Buoyancy, Balanced Rigs, Failures and Ditching – a comprehensive tool

[Fastmarc]

I'm just getting a chance to try this out. Pretty neat. Thanks.

I used it to check my current set up. I consider myself properly weight with my present set up. I'm 5' 8.5", 155 lbs. I use a jacket style BCD and carry 8lbs (4 trim, 4 intergrated ditchable). I dive in a 2mm shorty (old) and only dive Al80's.
I left my BCD at zero as I do not know what it is (Sherwood Avid a couple of years old, so pretty new).

This has me at the end of the dive at -1.2 @ surface and -2.6 @ 15'. This feels about right to me.
On my last dive, I was curious if I could reduce my weight, so I did a check with a tank at 1000 psi (no 500 psi available) without the 4lbs ditchable. Letting out all the air out of the BCD and lungs didn't have me going under much and as soon as I took a breath, I was up.

I'm planning on getting the Xdeep Ghost which they say is about 5lbs, so using this it seems I can remove 4lbs and just dive with 4lbs.

Do I have this correct?

 

[rsingler]

I'm planning on getting the Xdeep Ghost which they say is about 5lbs, so using this it seems I can remove 4lbs and just dive with 4lbs.

The Ghost is 2.3kg out of water weight, not in-water buoyancy. Looking at the plate and its cutouts, I would guess more like -1# in water.
Since by your description and the spreadsheet, you are about a pound or two heavy at present, I'd start by removing 2# and see how that worked. I'd bet you could remove 3# total, with your new Ghost.
I'm glad the tool worked out for you. Your experiment was perfect. By the numbers you are -2.6 at your safety stop. Removing 3# (+4#, but carrying an extra pound of air) worked, but you had to exhale to stay under. That tells me that you are a pound or two more buoyant than the spreadsheet predicted. That extra buoyancy could be personal buoyancy, a little trapped air in your bcd at the time of the test, or an inaccuracy in the spreadsheet.
But 1-2# is close enough for me! Thanks for the data!

 

[Fastmarc]

Thanks for the feedback. Much appreciated.

I'm a pretty lean, so I'm tempted to think it may be the BCD. When I get the chance I'll check to see how buoyant it is and report back.

 

[rsingler]

Thanks for the feedback. Much appreciated.

I'm a pretty lean, so I'm tempted to think it may be the BCD. When I get the chance I'll check to see how buoyant it is and report back.

Lol! So you noticed the subtle way I asked if you might be a little fluffy.
It's probably spreadsheet error. It gets harder and harder to be accurate with buoyancy predictions at the extremes of exposure suits. An old 2mm shorty and a thick Farmer John are problematic.
For personal buoyancy, jump in the ocean with a snorkel and a bathing suit and see where you float with a half breath. For me, about an inch of skull floats above the surface. So I call myself +1#. If you're more buoyant, guess at how much of your head is exposed, with your total skull weight about 8-10 lb.
If you're negative with a half breath, try to estimate how much you need to inhale to have just the top of your head awash at the surface. At 2.2 lb per litre inhaled, that's how negative you are.

Dive Safe!

 

[Fastmarc]

LOL

 

[DavidFL]

Just trying out the spreadsheet the first time. Noticed no data entry point that I could find for salt or fresh water. Is the whole spreadsheet assuming saltwater everywhere?

Edit; please disregard; just did the math and found the density of water in the CALCs tab...looks like it is set by default at a sort of average sea water density.

Second Edit: making changes, even big changes, in the Density of Water at D16 of the CALCS spreadsheet doesn't seem to have the affect I would have expected elsewhere in the sheet when using a Drysuit?

 

[rsingler]

Just trying out the spreadsheet the first time. Noticed no data entry point that I could find for salt or fresh water. Is the whole spreadsheet assuming saltwater everywhere?

Edit; please disregard; just did the math and found the density of water in the CALCs tab...looks like it is set by default at a sort of average sea water density.

Second Edit: making changes, even big changes, in the Density of Water at D16 of the CALCS spreadsheet doesn't seem to have the affect I would have expected elsewhere in the sheet when using a Drysuit?

No, the drysuit calcs do not reference cell D16. My error. They have a close approximation of D16 embedded in their buoyancy formula, and we'll incorporate what you found in the next version. So your conclusion is correct: this all assumes sea water.

For fresh water, "deduct 3-5 lb".

We are in the process of the next big change to this toy. There are overestimations of surface buoyancy that are currently under evaluation. That's the easy one - things like hood or no hood, gloves or no gloves. And a slight, general overestimate of the effect of body surface area.
There are also errors in wetsuit compression. To our surprise, there may be more retained buoyancy at depth than we had originally thought.
Stand by...We should have a new set of formulas in about four months. We haven't forgotten this toy. It's just a slow process, accumulating data.

 

[DavidFL]

Thanks for the reply! Like the other commenters, I really appreciate all the work to set this up and share. I've been certified for 50+ years, but just now getting ready to do some serious cold-water/drysuit diving. Will be doing a trip to the PNW in November, with partially rented gear and my new drysuit, so I'm finding the capability to plug and play and look at the results to be very useful.

 

[SJT1961]

Thanks for all the hard work! It's a really useful spreadsheet. Would it be possible to put a cell in the initial data entry that could be salt or fresh water and then calculations adjusted accordingly? You say subtract 3-5 lbs. for freshwater. Isn't it some percentage (~2.5% if memory serves) of total overall weight of person+rig? You know the weight of diver, tanks, gas, rig, weights, etc so this could be calculated more precisely. Would help those of us that go between salt and fresh in different rigs. Thanks again.

 

[rsingler]

Would it be possible to put a cell in the initial data entry that could be salt or fresh water and then calculations adjusted accordingly?

Of course! Already being implemented. Great suggestion, thanks!