Buoyancy?

Question
I am a new diver and have about 25 logged dives. I have been diving with my father and we have both been having trouble with buoyancy. We dive in Maine during the summer and are both under 180 lbs. During our first few dives we went swimming in a local quarry. We used 35 pound weight belts, these worked very well in fresh water. Our trouble occurred when we started diving in salt water. We couldn't get below the surface without holding rocks or other things to add to out weight. My question is what is affecting our buoyancy? Is it our aluminum tanks? We each have 7mm full body dive suits. I have thought about it and the only way I can see to conquer our problem is to add more weight to our belts. However I would also like to see if you think getting steel tanks would help.

Answer
Hello Evan,...To answer your first question..."what is affecting our buoyancy"...What you are experiencing is the difference between the buoyant force of salt water compared to fresh water. Have you heard the name Archimedes?...or Archimedes Principle...which states.."A body immersed in a liquid,either wholly or partially, is buoyed up by a force equal to the weight of the displaced liquid"...which simlpy means the buoyant force of a liquid (fresh water/salt water) is dependent upon its density, that is, its weight per unit volume. Fresh water has a density of 62.4 pounds per cubic foot....Saltwater is heavier, having a density of 64.0 pounds per cubic foot...therefore you are being buoyed up by a greater force in Saltwater, compared to being in fresh water...and thus the reason why you and your father were having problems getting submerged in Saltwater. You are going to have to add more weight...and even though steel tanks are less buoyant than aluminum ones...the switch to steel may still not be enough, but it will definitely help. I would first try adding more weight ....(a little at a time)...when you do your next Salt water dive. The thing to remember is...the weight belt is used to achieve NEUTRAL buoyancy. You should carry enough weight so that your buoyancy is slightly negative, with a full tank...and becomes slightly positive as air is consumed. The positive buoyancy provided by your dive suit is usually the largest contributing factor in determining your weight requirements.A good test to check for your proper amount of weight is to take a full lung of air at the surface....you should be able to maintain eye-level with the surface of the water....then with exhaling you should feel yourself slowly starting to sink. Don't forget Evan...the deeper you dive...less weight will be required to achieve your desired buoyancy, because of your dive suit's compressibility. I hope this answers your question(s)...it really comes down to understanding what Archimedes Principle is all about. Take care...and remember....PLAN your dive....dive your PLAN

                  Cheers, Gary.