Silly newbie weighting mistake

[Phishie]

I got my OW during my weddingmoon in Jamaica last September so most of my diving was just in my suit using 8 lbs. I then got my AOW in 3 more dives at home with an aluminum tank and wetsuit (no gloves) at a freshwater quary (Lake Rawlings, VA) using 24 lbs about a month later. No more dives since then :depressed:

Last weekend I took a trip out to Laguna Beach with a friend and got in 3 dives. Yay! None of our dives were more than 25' deep.

1st Dive just fine. Wetsuit, rental aluminum tank, 24lbs salt water. Went okay.

2nd Dive. Same, but using a second tank (steel). Was way overweight and couldn't figure out why. Had to keep my BCD almost full to stay afloat.

Not sure why, dropped 4 lbs (now at 20) before the third dive the next day.

3rd Dive. Aluminum tank again. Couldn't get underwater Ended up having to put back on an extra 3lbs on just to get under.

I had forgotten completely that an aluminum tank and a steel tank are different weighting with the steel being a lot heavier. Luckily we were doing easy shallow shore dives in good weather so there wasn't much risk.

Not a very exciting story but it might enforce for newbies like myself that read this forum like myself the importance of considering all your gear and any changes and how those changes could impact your dive. I was just so excited to be back in the water I didn't even think about the difference in the tanks.

Silly newbie mistakes

 

[vixtor]

If you have a full tank and you need to adjust your weighting, because you never dived it before, you can adjust your buoyancy in water, and then add approximately 1.2-1.3kg/cubic meter of air in your tank.

For example, if you adjust your weighting for a 15L steel tank, filled to 200atm, then you have 3m^3 of air inside, so you need to add 3-4kg of weights to compensate for when the tank will be empty.

After the dive, you can check the weighting again and record the details in your logbook for future reference.

Btw, adjusting your weighting would mean to check in the water that with the BC fully deflated, when you inhale a normal breath you are floating at eye level (and raise slightly more with a full breath), then sink when you exhale.

 

[snepdiver]

This chart my provide you with a little extra information in helping you determine weighting by tank use. Don't forget, As I am sure you have learned, Salt water and exposure protection also play just as important a role in weighting.​

This chart is for Worthington steel cylinders but you can get a general idea.​

​

http://www.divesports.com/Worthington-High-Pressure-X8-130-Steel-Cylinder-p/wor-x8-130.htm​

 

[Shasta_man]

just to add to Vixtor's weight testing, you want to do that with 500lbs in your tank, the amount at the end of your dive. As you use air, your tank becomes more buoyant. Not intuitive that lack of air weighs more,.

 

[vixtor]

just to add to Vixtor's weight testing, you want to do that with 500lbs in your tank, the amount at the end of your dive. As you use air, your tank becomes more buoyant. Not intuitive that lack of air weighs more,.

"Lack of air" weights less, not more - you are about 1.2kg lighter for each cubic meter (35 cubic feet) of air that you use. And you need to add more weight to compensate for this, in order to stay under water; otherwise you will become too buoyant and pop to the surface.

That's why I said that you can make a buoyancy test with full tank (if you don't have the opportunity to test the buoyancy with the empty tank before filling it) and add weight to compensate for the air that you will use (making you lighter at the end). Then, you can double check at the end of the dive whether you can drop some weight, to optimise your weighting for future dives.

 

[scubadude79]

Tank volumes remains same, more pressure = higher air density which is more weight.
As the pressure is reduced (air is used), the density of air is reduced causing the tank to be lighter.
Typically it is stated that a person using an AL tank should add a few lbs so you don't start to accent slowly and unexpectedly when making the safety stop.
They also sell tank boots for AL tanks with a couple pounds integrated to offset this.

 

[chucksaul]

Before we get into the higher math and bouyancy questions maybe we should ask a couple of questions regarding dive times... I am willing to bet changes in weight have more to do with water comfort, relaxation, etc... If my theory holds water then times should have incresed and I would bet better bouyancy and he probably didnt even realize that was it... The more you dive the better you get...