Misconceptions and Fallacies

[alcina]

I haven't dived for 12 months. I'm confident in my skills and abilities and am ready to dive.

 

[sbrooks]

Misconception by non-divers: "Hey Man... How long do those oxygen tanks last?"

 

[CAPT HOOK]

All the posters on Scuba Board actually know what they are talking about - FALLACY!!

 

[RJP]

Getting toasted every night, greatly lessens the effect of "getting narced".

No, but it does lessen the ALLURE of getting narced!

 

[Stu S.]

Me and the guys I dive with have created a misconception. Every so often, person will ask "Do you pee in that suit?". I got asked that at work once, too. The new answer is: "No. The pressure around a diver prohibits it". Nobody has questioned that response.

 

[SparticleBrane]

Someone at the dive shop where I used to work said that it's "impossible to not pee in your wetsuit, because the shock of the cold water makes your bladder contract."

...

 

[miketsp]

Mike,

This is one with which I must disagree with you. Buoyancy and mass are two different things. You can be neutrally buoyant with a 2000 pound anchor attached to you, but try moving that thing. Takes quite a bit of effort.
It's no different than having 50 pounds of lead on you. You may be neutrally buoyant, but you have to make that mass move and you have to stop it. It takes more energy (ie more calories which require more oxygen) to move 2000 pounds than it does 10 pounds.

The laws of inertia at rest and at motion . . . and so forth . . .

the K

I will agree with you that if I had a 2000 lbs anchor as weight and a BC big enough to lift it then, yes, my SAC increase would be noticable.

However without splitting hairs, if we consider the more normal 5-10 lbs overweight and normal slow swimming speeds around a reef or a wreck then I have never noticed any perceptible increase.
Inertia also comes into play. Once you get the mass moving it keeps moving without any additional effort.

As for some other posters commenting about streamlining.
I dive with both a jacket BC and a back inflate wing style according to conditions.
With the jacket BC an extra 10lbs adds almost nothing to my cross-sectional area, the growth due to expansion is practically all in the streamlining shadow area under my armpits.
With my wing I actually suspect there is a streamlining improvement by carrying more weight as when it is empty the two lateral flaps of empty material tend to cause a lot of induced drag if I increase swimming speed. I had an experience once where I picked up a weight belt that someone had dropped near the start of a dive and this must have had around 20 lbs. I completed another 45 minutes of dive with this extra belt and the impression I had is that my consumption actually decreased. The air that I put into the wing actually made the bag more streamlined.
The only side effects after I got out of the water were a bit of back ache and a little bit of chest pain. The former for obvious reasons and the latter because I normally adjust my buoyancy by adjusting the residual air in my lungs and with the increased sensitivity I was using a wider range of adjustment than normal as I moved up & down thoughout the dive.
For me the whole issue is whether or not you can get a good horizontal trim with the extra weight.

 

[The Kraken]

Mike stated . . .

"if we consider the more normal 5-10 lbs overweight and normal slow swimming speeds around a reef or a wreck then I have never noticed any perceptible increase"

. . . and I agree.

My SAC rate on a warm water dive with a thin wet suit tends to run about 0.45. Cold water (45 degrees or so) and a dry suit tends to push it up to about 0.55.

the K

 

[spankey]

What water in the BC does is decrease the available lift of the BC by the weight of the water in it. That is, a BC with 22 pounds of lift that has a quart (about two pounds) of water in it will have the potential of only 20 pounds lift, because the two pounds of water inside the BC displaces (surprise!) two pounds of water outside the BC.
So, in addition to being sloppy, poor technique, a whole bunch of water in your BC isn't a good idea.
Rick

Does it really matter? What do you think will happen to the water when you add air to the BC? it should be expelled when the bc reaches its limit.

 

[wettek]

Mike stated . . .

"if we consider the more normal 5-10 lbs overweight and normal slow swimming speeds around a reef or a wreck then I have never noticed any perceptible increase"

. . . and I agree.

My SAC rate on a warm water dive with a thin wet suit tends to run about 0.45. Cold water (45 degrees or so) and a dry suit tends to push it up to about 0.55.

the K

How can you agree with this comment, you have just said your SAC increases when you are in a drysuit?