Weighting question

[NorthO]

I have a little technical question about weighting and buoyancy...

I'm going to dive this weekend in a place where there's both fresh and salt water... I think it's called a halocline. Basically there's about 40-50ffw on top, and then only salt water. I was wandering how I should adjust my weighting? Do I have to keep the same amount I have in salt water only, fresh water only or somewhere in between?

Thanks!

 

[k ellis]

unique situation but Id do the weighting based on salt water. You can adjust the excess with air in your bcd for fresh water but its kind of hard to add weight when you dont have extra weight on you. So Id say base it off salt water

 

[The Kraken]

Well, you should be heavy at the start of your dive because of a full tank.

I'm guessing you'll be diving an aluminum 80.

Your weighting needs to be adjusted so that you're ever so slightly heavy in order to maintain a safety stop.

So, putting all things into consideration and given the swing in buoyancy between a full and empty tank, I'd say adjust your weighting for fresh water . . . that's where you'll be doing your safety stop.

the K

 

[Louma]

I agree with The Kraken and then add a slightly different twist. As you descend you would need less weight because the pressure of the depth will start taking over. So I would weigh for the freshwater part of the dive and you should be OK when you hit the 40 to 50 ft mark. I would say the best suggestion would be ask others who you are doing the dive with on how they handle this situation.

 

[spectrum]

I'm inclined to agree with salt water as long as the fresh water extends that deep.

At the start of the dive it may be a toss up since cold water exposure protection is sure to loose the 6-8 pound difference you need to be OK in salt water at 40+ feet. If you are controlling your drop you may need to suddenly dump some BC air to penetrate.

At the end of the dive you will of course have dropped a good 5 pounds due to air consumption. As long as you get back into fresh water before your suit rebounds too much the less dense water will relax your weight requirement.

The real question I have is how predictable is this threshold.

Pete

 

[Charlie99]

Kraken is onto something, but you also need to have enough weight to be under control in saltwater at 40-50' with a mostly empty tank. If this is greater than your fresh water safety stop weighting, then that's what you need to bring.

If you have a 7mm wetsuit, then the freshwater safety stop needs the most lead. (This assumes wetsuit compression between 10' and 40' is greater than 6 pounds).

If you have a 3mm wetsuit and therefore less loss of buoyancy at depth, then most likely you saltwater weighting at 40' is the determining factor.

Personally, since it is much more difficult to be underweight than overweight, I'd split the difference and go 3 pounds more lead than my freshwater weighting. Since my fresh to saltwater adjustment is 6 pounds, this means that I'll be 3 pounds less than my saltwater weighting. I'm quite sure that this would still leave me in control at 40' in saltwater.


Charlie Allen

 

[NorthO]

Thanks guys

I'm going to dive dry since water temp wil be around mid-to-high 30's. I have a CD4 dry w/ polar hiloft underwear, I usually have 32 pounds of lead and a steel 80 in (cold) salt water and I usually don't put air in my BC during the dive, only in my suit.

Like spectrum said, I think I'll try to know how reliable the limit between fresh & saltwater is... I guess once I'll hit that limit, the compression of the suit should compensate for the loss of buoyancy in salt water... I'll keep you posted after my dives!

 

[NorthO]

Well, it turned out the limit between fresh & saltwater changes a lot! We went with our usual saltwater weighting.

First 2 dives were on a wall, water temp 34F. We descended at 80 feet in the afternoon, tide was rising. We met the halocline at around 35 feet, viz was poor until then and after that limit everything cleared up! There was absolutely no light left at 40 feet, but the viz was great! With our lights covered we were floating in absolute darkness... It was a great feeling!!!

On our second dive (night diving, but it made no difference at depth), the tide was low and we met the halocline at 15 feet.

On sunday morning we made a shore dive, but the conditions were pretty similar except visibility, which was still poor at depth, around 10 feet. Halocline was at 30 feet, and with the rising tide in the bay, the currents were very unpredictable!

As for our buoyancy, our safety stop was pretty easy to do except when we met the halocline at 15 feet... We managed to stay between 13 and 15, but the change in salinity made it a bit harder than usual. But it was definitely a good exercise to practice my buoyancy!

 

[scubadiver888]

Just curious, where was this?

 

[NorthO]

Just curious, where was this?

In the Saguenay fjord, about 3 hours NE of Quebec city.