Confused about PADI weight check

[Chris66]

I am somewhat confused by the PADI weighting check. As I understand it part of the idea behind weighting is meant to prevent you from becoming too bouyant at the end of the dive when your aluminum tanks starts emptying. According to PADI one checks weighting by floating eye level with full lungs and empty BC however that is on a full tank. When that tank empties should you not based on that theory suddenly bob to the surface and in my experience with "optimal" weighting" my bouyancy is always off at the end of the dive as my tank empties and at the time it is most important to be properly weighted. Can some one explain

Second question : does the weighting check change for steel tanks or is it done the same way

Finally , I have been diving in pool water and with my new BP halcyon and a 3mm needed only the 6 lbs inherent in the ss plate. I am going on a cruise (and likely cattle boat dive) in the carribean next week and am concerned they will just over weight me for simplicity and I am wanting to get an idea of how much I will need. I am assuming that if I did 6bs in 3mm suit in pool water that I will need 6+5 for a total of 11 in the ocean and for safety should do 12 pounds. I weigh 160lbs

Thanks

Chris

 

[UWupnorth]

If you do your bouyancy check with a full tank, add 5lb to offset the air you use. When I dive warm water in my ss BP and unweighted single tank adapter I only added 4lb of lead, I was in a 3/2 mm suit and weigh 175lbs

 

[rockjock3]

I am somewhat confused by the PADI weighting check. As I understand it part of the idea behind weighting is meant to prevent you from becoming too bouyant at the end of the dive when your aluminum tanks starts emptying. According to PADI one checks weighting by floating eye level with full lungs and empty BC however that is on a full tank. When that tank empties should you not based on that theory suddenly bob to the surface and in my experience with "optimal" weighting" my bouyancy is always off at the end of the dive as my tank empties and at the time it is most important to be properly weighted. Can some one explain

Second question : does the weighting check change for steel tanks or is it done the same way

Finally , I have been diving in pool water and with my new BP halcyon and a 3mm needed only the 6 lbs inherent in the ss plate. I am going on a cruise (and likely cattle boat dive) in the carribean next week and am concerned they will just over weight me for simplicity and I am wanting to get an idea of how much I will need. I am assuming that if I did 6bs in 3mm suit in pool water that I will need 6+5 for a total of 11 in the ocean and for safety should do 12 pounds. I weigh 160lbs

Thanks

Chris

You got the bouyancy check wrong. PADI tells you to do it with a near empty tank (500psi). The best time is at the end of your dive. If you have a little more then just bleed the tank down before your check. You take in a normal breath and when you hold it you should be float at eye level, when you exhale you should sink.

It is the same test for steel tanks.

A good starting point will be to add 5 lbs when moving from fresh to salt, but that will vary with everybody so you just have to experiment to see what works good for you.

 

[jpcpat]

You got the bouyancy check right, but you missed the last step. Once you are floating at eye level, holding a normal breath, with an empty BC, you need to add 5 pounds to account for the air you will use.

For steel tanks, the weight of the air you use is the same, but the tank itself is more negative.

Virtually all rental tanks are Al 80's, about +6 at the end of the dive. For someone your size diving in the Carribean wit ha 3mm suit, 12-15 pounds is the right ballpark. Retest at your safety stop. Oh, and yes, they probably will overweight you a bit at first. It's your dive so adjust as needed

 

[Blackwood]

I... am concerned they will just over weight me for simplicity

They should not be weighting you. If some crew member tells you do something that you know (or suspect) to be wrong, say no.

 

[diverdown247]

You got the bouyancy check wrong. PADI tells you to do it with a near empty tank (500psi). The best time is at the end of your dive. If you have a little more then just bleed the tank down before your check. You take in a normal breath and when you hold it you should be float at eye level, when you exhale you should sink.

It is the same test for steel tanks.

A good starting point will be to add 5 lbs when moving from fresh to salt, but that will vary with everybody so you just have to experiment to see what works good for you.

Let's clarify the bold words for this particular post. What Chris is referring to is a surface weight check (done for proper weighting), which should not be confused with a bouyanch check. A bouyancy check can and should be done at the surface once properly weighted. It can also be done just above the bottom (hover) or on the bottom (fin pivot).

