Buoyancy challenges in fresh water vs salt. Why ?

[astralmind]

We just got back from an amazing 16 days trip to North Sulawesi, diving Bangka 43 times! It was a real blast (Nomad divers rock!) and we managed to hit dive #100 over the course of that vacation. We were very happy with our own personal progress both air consumption and buoyancy wise. Without being anywhere near perfection, I think we reached a point where it was all natural and we felt very much in control.


Over 80 of our dives where done in warm salt water. In between trips we dove a bit locally (Canada, Ontario) but will now be diving on a more regular basis in those conditions. Coming back from our trip, we thought our diving practice would have helped quite a bit. Unfortunately, we found out that cold, fresh, drifty water were not playing nice with us and had a real challenge with our buoyancy. Air consumption is a whole different story with the dives being much more demanding physically but for some reasons, we felt awkward and heavy with a very poor buoyancy.


We're diving with the same gear besides the tank (steel HP100 vs Al80) and wetsuit (full 7mm vs 3mm no hood). For me, at 6'3 230 lbs I'm using 14 lbs, my wife is using 8.


The most obvious difference we could both observe was how much more we needed to rely on our BCDs. In Indonesia I barely touched it, even at greater depth 90' + - my lungs were doing plenty in terms of adjustment. Here I feel like I need to inflate it even for our safety stop! On top of that, we both found that the variation in pressure was much greater in local waters - Going from 40' to 60' in Indonesia practically didn't change a thing while the difference is much more observable here.


Is this simply normal or are we doing something wrong besides having too much weight (we'll shed 2 lbs next time but it definitely feels like there is more to it).


Thanks,

 

[rivers]

You'll need more weight in salt water than in fresh (and depending on the "saltiness" of the salt water, weight requirements can vary between different bodies of salt water). You'll also need more weight with an ali tank as opposed to steel. Ali80s are a couple of kilos positive when empty, as opposed to steel tanks which are negative when empty. So if you are using the same weighting in Ontario that you were using in Indonesia, there is a good chance you are a few kilos overweight, which would explain the buoyancy problems. The difference would be more noticeable as you are having to put more air in your BC to compensate for the additional negative buoyancy, which means more air to expand as you ascend. Hope that helps.

 

[Martigan]

Density of fresh water is less than density of warm water. If nothing else, pressure difference between 40' and 60' should be felt less in fresh than in salt water, all other things considered...
But many things are different... The most important one being, even more than weight (which is important), the wetsuit!
Your tank, your weight are fixed parameters. They change the way you have to dive, your trim, your habits etc. But not your buoyancy while changing depth.
When you dive, your wetsuit is being compressed when you change depth. The thicker your wetsuit, the more air bubbles inside, the bigger its volume, and the bigger the impact of pressure change induced by depth changes are.
You need to compensate for it with your BC more with a thick suit than a thin one. Your lungs are not enough in that conditions.
If you had the other factors:
- you are maybe overweighted, leading to not optimal trim
- your general balance is perhaps different (hood, other tanks), your trim may be affected as well...
- your have more "physical" dives, leading perhaps to more small depth variation than in warm waters with lighter dives...
Try to optimize your weight, to work on your general balance and trim, it will help.
But you won't get rid of the added volume of your wetsuit (except dry suit, which is another story).

 

[TSandM]

If you are diving the exact same configuration in fresh water as in salt, you are at least five pounds overweighted in the fresh water. This is going to magnify any buoyancy issues you have to a very noticeable degree. In addition, strong currents will make you less stable, especially if you are not perfectly horizontal (or if the current isn't!).

 

[DivemasterDennis]

I agree with TSandM. You are, based on your report, overweighted. Being overweighted means more air in the bcd, and more tuning of the bcd as you vary your depth. D a good weight check ibefore you next dive, If you are not shedding 5 pounds, you will at least shed some and it will make a HUGE difference.
DivemasterDennis

 

[Tigerman]

If you are diving the exact same configuration in fresh water as in salt, you are at least five pounds overweighted in the fresh water. This is going to magnify any buoyancy issues you have to a very noticeable degree. In addition, strong currents will make you less stable, especially if you are not perfectly horizontal (or if the current isn't!).

