Weight belt equation

[NewportDiving]

This may be accurate for your area, but here it is very different, for example a:
1/4" (6mm) 2-piece w/ hood, gloves, boots: 10% + 15# women and heavy people need more. I will not do the rest.

Back when I started the LDS provided the following charts as starting points. This is more info than you asked for, but I thought it might help you (and possibly others). Of course, a bouyancy check must then be done. # = US pounds.

In saltwater

Swimsuit or dive skin: 1-4#
1/16" (3mm) one-piece or shorty: 5% of bodyweight
3/16" (4.5mm) 2-piece (john and jacket): 10% of bodyweight
1/4" (6mm) 2-piece w/ hood, gloves, boots: 10% + 3-5#
Neoprene drysuit: 10% + 7-10#
Shell dry suit: 10% + 7-14#

For fresh water, subtract weight according the following:

If you weigh between 100 and 125#, subtract 4# of lead
If you weigh between 126 and 155#, subtract 5# of lead
If you weigh between 156 and 186#, subtract 6# of lead
If you weigh between 187 and 217#, subtract 7# of lead

This is just a rough generalization, but hope it helps.

 

[O2BBubbleFree]

This may be accurate for your area, but here it is very different, for example a:
1/4" (6mm) 2-piece w/ hood, gloves, boots: 10% + 15# women and heavy people need more. I will not do the rest.

10% + 15#?? Wow, glad I'm not in your area! Like I said, the chart's a rough starting point, but it's been pretty accurate for me.

 

[miketsp]

Surface air pressure does affect water density, but more on that later...

.... snip....

Surface air pressure affects water density in that the weight of the air pressing down on the water is much less at high altitudes. In my physics classes I was taught that the density of pure water can be taken as a constant only at sea level.

I think you'd better go back to your physics classes if you think that the reduction of density of water due to altitude is anything significant.

"Water is but slightly compressible, and for all practical steam engineering calculations, may be considered as non-compressible. The coefficient of compressibility ranges from 0.000051 per atmosphere of pressure at 35oF to 0.000044 per atmosphere of pressure at 128oF, decreasing as temperature increases."

An impact in the fifth decimal place is of no interest to divers.

Values taken from
http://energyconcepts.tripod.com/energyconcepts/water.htm

There is an increase in density at depth but in this case the pressure change due to the column of water is much more significant.

 

[Groundhog246]

I'm going to agree and disagree. I can agree with the effect of lowered air pressure on your exposure suit (will affect the air in your drysuit also). And certainly the total pressure at a given depth will be slightly less than at sea level. BUT, the DENSITY of fresh water does NOT change. The change in air pressure will also affect your depth gauge causing an error, but the error will be constant. If it causes it to read 1 foot less at 33 feet, it wall also read 1 foot less at 60 feet.

 

[O2BBubbleFree]

I confess that my above explanation was a bit simplistic. My point was that there are many considerations to diving at altitude, and it shouldn't be done lightly or without proper education.

I guess I should have gone on to mention that the differences shown by a standard depth gauge are a function of both barometric pressure and salinity. As mentioned above, high-altitude salt water diving is pretty rare, so conversion tables, available literature, and altitude-correcting dive computers generally assume fresh water. (Maybe I shouldn’t generalize; the texts and articles that I have read, along with my dive computer, assume fresh water. In the case of my particular computer, fresh water is assumed above 500’).
Standard depth gauges are calibrated for sea level salt water, and will have significant error if used in fresh water above 1000’.

Here’s a web site that goes into detail, should you be so inclined:
http://www.mindspring.com/~divegeek/altitude.htm

On a less technical scale, if you look at your dive table there should be a note mentioning that the table should not be used above 1000'. If you go to your LDS and look at a conversion table that gives theoretical depth at various altitudes, you will see that the differences are more than a straight 'add 1' at all depths' assumption can account for. For example, diving at 6000' with an actual depth of 20' your theoretical depth for dive planning is 25', and with an actual depth of 100' your theoretical depth will be 124'!

