Water in BCD

[Panama Jones]

Panama Im not being a smart alec here but that is EXACTLY why you use a log book.Note your weight,water type,exposure suit etc.That gives you a baseline for the next dive in the same conditions.
You have probably worked out that basic dive training has to be taken down to the lowest common denominator.That being a 10 year old child.
The other thing is that because there are just so many variables you can introduce into any situation the manual would be about three feet deep if it covered everything off.
I recently did my rescue diver course and yes theres a lot of it that there is THE way to do things.Theres also a lot of it where its so dependant on the individual situation you can't have a fixed rule.Your best bet is to have training to cover the options and do what you feel is the best.

No offense taken. I log every dive and by now I am getting it down quite well. But I travel and dive by myself so you realize that I often get to a place, get on a boat with other divers, buddy up, guess the proper weight given the water temp, exposure suit etc and get in the water. In my experience with now a number of dive shops, they really don't want take time to weight check everything.

I'll give you a 3 month example.

1, Cuba, salt 85 degrees 3mm shorty, steel 80
2. Italy. salt 60 degrees 5mm full, Steel 100
3. Alberta fresh full 7mm booties hood and extra vest 14mm on core 45 degrees, Al80
4. San Diego straight 7mm, Al 80, 55 degrees

As I said, I log every dive and now am getting pretty good after about 35 or 40 dives but when the shops push you through, what I log in my book is the gear, the weight and comments like a little overweighted, little under" and adjust for the next time.

Another thing you said was "That gives you a baseline for the next dive in the same conditions." Being slightly "ha ha" older than 10, I do refer to my log and equip properly if I have dove in the same conditions before.

I know... I should demand to do a weight check but there is peer pressure when you got 6 or 7 in the water wanting to go down, especially when you are the junior diver. BTW, have I said I believe I got it by now. Still learning other stuff though. Thanks.

 

[Crass3000]

Forgetting the specifics of weight of air, amount of PSI, etc..... It's the total weight of the tank, no? ei., water weighs a lot more than air, so a tank completely full of water would weigh WAY more than one full of air and would sink you like a rock, no? Same as a tank full of air is heavier than one that's almost empty. So it's the total weight of the non-flexible container that counts....whatever is inside the tank becomes part of the tank's total weight....With the BCD, the water inside it is pretty much in contact with the water outside it--not separated inside a non-flexible container, thus no effect on the diver's weight. Correct?

I still think it's the same. The tank weighs what the tank weighs. If you have water in the tank that is the same density in the tank as the water outside the tank it will weigh more on land obviously. But if you put that tank full of water in the water I bet it weighs about the same as if it had no water in it if it is in the water.

Edit: I'm an enginerd and I am really doubting my logic but for some reason it makes sense to me.

 

[dfx]

I still think it's the same. The tank weighs what the tank weighs. If you have water in the tank that is the same density in the tank as the water outside the tank it will weigh more on land obviously. But if you put that tank full of water in the water I bet it weighs about the same as if it had no water in it if it is in the water.

Edit: I'm an enginerd and I am really doubting my logic but for some reason it makes sense to me.

Well, you're wrong

The weight of a tank is the sum of the weight of the tank itself plus the weight of its contents.

Obviously the weight of the tank itself doesn't ever change, so you can forget about that. What remains is the weight of its contents.

To offset its weight, there's buoyancy. Buoyancy is the weight of the substance the (outer) volume of the tank displaces. In air, the weight of the air that the tank displaces is negligible. In water, the weight of the water that the tank displaces is significant, because water is that much heavier. However, in both cases the displacement remains the same independent of the tank's content, because the outer volume of the tank never ever changes. So you can forget about that too.

The only significant thing that remains to determine a tank's weight (and/or buoyancy) therefore is the weight of that tank's contents. Since water weighs more than air, a tank containing water will be heavier than one containing air. No matter if it's on land or in the water.

 

[Crass3000]

Well, you're wrong
.

Yah I know you're right. My mind just isn't working at this time.

 

[T.C.]

I still think it's the same. The tank weighs what the tank weighs. If you have water in the tank that is the same density in the tank as the water outside the tank it will weigh more on land obviously. But if you put that tank full of water in the water I bet it weighs about the same as if it had no water in it if it is in the water.

Edit: I'm an enginerd and I am really doubting my logic but for some reason it makes sense to me.

That doubt is called common sense. Listen to it.