But they arent diving the same gear..
They dive a Steel tank and 7mm suit at home and al80 and 3mm in saltwater according to the op...

 

[KeithG]

it's your change of wet suit. that is why you must use your bcd more at home.

a 7mm provides more bouyancy, but then also looses more bouyancy as it crushes at depth. so the bouyancy swing of a full 7mm will be much larger a 3mm.

this means you will now need to depend on your bcd much more to compensate for the larger swing of your wetsuit as you go deeper.

 

[astralmind]

But they arent diving the same gear..
They dive a Steel tank and 7mm suit at home and al80 and 3mm in saltwater according to the op...

Thanks everyone for the replies. As Tigerman pointed out, we are not diving the same configuration at all. I'd say we use roughly the same weights but that is to deliteberately overweight us a tiny bit to help out with the camera work. So

Warm saltwater: al80, 3mm suit, 14 lbs, slightly overweight (probably 2 lbs too much)
Cold fresh water: steel HP 100, 7mm with hood+thick gloves, 14 lbs

In the end doing a proper weight check is a must and I'll do that as soon as I can (on a charter boat with a group in crazy current, not so easy). That being said I was wondering from your experiences if the environment had an impact (fresh, cold, drift etc... other ?) on buoyancy challenges that went beyond proper weighting.

I think Martigan really highlighted a great point here : "When you dive, your wetsuit is being compressed when you change depth. The thicker your wetsuit, the more air bubbles inside, the bigger its volume, and the bigger the impact of pressure change induced by depth changes are.
You need to compensate for it with your BC more with a thick suit than a thin one. Your lungs are not enough in that conditions. " That definitely would explain the larger variations at least a bit.

Thanks again

 

[beanojones]

What you noticed is why I encourage divers who have the time to give things a try to go with the minimum possible wetsuits in the tropics. Thermal comfort matters but the amount of fiddling needed for no wetsuit (no BCD even require for many) versus the amount of fiddling for even just a full 3 mil suit is huge.

Full 3 mi suits are surprisingly rare since most "3" mil suits actually are 3/2 rentals which have been crushed by repeated usage to bascially 1 mil for much of the suit.

Here's another thing that matters with a steel tank versus an aluminum. The AL80 tanks will be only little negative at the beginning.

Steel tanks will always want to make you turn turtle.When I used steel tanks, I always made a point of demoing hovering by letting the tank gradually roll me over on my back.

A huge hidden factor, though, is that high visibility in nice conditions allows people to run through the initial adjustment to diving quite easily (That initial adjustment of a land animal going underwater, is there on every dive pretty much no matter how much experience one has.) Add a stress factor (cold water and low vis are way up on those lists of stressors) and that period of adjustment becomes extended.

In the tropics, I often do a quick Mask R+R at depth, to start a dive. I did that in cold water (almost 68 degrees F!!), and felt slapped by God for my hubris. It gave me much respect for people who do cold water diving. And of course, that is just not cold at all to many active divers.

In 100'/30m visibility, everyone can check with everyone from the top off the anchor to the bottom. In cold and low viz, you maybe cannot even get visible confirmation at all, and have to rely on touch to check in at the surface.

We do say stumbling along blindly as an metaphorical expression to express being uncomfortable. In cold, low viz, it's not just an expression. You will get used to it in time, and if SB is any gauge come to love it for what it is.

But that's also why people do so much dive travel to warm water, and high viz.

 

[Peter_C]

Here I feel like I need to inflate it even for our safety stop!

You are overweighted if you are adding air at the safety stop with 500 psi in the tank. Empty the wing and start chucking weights, until you are neutral. Shore diving works best for this. You may have to bleed off air from the tank to get to 500 psi.