Chris, you can perform the surface weight check with a full tank. It is generally a good idea to double check it on a near-empty tank if you have changed your weighting for the dive. Perform it exactly as you would for aluminum tanks as UWupnorth and rockjock3 had stated.

Since aluminum tanks are generally lighter than steel, you may have to make a couple of dives to "fine tune" the weighting. As with any weight adjustment, as you build experience with the changes you may be able to shed some of the weight. I would suggest having both types of tanks weighed with identical amounts of air (weigh the aluminum tank first) to see how much of a difference there truly is. The difference should give you an idea of how much weight you should expect to make the change with.

 

[Charlie99]

Second question : does the weighting check change for steel tanks or is it done the same way

The weighting check you describe (full lungs, full tank, float eye level) only works because the extra buoyancy from full lungs is about the same as the weight of the 6 pounds of air in an AL80.

The same weighting check can be used as long as the tank holds around 80 cubic feet. It doesn't matter whether it is aluminum or steel, the air in the tank weighs the same and the change in buoyancy of the tank is the same as it empties.

The method you describe is a pretty crude one and just gets you in the right ballpark. You should check your weighting again with a near empty tank.

I prefer to be be neutral at 10', full lungs, and near empty tank. That makes it so that I can control my ascent easily all the way up to the surface.

am assuming that if I did 6bs in 3mm suit in pool water that I will need 6+5 for a total of 11 in the ocean and for safety should do 12 pounds. I weigh 160lbs

If you are changing nothing other than going from fresh water pool to the ocean, then the added weight will be pretty close to 1 pound per 40 pounds total weight --- this will be 4 pounds for you, another 1 for the tank, and another fraction for the BP/W and lead. Adding 6 pounds (12 pounds total) will be right, provide you stay with the same sort of tank, the same wetsuit, BP/Wing and other gear.

Most likely the resort boats will provide you with AL80 tanks that are 4 to 4.5 pounds positive buoyant. If that's what you used in the pool, then no adjustment is needed. If you are now using a different tank, then you need to check the empty tank buoyancy and make an adjustment.
http://www.huronscuba.com/equipment/scubaCylinderSpecification.html is a typical listing of tank specs.

Charlie Allen

 

[Charlie99]

Once you think you have the right amount of lead, and easy way to check it with an arbitrary amount of air in an 80 cubic foot tank is to realize that the air in a 3000psi AL80 weighs about 6 pounds, or about 1 pound for every 500psi.

Look at your SPG. Remove 1 pound for every 500psi in your tank.

Now with a fully deflated BCD, and with gear and wetsuit fully saturated with water (i.e. you have been floating around for a few minutes) see if you can descend by exhaling forcefully. Then see if you can gradually ascend back up to the surface by just changing your breathing pattern. Play around with adding an extra pound or two and taking off a pound or two to see the effect on how easy it is to both descend and ascend using just your lungs to control buoyancy. When you have found the sweet spot that you like, check your SPG again. Your ideal weighting will be the amount of lead you have on at that instant, plus 1 more pound for every 500psi in the tank.

Obviously, you could do the same sort of testing with any other sort of tank, but the calculations on how many pounds of air vs SPG reading will be different.

 

[halocline]

I also like to be neutral at 10-15 feet as opposed to the surface; this usually lets me shave a pound in a 3mil suit. Think about it; you want just enough weight to stay neutral at your safety stop; in fact, that's when I usually end up doing the fine tuning for the next dive. This makes you very slightly positve at the surface. You also want enough weight to easily make your initial descent. With the BP/W this should be a little easier than with a standard BC that usually has some inherent buoyancy, or at least has more fabric to trap air bubbles.

I'm also 160 lbs, and use 2 lbs plus my SS plate with a 3 mil full suit in salt water. I used 10 (sometimes 12) with my old jacket style BC; started with 4 lbs using the plate and soon was able to drop to 2. By the middle of a dive week, I can usually drop to no lead with the 3 mil. The point is you might find your necessary weight dropping as you get more used to the plate.

 

[Chris66]

Thank you all for your advice. I feel more confident in bieng able to make the right weight determination. I agree that my weight needs will likely change as I go along as I become more comfortable in the water

Chris