That "empty" tank is really not empty. It has air in it at ambient pressure- which is barely enough to offset the weight of the aluminum (or steel, or carbon fiber). Now, if you fill that tank with water, you lose that buoyancy. You have, basically, an aluminum slug. It doesn't matter it's shape in the water, it will be heavier in the water because you don't have the "empty" air contributing to the total displacement.

Think of how a submarine dives. It has air in the ballast tanks while on the surface, making it's displacement high. It opens the valves, floods the tanks, and becomes negatively buoyant, since it now weighs more, compared to the water around it.

 

[boat sju]

Buoyancy is determined by the density of an object vs. the volume of water it displaces. An empty tank, full tank, or tank full of water all displace the exact same volume of water. Essentially an empty tank is (overall) not as dense as a full tank, which is not as dense as a tank full of water.

If Archimedes is indeed rolling over in his grave it's probably due to the way I just tried to explain buoyancy.

 

[oldschoolto]

No offense taken. I log every dive and by now I am getting it down quite well. But I travel and dive by myself so you realize that I often get to a place, get on a boat with other divers, buddy up, guess the proper weight given the water temp, exposure suit etc and get in the water. In my experience with now a number of dive shops, they really don't want take time to weight check everything.

I'll give you a 3 month example.

1, Cuba, salt 85 degrees 3mm shorty, steel 80
2. Italy. salt 60 degrees 5mm full, Steel 100
3. Alberta fresh full 7mm booties hood and extra vest 14mm on core 45 degrees, Al80
4. San Diego straight 7mm, Al 80, 55 degrees

As I said, I log every dive and now am getting pretty good after about 35 or 40 dives but when the shops push you through, what I log in my book is the gear, the weight and comments like a little overweighted, little under" and adjust for the next time.

Another thing you said was "That gives you a baseline for the next dive in the same conditions." Being slightly "ha ha" older than 10, I do refer to my log and equip properly if I have dove in the same conditions before.

I know... I should demand to do a weight check but there is peer pressure when you got 6 or 7 in the water wanting to go down, especially when you are the junior diver. BTW, have I said I believe I got it by now. Still learning other stuff though. Thanks.

Easy fix.... When on the boat ask if they mind if you do a weight check.... If they say yes, You do it.... If they say NO, You'll be fine.... Guess your weight as best you can and than grab a few one and two lb weights to stick in your BCD pockets... If your heavy just start throwing weights till you get good and trimmed... If they give you crap... Just tell them a Northeast wreck diver told you to do it...

Jim...

 

[Frosty]

I think the point with the weight-adding post is that, if the student can sink, she is not terribly underweighted. She may not be carrying enough weight to sink with an empty tank, but she's certainly carrying enough to sink with a full one. If she immediately floats back up, it's probably either a technique problem with BCD usage, or badly controlled breathing. Adding more weight doesn't help with either, because although it can make up for poor breathing, the ensuing buoyancy instability due to the large gas volume in the BCD is worse than the original problem.

TSand M--I had a slightly different mental image to be honest.
I imagined that the student had hand opver handed it down to the bottom.Then had let go of the line and floated. I saw just that happen recently.
BUT the first thing to do is to dump the air from the BC -If its empty then the student might be underweighted

 

[oldschoolto]

The other is to grab the front neck of the wet suit to bleed any trapped air also the back of the BC can trap air....

Jim...

 

[Nemrod]

You know, as a new diver, I find scuba diving, with relatively simple (albeit very important) concepts, somewhat difficult to understand.

In my line of work, the concepts are airtight (no pun intended). Anywhere I go in the world, any professional in my line of work will fully understand that concpet and any related process in EXACTLY the same way.

In Scuba diving, I can ask the same question on a basic concept of 5 different scuba instructors and get anywhere from 2 to 5 different answers. If anyone wants examples just ask.

After my last dive, I was emptying water from and I made some sort of jike to an instructor who had just finished a class. His words to me were, don't worry, everyone gets a lot of water in their BCD and I often add some on purpose if I need more weight.

Forget the point that I wondered why someone with an instructor rating would need to do something like this to correct a weight issue, my real question is that I thought that the weight of water, in water was ambient weight.

The water in my BCD at depth should not actually add weight below the surface... would it?

You have just learned a valuable lesson, do not go around saying things like "my instructor said" to divers who do know what they are doing because then they will know that both you and your instructor know nothing quite right.

N