Proper Weighting

[donacheson]

I typed up some guidelines that I was given by my LDS and posted them on the above-mentioned thread.

http://www.scubaboard.com/t61010.html

These are fine for a starting point. Work for me as well as others that have used 'em. Of course, this just gives a rough starting point and must be fine tuned for different dive conditions (salinity, altitude, etc..)

Altitude!?

 

[O2BBubbleFree]

Altitude!?

Um, yup. Please read the above-referenced thread, which discussed this pretty aggressively.

Short answer, due to the lower barometric pressure the bubbles in your suit expand, requiring more lead to sink it.

Long answer, there are LOTS of considerations if diving 1000' or more above sea level, including adjustments to the dive tables (lower NDL limits and shorter times), different safety stop times and depths, etc... And old-fashioned depth gauges are not accurate at altitude.

If you're planning to dive above 1000’, please get the training or education required.

 

[Don Burke]

Short answer, due to the lower barometric pressure the bubbles in your suit expand, requiring more lead to sink it.

Based on some measurements I made with some neoprene at a fifteen foot pool, I would say that it would take a frightful amount of neoprene to make more than one pound of difference on an ascent of five thousand feet.

 

[The Kraken]

Have to agree with Don. The increase in flotation based on increase in altitude would be physically insignificant.

 

[Boat]

My friend had me adjust weight at the end of a dive in shallow water with about 500-700psi left. I had been diving with about 8lbs. I'm down to 4 now, and probalby would be fine with 2. I'm right at 6ft/160ish pounds, Seaquest Pro QD+, Catalina (compact)AL80's, 2mil shorty (wet suit). He had me empty my bcd and I took off weight (all but the 2lbs in the trim pockets) till I could barely stay down...worked like a champ for me. Feels much better getting the weight off and I think it's improved my air consumption rate significantly. Take the time to check the weight...I wish I'd done it earlier and reaped the benefits.

 

[O2BBubbleFree]

Based on some measurements I made with some neoprene at a fifteen foot pool, I would say that it would take a frightful amount of neoprene to make more than one pound of difference on an ascent of five thousand feet.

As I mentioned on the other thread, I used to live in Sacramento, CA. I dove several times in Monterey, CA, and in Lake Tahoe, NV. For both locales I used a 7mm john and jacket, 3mm hood, 3mm gloves, booties, and al80.

I used the same amount of lead for both locations, so with that much neoprene the difference in altitude was offset by the saltwater to freshwater change.

So, depending on the amount of neoprene in question, I guess if salinity is significant, then altitude can be.

I’d be interested in hearing how you conducted your tests.

 

[Don Burke]

I’d be interested in hearing how you conducted your tests.

I went to Dive Quarters in Virginia Beach and put my wetsuit, booties, and gloves in a mesh bag. I then used various dive weights to find out how much weight it took to sink the gear at various depths.

At the surface, it took sixteen pounds to sink the gear. As I got deeper, the amount of weight needed was proportional to the change in total pressure with a one pound offset. I attributed that to stiffness in the neoprene bubbles.

I tossed the raw data years ago. I posted it to the VBtech list at the time and can probably retrieve it if I work at it long enough.

I've actually worked the figures out and an ascent to five thousand feet is a total pressure change of 1.2, so I will retract what I said before and guess at about four pounds of difference for weighting.

If you weigh about two hundred pounds fully geared up, that would come up about even with a change in salinity from 35ppt to zero.

 

[donacheson]

Um, yup. Please read the above-referenced thread, which discussed this pretty aggressively.

Short answer, due to the lower barometric pressure the bubbles in your suit expand, requiring more lead to sink it.

Long answer, there are LOTS of considerations if diving 1000' or more above sea level, including adjustments to the dive tables (lower NDL limits and shorter times), different safety stop times and depths, etc... And old-fashioned depth gauges are not accurate at altitude.

If you're planning to dive above 1000’, please get the training or education required.

Your long answer is correct, but irrelevant. As for your short answer, it's hypothetical until someone has some real numbers on the decrease in density of closed cell neoprene foam with decreasing atmospheric pressure. Absent these numbers, I'll assume that the density change is small enough to require, at most, an insignificant change (less than one pound) in weighting.

 

[donacheson]

I went to Dive Quarters in Virginia Beach and put my wetsuit, booties, and gloves in a mesh bag. I then used various dive weights to find out how much weight it took to sink the gear at various depths.

At the surface, it took sixteen pounds to sink the gear. As I got deeper, the amount of weight needed was proportional to the change in total pressure with a one pound offset. I attributed that to stiffness in the neoprene bubbles.

I tossed the raw data years ago. I posted it to the VBtech list at the time and can probably retrieve it if I work at it long enough.

I've actually worked the figures out and an ascent to five thousand feet is a total pressure change of 1.2, so I will retract what I said before and guess at about four pounds of difference for weighting.

If you weigh about two hundred pounds fully geared up, that would come up about even with a change in salinity from 35ppt to zero.

I'd love to see the data.

When you say" ... the amount of weight needed was proportional to the change in total pressure with a one pound offset", do you mean that the weight required to just hold the wet suit at depth decreased linearly with depth?

A simple model would have the density decreasing inversely with absolute pressure, but since neoprene foam isn't all gas, there are more complex limits at pressures both much greater and much less than 1 atmosphere.

 

[Don Burke]

I'd love to see the data.

I'll track it down this week. Don't get too excited. There were only three datapoints, one foot, nine feet, and fourteen feet.

When you say" ... the amount of weight needed was proportional to the change in total pressure with a one pound offset", do you mean that the weight required to just hold the wet suit at depth decreased linearly with depth?

Actually, the weight decreased with total pressure with the one pound offset. So sixteen pounds at the surface would translate to nine pounds at thirty three feet instead of the theoretical eight pounds.

A simple model would have the density decreasing inversely with absolute pressure, but since neoprene foam isn't all gas, there are more complex limits at pressures both much greater and much less than 1 atmosphere.

I've dealt with neoprene that is not foamed. The stuff sinks. That tells me there is a depth where the suit would actually go negative, which also means my simple "reciprocal of absolute pressure plus one pound" is not true for all cases.