Advice please

Hi all this is my first post on here and i have a question, i am a bit of an air pig so i bought a 15L tank to use this weighs 3.5kgs more than ,my previous tank which was a twelve litre dumpy, i dropped my weight 2k , thinking the weight of tank would compensate for me dropping weight , but no i couldnt stay down and had to go back up to my original weight , any answers

cheers Skidmarc

If the cylinder is Aluminum, it will typically swing from about 4 lbs negative to 4 lbs positive buoyancy, making it difficult to stay down once it is less than 69 bar. Even a steel cylinder, there will be some swing in buoyancy, though not nearly as much & usually remain negatively buoyant.

The top-side weight of the tank isn't as important as the buoyancy characteristics of the tank when full and empty. Do you have the make/model of the tanks? There are a number of good charts online that describe how positive or negative a tank can be - that can help to establish how much additional lead you'd need to carry. It's entirely possible that the larger tank is less negative than the smaller tank (as is the case with the PST HP80 vs. HP100 tanks).

No they are both steel ,many thanks for your reply

Simple - In general a 12 and a 15 will have similar buoyancy characteristics.

The 12L is smaller and lighter. The 15L is larger and heavier. But, in both cases the ratio of volume:weight is about the same when empty, making them similar in buoyancy empty, although the 15L will have the larger change in buoyancy between full and empty because of the more air.

Check out your cylinders here:
UK Scuba - Cylinder Buoyancy Calculation

I tend to swap between a 12 and 15 without changing my weights. Many dive boats do this - give you a 15L for the first dive, and a 12 for the second. I will in general finish both dives just able to hold a 5m safety stop without being overweight (or underweight).

Hope this helps

Jon

PS remember, what counts is your buoyancy at the END of the dive. The larger air capacity, and hence larger buoyancy swing of the 15 will mean that you have to be more negative at the start of the dive to finish at the same neutral buoyancy as with the 12.

Dry weight can be deceiving -- remember that things, as they are put into water, displace their volume. The volume of water they displace, compared with the density of the object, results in a certain degree of lift or buoyancy. With tanks, this massively reduces the effect "weight" of the tank, once it is underwater. It's not particularly easy to predict what the net effect will be for a given tank (unless you go through a great deal of math and know a LOT about the tank in question) but there are charts available of the buoyancy characteristics of various tanks, because somebody has done the experiment for you. It is the buoyancy empty about which you care, because that's what determines how much weight you need to carry to hold a stop when your tank is empty. Your two tanks, though quite different on land, may behave very similarly underwater and when empty.