included in sites like that is BCD type. A recent packing error for a trip forced me to rent and wear a jacket BCD in the ocean for the first time since I started diving. I needed far more weight than I would normally use because of its increase bulk. I had to use 12 pounds, which is much more than I needed even back when I used a back inflate BCD (8 pounds). I felt like I was diving with an inner tube around my waist. Even so, dive buddy said I needed 21 pounds.

 

[James79]

The best results for any calculator I've had are the Buoyancy Computer spreadsheet that RSingler wrote; Info - Optimal Buoyancy Computer . As I understand it, it is mostly formulas for pure physics, with some estimates and empirical data thrown in (like compression rates for neoprene). I have the luxury of a small pool in the back yard, so I initially worked the spreadsheet from both ends. I verified my rig is neutral in freshwater by itself in the pool, put in estimates for the reg, told it what tank I had and then adjusted my "personal buoyancy" until the lead required matched what I actually needed. Once I had that in, I've used it to calculate needed lead for wetsuits, changes from salt to fresh water, and different tanks and it's always been within a pound or so.

Respectfully,

James

 

[boulderjohn]

A little more data:

If the wind ever stops blowing here, I will dive again. I went to Dive Buddy and put in the data for the dive I plan to do with the equipment I plan to use. With my 5mm suit and steel tank in the ocean, it says I need 19 pounds.

I will count my steel backplate as 6 pounds. I will carry no weight whatsoever, and I will be a little overweighted.

 

[bradlw]

very old discussion I know, but dredged it up doing research

I'm curious - especially from those of you that are active OW instructors.... what is going through your minds when you estimate lead needed for students? rules of thumb, etc....?
I'm wondering your estimating strategies for use as a quick "dummy check" of my checks and calculations
but also to have a double check in mind for reality checking what my wife and kids end up with

My family recently all went through OW class, and while they are surely not weighted perfectly, they were weighted pretty well considering and were able to gain very good bouyancy control straight away.
Just sort of based on what seemed like off-the-cuff guesses by the instructor.

So it's gear dependent to at least some degree.... the instructor is familiar with the rental gear that the students are wearing
and I'm sure that they are compensating for the average student not fully exhaling, shallow breathing, and all of that...

The real reason for my question is that we have a dive trip planned to Key Largo, coming up soon
It's been a long time since I dove in seawater, and I'm thinking through my weighting.
And I'm also thinking through how to help the others in my family.

My body has changed since I was an active diver, and a lot of my kit/gear has changed too ...so notes I have from back in the day about what worked aren't exactly correct.

We all know what we used in the pool and for the freshwater checkout dives and that it worked well enough
but for all of us that was with full wetsuits and different rental gear that's what they'll have there
AND I've changed my wing and regulator since then too.
+ this will be warm salt instead of cold freshwater

For me, I had 8# in the springs and was a bit heavy. I rekon 6# was about right for my full 3mm farmer-john

Yesterday I jumped into my pool with my new kit, my 1mm/2mm rashguard "shirt", and a full Catalina S80.
I didn't have my full kit with light, SMB, etc... but close enough
With zero lead I was slightly negative...I'd guess -2#. Very diveable!
minus about 5.5# of gas at the end of dive, that means I need about 3.5# of lead in freshwater, call it 4# since I don't have 1/4# weights. I tried 6# and it seemed fine.

and then just spitballing my total geared up dry weight ready to giant stride at about 214#
214# x 0.025 delta density = 5.35#
so I'll need to add about 5.5# for seawater, bringing me to maybe 9# of lead required in salt

just hoping for help to reality-check that number

and if that's about right for me, that's about the same as what I dove in the springs with then
and so what my family used in the springs with full suits on will probably be about right in saltwater with rashguards or a little thin jacket or shorty.

 

[James79]

very old discussion I know, but dredged it up doing research

I'm curious - especially from those of you that are active OW instructors.... what is going through your minds when you estimate lead needed for students? rules of thumb, etc....?
I'm wondering your estimating strategies for use as a quick "dummy check" of my checks and calculations
but also to have a double check in mind for reality checking what my wife and kids end up with

My family recently all went through OW class, and while they are surely not weighted perfectly, they were weighted pretty well considering and were able to gain very good bouyancy control straight away.
Just sort of based on what seemed like off-the-cuff guesses by the instructor.

So it's gear dependent to at least some degree.... the instructor is familiar with the rental gear that the students are wearing
and I'm sure that they are compensating for the average student not fully exhaling, shallow breathing, and all of that...

The real reason for my question is that we have a dive trip planned to Key Largo, coming up soon
It's been a long time since I dove in seawater, and I'm thinking through my weighting.
And I'm also thinking through how to help the others in my family.

My body has changed since I was an active diver, and a lot of my kit/gear has changed too ...so notes I have from back in the day about what worked aren't exactly correct.

We all know what we used in the pool and for the freshwater checkout dives and that it worked well enough
but for all of us that was with full wetsuits and different rental gear that's what they'll have there
AND I've changed my wing and regulator since then too.
+ this will be warm salt instead of cold freshwater

For me, I had 8# in the springs and was a bit heavy. I rekon 6# was about right for my full 3mm farmer-john

Yesterday I jumped into my pool with my new kit, my 1mm/2mm rashguard "shirt", and a full Catalina S80.
I didn't have my full kit with light, SMB, etc... but close enough
With zero lead I was slightly negative...I'd guess -2#. Very diveable!
minus about 5.5# of gas at the end of dive, that means I need about 3.5# of lead in freshwater, call it 4# since I don't have 1/4# weights. I tried 6# and it seemed fine.

and then just spitballing my total geared up dry weight ready to giant stride at about 214#
214# x 0.025 delta density = 5.35#
so I'll need to add about 5.5# for seawater, bringing me to maybe 9# of lead required in salt

just hoping for help to reality-check that number

and if that's about right for me, that's about the same as what I dove in the springs with then
and so what my family used in the springs with full suits on will probably be about right in saltwater with rashguards or a little thin jacket or shorty.

Not an instructor, but a bit of a nerd... My experience is that freshwater/3mm fullsuit weighting is usually just right for saltwater/no neoprene. But a better way is to use the the buoyancy calculator here ; Info - Optimal Buoyancy Computer
I work it from both ends (put in the knowns like equipment, salt or fresh water) then adjust the unknowns (personal buoyancy) till the answer aligns with reality (actual lead needed on a known dive). Then save it, and adjust for changes to get a pretty accurate estimate of needed lead. For example, recreate your recent freshwater dives so it reflects the weight you needed, then just change the exposure protection and salt/fresh sections to get a good estimate for your upcoming dive.

Respectfully,

James