DiveMom1
Guest
Just curious Rick.. what would the difference be for being neutral at the surface Vs 10' with a near empty tank. I'm sure there is a reason, just trying to learn here.
Best time to do the weight check
- Thread starter k374
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
dkktsunami
Contributor
Noooo........ This bad procedure made it into at least one major agency's weighting advice a few years back (so dkktsunami can be excused for recommending it) - it was bad advice then and it's still bad now. You want to be able to control your buoyancy all the way to the surface! You need just enough weight to be neutrally buoyant at the surface at the end of the dive - which can be a bit (up to a couple pounds) more than the minimum needed at 10', depending on your anti-exposure suit.
Rick
Good point. With a good exhalation from 10' I have no problem moving in a slow and controlled fashion to the surface, but I understand your logic.
BabyDuck
Contributor
no.
*you* are the changing factor! as you get better or more comfortable or gain or lose weight or many other things, you might find that your weighting needs change. however, what you needed last dive is a fine place to start if what you had last dive worked fine.
for example, i started diving dry this january. i needed 14 extra pounds the first 3 or 4 dives, then didn't need it anymore as i got more comfortable diving in the drysuit. another example, i needed huge amounts when i was taking my ow class. my first non-class dive, i took off 20lbs! (and the gear was the same.)
The difference is being in control.
You don't want to be shooting up to the surface after a long dive through those last 10 feet. Percentage-wise there is a lot of pressure change going on there. You need to ascend even slower there than up to that point.
And you are right, you need to do the test with a near empty tank rather than 500 psi. What if you stay at depth too long and incur a deco obligation and end up getting down to 200 psi at your stop. The last thing you need is to shoot up to the surface.
dkktsunami
Contributor
I would rather focus on having good procedures (not accidentally slipping into deco, and planning/monitoring my gas supply) than carrying around extra weight to account for the low possibility of bad diving. I assume it to be a given that no one is "shooting to the surface" and the weight carried needs to be enough to control final ascent. Doing the check with 500 psi at 10' is probably a little safer than floundering around on the surface with an "empty" tank and maybe being a little too heavy. Just my opinion. Supposed to be "on the boat" with 500 psi anyway.
Ok... let's do the math.
With a typical 7 mil farmer john, the surface buoyancy of the suit is around 20 pounds - all of it from nitrogen bubbles in the neoprene. Those nitrogen bubbles obey Boyle's law, so at 10' they will be compressed by .3 ATA water pressure in addition to the 1 ATA at the surface, and the suit will require only 1/1.3 X 20, or 15.4 pounds of offsetting weight to hold it down. If you establish weighting for neutral buoyancy at 10' with an empty BC, then as soon as you begin your ascent the nitrogen bubbles in the suit will begin to expand and you'll be positively buoyant... by the time you're at 5' or so you won't be able to stop your ascent with breath control alone and you'll have to actually swim down to stay level... and at the surface you'll find that you need another 4.6 pounds to get neutral/descend.
Bottom line: to asssure buoyancy control at all depths, do your weighting checks at the surface.
As for tank pressure, we can apply math there, too. For an AL80, 500 psi weighs about a pound, so if you want to weight for an absolutely empty tank, after you do the surface check with 500 psi, add a pound and you'll be all set.
You can do the weight check with a full tank, for that matter, and then add the weight of the air in the tank at the rate of .08 pounds per cubic foot. The swing from full to empty for an 80 is a bit over 6 pounds, while the swing from full to empty on a set of 130's is about 20 pounds.
---
All that said, if you'll weight all your gear once and write it all down as I mentioned earlier in this post, then weighting becomes a matter of simply adding up the values you've already established - and you'll be dead on the money every time.
Rick
GrumpyOldGuy
Contributor
Your logic is assuming Boyle's law acts instantaneously on a crushed wet suit. I found this is not really true, it take time to return to full bouyancy and being a bad boy, also set my weight for neutral at 10' with 500PSI. I would rather avoid the extra 5lbs and risk having to fin a bit at 5'. At no time do I find myself out of control.
Don't do deco diving.
Rick
GrumpyOldGuy
Contributor
This is a BASIC SCUBA forum, deco issues are moot.
Bubbletrubble
Contributor
That may be, but one of the purposes of the forum is to teach good diving practices to newer divers. Under-weighting on purpose is simply poor diving practice IMHO. Consider the implications of under-weighting at a dive site with lots of boat traffic. Besides, any air you save by not schlepping that extra 5 pounds on the dive will be lost while you are finning down at shallow depths. Be comfortable and safe at the end of the dive. Weight yourself appropriately.
BTW, I love your username.
Similar threads
