Buoyancy calc of the day

[bkotheimer]

Since I know that Tobin is the god of buoyancy, I usually post such questions here on the DSS board where I know that he or his disciples will have the answer.

To the point... I have a new Henderson 7mm and a Henderson 7/5 hood. AL80, very small backup light (no can light or anything).... a video camera that is only slightly negative (and not part of my weighting anyway).... and Biofins, which I just found out are -5 in salt water.... SS plate with a Torus 26. I'm 5'9"/140#, so I sink like a stone.

I've been diving with 10# (2x5) lead on a belt and another 6# (2x3) lead on my upper cam band.

I usually dive Anacapa, places like that, not particularly deep. So I rarely get to 500psi before I'm headed back up. I FEEL like I might be overweighted at my safety stop, but I could contribute it to still having about 1k-1200 psi in the tank.

I believe that I recently saw a discussion regarding the buoyancy check at surface before the dive being inaccurate due to the weight of the gas in the tank. However, I also recall Tobin's contention that the difference in gas consumed at the end of the dive is about equal to the compression of the suit between the surface and 15' (safety stop), making the surface check before the dive relatively accurate.

So a couple questions I've been struggling with...

1. First of all, is this true that the surface check with a full tank before the dive is about equal to the safety-stop check with 500 psi, due to the suit compression offsetting the weight of the gas?

2. On the average, does a new 7mm suit + hooded vest have the 24# of buoyancy (approximately) that I am compensating for with 6# (SSBP) + 2# (reg) + 16# (lead), or am I overweighting even in theory?

3. Should I be counting my Biofins as ballast and losing another 5 lbs of lead once I have the numbers right?

I appreciate any input. I'm trying to get at least in the ballpark before doing further checks in the water.

bk

 

[cool_hardware52]

Since I know that Tobin is the god of buoyancy, I usually post such questions here on the DSS board where I know that he or his disciples will have the answer.

To the point... I have a new Henderson 7mm and a Henderson 7/5 hood. AL80, very small backup light (no can light or anything).... a video camera that is only slightly negative (and not part of my weighting anyway).... and Biofins, which I just found out are -5 in salt water.... SS plate with a Torus 26. I'm 5'9"/140#, so I sink like a stone.

I've been diving with 10# (2x5) lead on a belt and another 6# (2x3) lead on my upper cam band.

I usually dive Anacapa, places like that, not particularly deep. So I rarely get to 500psi before I'm headed back up. I FEEL like I might be overweighted at my safety stop, but I could contribute it to still having about 1k-1200 psi in the tank.

I believe that I recently saw a discussion regarding the buoyancy check at surface before the dive being inaccurate due to the weight of the gas in the tank. However, I also recall Tobin's contention that the difference in gas consumed at the end of the dive is about equal to the compression of the suit between the surface and 15' (safety stop), making the surface check before the dive relatively accurate.

So a couple questions I've been struggling with...

1. First of all, is this true that the surface check with a full tank before the dive is about equal to the safety-stop check with 500 psi, due to the suit compression offsetting the weight of the gas?

2. On the average, does a new 7mm suit + hooded vest have the 24# of buoyancy (approximately) that I am compensating for with 6# (SSBP) + 2# (reg) + 16# (lead), or am I overweighting even in theory?

3. Should I be counting my Biofins as ballast and losing another 5 lbs of lead once I have the numbers right?

I appreciate any input. I'm trying to get at least in the ballpark before doing further checks in the water.

bk

Your ballast totals agree with mine, 24 lbs. (less the fins) 24 -4 for your empty al 80 leaves ~20 for your suit. That's reasonable for all the neo you are using.

If your fins are actually -5 lbs they are part of your ballast, ballast is after all everything you take that does not float.

In a buoyant suit and single tank I like to use "eye level with no gas in the wing and a full cylinder" mostly because it's much easier to adjust ballast at the beginning of the dive at the surface where one can easily add or remove lead.


Remember the absolute minimum ballast required is equal to the buoyancy of your suit at your shallow stop with a near empty cylinder.

I'd suggest breathing your tank down to ~500 psi at your shallow on your next dive and noting how much gas you need in your wing to stay neutral.

Reach back and squeeze the wing. If you have a bunch of gas in your wing you can shed some lead.

Tobin

 

[bkotheimer]

Thanks Tobin! I'll keep all that in mind and fine-tune both at the beginning and end of the dive, including trying to get down to 500 psi for the safety stop.

Yeah, 20# for the neoprene and 4# for the empty AL80 tank, right. Then I'm probably on the right track. However, my fins may very well be pulling me down and may justify losing a couple lbs if not an entire 5.

Thanks again!
On a separate note, there's Torus 26 coming your way for repair soon in 2010!

 

[climbr1]

bk I hope Im not hijacking but it looks like you got your question answered. I have a calc question for Tobin as well.
I was certified 15 years ago and am just now getting to buying gear. I am going to go the Bp/w route and have been doing a lot a reading and think I have narrowed my selection down to one of your rigs. My set up will include a TLS 350 with 5 mm neo hood and wet gloves. My issue is im going with a completely new system. My drysuit is new and I have not yet had it in the water. Being winter here its hard to get into the water and I want to get a bc and do my drysuit class and work out the kinks in a few pool sessions before open water. I hope to one day get into doubles but I will just buy a new wing when that time comes. But I am also on a rescue team and we end up in fresh water of any depth summer or winter so I would like some what of a wider ranged lift.
So I am not sure the questions I need to ask, except that I am looking at the single tank with pro-fit harness and SS plate. Weighted STA and either the torus 35 or lcd 30 wing. I have been using an Al80 currently but plan on buying a steel 95 or 105 by year end. I dont expect a turn key system that will work for everything and its ok if I need to add gear and adapt, thats part of the game. Any advise? Im 5'11 and 170 lbs. Anything else I can tell you? unfortunately I dont know what my current gear system demands in terms of weight. In warm fresh water without any suit I use 5 lbs and when I dove catalina in a 6mm wetsuit I was using about 25 lbs total.

 

[onemorefalcon]

double post

 

[cool_hardware52]

bk I hope Im not hijacking but it looks like you got your question answered. I have a calc question for Tobin as well.
I was certified 15 years ago and am just now getting to buying gear. I am going to go the Bp/w route and have been doing a lot a reading and think I have narrowed my selection down to one of your rigs. My set up will include a TLS 350 with 5 mm neo hood and wet gloves. My issue is im going with a completely new system. My drysuit is new and I have not yet had it in the water. Being winter here its hard to get into the water and I want to get a bc and do my drysuit class and work out the kinks in a few pool sessions before open water. I hope to one day get into doubles but I will just buy a new wing when that time comes. But I am also on a rescue team and we end up in fresh water of any depth summer or winter so I would like some what of a wider ranged lift.
So I am not sure the questions I need to ask, except that I am looking at the single tank with pro-fit harness and SS plate. Weighted STA and either the torus 35 or lcd 30 wing. I have been using an Al80 currently but plan on buying a steel 95 or 105 by year end. I dont expect a turn key system that will work for everything and its ok if I need to add gear and adapt, thats part of the game. Any advise? Im 5'11 and 170 lbs. Anything else I can tell you? unfortunately I dont know what my current gear system demands in terms of weight. In warm fresh water without any suit I use 5 lbs and when I dove catalina in a 6mm wetsuit I was using about 25 lbs total.

The plate and harness part of your gear is easy, medium SS plate.

To size the wing I need to know how buoyant your suit is, info on your wetsuit doesn't really help.

I can make an educated guess, but it's still a guess.

What undies will you be using?

Tobin