A bourdon tube depth gauge will read actual depth at altitude (if you can find one), but they are difficult to use. The scale gradient is not linear, and gets so compact below about 20' that they are very difficult to read accurately. An altitude-compensating computer will read actual depth, but the above-mentioned conversion table for theoretical depth must be used in conjunction with your standard dive tables for dive planning above 1000'.

Based on available literature (PADI, NAUI, etc.), your safety stop at this altitude should be at 12', not 15', your ascent rate should not exceed 30'/min., and your max and recommended depths are reduced. Also, if you drove over a 7000’ pass to get to a 6000’ lake, you must figure your dive as a second dive at 7000’, where all the above numbers are different.

As I mentioned earlier, there are many considerations to diving at altitude, and this post is in no means meant to be an all-encompassing short course. The numbers I have provided are specific for one particular scenario, and given for illustration only. If you are going to dive at altitude, get the proper education.

 

[ShakaZulu]

Yes, it was nice to know MiketSP..................Altitude diving is a completely different game bro.

http://www.sierradive.com/Altitude.htm

 

[O2BBubbleFree]

Just caught a mistake I made in my earlier post. A bourdon tube-type depth gauge gives theoretical depth directly, so is used w/o conversion. I stand by what I said about 'em being hard to read, though.

Thanks for chiming in, ShakaZulu, I was beginning to feel I was diving solo on this thread...

 

[Big-t-2538]

This is more info than you asked for, but I thought it might help you (and possibly others). Of course, a bouyancy check must then be done. # = US pounds.

In saltwater

Swimsuit or dive skin: 1-4#
1/16" (3mm) one-piece or shorty: 5% of bodyweight
3/16" (4.5mm) 2-piece (john and jacket): 10% of bodyweight
1/4" (6mm) 2-piece w/ hood, gloves, boots: 10% + 3-5#
Neoprene drysuit: 10% + 7-10#
Shell dry suit: 10% + 7-14#

For fresh water, subtract weight according the following:

If you weigh between 100 and 125#, subtract 4# of lead
If you weigh between 126 and 155#, subtract 5# of lead
If you weigh between 156 and 186#, subtract 6# of lead
If you weigh between 187 and 217#, subtract 7# of lead

This is just a rough generalization, but hope it helps.

FWIW...this seems to be a pretty accurate stab for me (I dive an AL80 BTW)

For instance, I dive a 6.5mil 2-piece, and I weigh 245.

In FW I dive with 18lbs.....so using the chart....that's 10% (24) + 3 = 27. add 1 category (218 to 248 = subtract 8 pounds for FW)...and I get 27 - 8 = 19lbs.

I recently dove SW and I dove with 21, but I also compensated for the fact I switched to a Steel tank, so that's another 4...essentially I was diving with 25. So, I'd say this little chart seems to be pretty close for thick we-suit diving.

 

[fldivenut]

So what is the formula or method to calculate the required weight one needs diving from a 14mm suit to a 5.6mm suit?

Go to a pool. Throw the wetsuits in the pool and start to throw weights on them. When they fully sink you know how much weight to add for that particular suit. You don't need to compensate for salt/fresh water either since the difference is so small.

If you then have figured out your correct weighting "naked" (i.e. without wetsuit and an empty tank) you always know how to change your weighting due to change in wetsuit.

And if your should be really anal about it and maximize your bouyancy control you should be absolutly neutral at 10ft with an empty tank and no air in your BC. That's your last stop, no need to be able to stay down shallower than that.

Have fun throwing lead in the pool. It is actually really educational. Suits of same thickness from different manufatures has different lift, and also an older more used suit looses some of its lift.

--A

 

[O2BBubbleFree]

Go to a pool. Throw the wetsuits in the pool and start to throw weights on them. When they fully sink you know how much weight to add for that particular suit. You don't need to compensate for salt/fresh water either since the difference is so small.

For me the difference between salt and fresh is 8#, and I don't consider that insignificant. Don't forget hood, gloves, and booties, if you use 'em.