Proper Weighting

[O2BBubbleFree]

Your long answer is correct, but irrelevant.

True. Pardon my drift. I've been surprised lately by the number of divers on this board who don't realize that extra training/education is required for diving above 1000', and I guess I was trying to emphasize this point for anyone reading the thread.

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.

In the situation I cited above, the change in lead I required at 6000' offset the change from saltwater to fresh water, all other factors being equal. At least real-world equal; I was using the exact same wetsuit and gear, but I don't know that the barometric pressure and water density were the same each time I dove at either location, and my weight might have fluctuated a pound or two, but I consistently needed the same weight at Lake Tahoe and at Monterey. I dove the American River (sea-level fresh water) a couple of times in that same timeframe, but unfortunately I was using a different wetsuit combination, so can’t directly compare freshwater sea-level to saltwater sea-level.

I feel that this is about as ‘real’ as it gets for me. Equations comparing compression of neoprene WRT altitude, IMHO, would be theoretical since lab results would have to be proven, and more than likely modified, by experiential data to be useful as a guide for real-world divers.

When I took my altitude diving specialty course, they taught that changing altitudes makes a significant enough difference that you will have to add additional weight to get neutral, and my limited experience supports this.

p.s. I will be offline for the next week or so, so will not be resonding to any more questions regarding my posts. The basic information I have presented is available in published SCUBA liturature and altitude diving certifications classes.

 

[String]

Diving at altitude IS taught here as part of our equivalent open water. The tables also contain altitude and transfer tables so pre dive planning can be done for any altitude (and change of altitude pre dive).

Its not THAT complicated. 10 mins work if that.

 

[donacheson]

In the situation I cited above, the change in lead I required at 6000' offset the change from saltwater to fresh water, all other factors being equal. At least real-world equal; I was using the exact same wetsuit and gear, but I don't know that the barometric pressure and water density were the same each time I dove at either location, and my weight might have fluctuated a pound or two, but I consistently needed the same weight at Lake Tahoe and at Monterey. I dove the American River (sea-level fresh water) a couple of times in that same timeframe, but unfortunately I was using a different wetsuit combination, so can’t directly compare freshwater sea-level to saltwater sea-level.

I feel that this is about as ‘real’ as it gets for me. Equations comparing compression of neoprene WRT altitude, IMHO, would be theoretical since lab results would have to be proven, and more than likely modified, by experiential data to be useful as a guide for real-world divers.

When I took my altitude diving specialty course, they taught that changing altitudes makes a significant enough difference that you will have to add additional weight to get neutral, and my limited experience supports this.

p.s. I will be offline for the next week or so, so will not be resonding to any more questions regarding my posts. The basic information I have presented is available in published SCUBA liturature and altitude diving certifications classes.

I was wrong - altitude will make a detectable difference. I found some data on neoprene foam which indicates it would expand by about 6 percent in going from sea level to 6000 feet. My own wet suit displaces about 27 pounds of water at sea level, so at 6000 feet it would displace another 1.6 pounds. If I were minimally weighted at sea level, with the same weight I'd be noticably (1.6 pounds) light at the end of a dive at 6000 feet in water of the same salinity.

If I went from minimally weighted in salt water at sea level to fresh water at 6000 feet without changing weight, I 'd expect to be about 6 (.025x300-1.6) pounds heavy at the end of a dive.

Of course, if one dives a shell dry suit, this discussion doesn't apply.

 

[donacheson]

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

I remembered having seen some data on closed cell neoprene foam and this morning found some notes that I made. Unfortunately, I can't recall where I found the data except it was on the Web. In any case, the data given was the reduction in insulating value of the foam at several depths. Assuming that the reduction in insulating value is directly proportional to a reduction in thickness , the following table obtains:

Depth Foam Thickness
(feet) Relative to that
at 1 atmosphere

20 0.8
40 0.65
60 0.50
100 0.30

When worn on the body, I'd expect that the density of wet suit foam is inversely proportional to thickness since the surface area of the wet suit doesn't change significantly. (I haven't thought this through, but I wonder if comression in three dimensions, not necessarily the same in each dimension, would affect data taken with a wet suit unconstrained by a body inside it.)

In any case, when the data is plotted an extrapolation to 0.8 atmosphere (the pressure at about 6000 feet) indicates the foam thickness increases by about 6 percent.

 

[goodknight411]

10% is a good starting point, but it probably won't be right for you.
I've noticed that people vary greatly, and you can't always find a logically explination for the amount of weight you'll use. example, my beanpole brother uses about 14lbs. He's super skinny, and really tall. I use 6lbs. I'm short, but weight the same amount as him. Go figure.
All i can say is get in the water and play around for a while til you get it right.