Misconceptions and Fallacies

[Charlie99]

A thread about buoyancy got me thinking about a time dumped a large amount of water out of my BCD after a dive and an instructor commented about how "All that water must have made me very negatively buoyant during the dive". Of course, it made absolutely no difference, but it seems to be a common misconception.

Anyway, here's a few misconceptions and fallacies that I've seen. Please add the ones you have run into.

 

[Charlie99]

Water in a BCD doesn't affect buoyancy one way or the other. There is a big difference between "weight", which water does have, and "negative buoyancy", which is doesn't have.

The instructor was convinced that all the extra water in the BCD would make me sink. She finally saw the light when I asked her to think about a gallon jug of water and how it behaves in the water.

Charlie Allen

p.s. Yes, I've no longer keep trying to dump air that isn't there, and no longer get excessive water in the BCD.

 

[Charlie99]

That was my impression also until reading the course book for the nitrox course. Only then did I find out about the big gotcha with high oxygen content gases ---- O2 toxicity.

Where nitrox really excels is in the 50' to 120' range. On very shallow dives you will run out of air before hitting NDL limits. On much deeper than 120' you need to reduce the fraction of O2 back to the point where you don't gain much advantage.

As to the old debate as to whether nitrox adds safety, increases NDL, or reduces SI, the real answer is that is reduces nitrogen absorption and you can use that reduction to do any one of the above three, or any combination you desire.

 

[JeffG]

One of my favorites Misconceptions and Fallacies

"The water that is trapped between you and your wetsuit is what keeps you warm."

 

[Charlie99]

Not true. The air in a steel tank weighs just as much as the air in the same capacity aluminum tank. You will have additional positive buoyancy of the same amount towards the end of the dive, whether the air you consumed was in a steel tank or an aluminum tank.

There are lots of good reasons for steel tanks, but buoyancy SWING or CHANGE is not one of them.

 

[jepuskar]

Misconception:

Ankle weights are necessary.

 

[miketsp]

..snip..
Anyway, here's a few misconceptions and fallacies that I've seen. Please add the ones you have run into.

Misconception:
"Being overweighted automatically increases air consumption."

The fact is that if your buoyancy control is good, being overweighted has a zero or negligible impact.

 

[Charlie99]

Misconception:
"Being overweighted automatically increases air consumption."

The fact is that if your buoyancy control is good, being overweighted has a zero or negligible impact.

Yeah. When people talk about divers going around feet down at a 45 degree angle finning heavily just to keep depth, they normally refer to them as "overweighted", when in reality the true problem is that they don't have enough air in their BCD to be neutral. They may or may not be overweighted.

 

[miketsp]

Dare I?

MOF

Nuff said.

 

[ShakaZulu]

Misconception:
"Being overweighted automatically increases air consumption."

The fact is that if your buoyancy control is good, being overweighted has a zero or negligible impact.

Really? If you carry a lot of weight, you have to add more air to become neutral at a certain depth. Then, if you are changing depths constantly, you will have to add/remove more air on every up/down cycle to become neutral, and hence an increase in air consumption.....