Density altitude and diving

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Where do you think that pressure comes from? It is caused by the amount of air above the ground. A one-inch square column of air from the surface to the sky weighs 14.7 pounds.....thus 14.7 pounds per square inch pressure.

No need to defend yourself, and no need to try to get him to understand. What you describe, the weight of a column of air, is the standard way to explain the concept of atmospheric pressure and has been since Torricelli did it back in the 1500's.
 
you are on the right track, but density alt corrects for temperature and humidity. Calm air has nothing to do with this. But it is exactly the density of the surface air that matters to divers.

I think it does matter, but it’s a hard concept to understand and goes above the math ability needed from the various agencies. .

I'm pretty sure that the motion of air affects the atmospheric pressure. Calm vs not calm does make a difference. But your lectures are entertaining... please continue.
 
A typical summer day in key largo is say 90 degrees F and 70% humidity. That gives a pressure altitude of 2400 feet. So standing there on the dive boat is the same ambient pressure as being at 2400 feet elevation. .

This is untrue. We've tried to answer your question, but you don't seem interested. You asked why established protocol for altitude diving does not consider temperature and humidity. Here are the facts:

--- Many people dive in Florida in conditions similar to those you describe above, but do not treat the dives as "altitude" dives.

--- The rules for diving have a long history of being thoroughly tested.

--- Which would lead us to conclude that diving in Florida does not present the same kind of situation as diving at an altitude of 2400 feet.

And here is why:

--- You admitted you are a pilot.

--- The performance of a plane is entirely dependent on the density of air, which varies with altitude and temperature.

--- Pilots, just like divers, have tables and simple calculations they use so they can plan their flights.

--- "pressure altitude" is something that pilots use to help with their planning.

Conclusion: pressure altitude calculations are designed to give pilots a simple way to determine how their plane will perform, which means while the name says "pressure altitude", the calculation is really about the density of the air. Divers are only concerned with the pressure of the air, which is not affected by temperature ... the pressure of the air is essentially the same (with negligible variations) at a given elevation.

So... if you want to know why the things that pilots do for flight plans are not considered by divers making diving plans, it's because one is only concerned with air density while the other is only concerned with air pressure. You may not like this explanation, but I have decades of divers and pilots using their systems successfully to support me.
 
The density will affect if you sink (air), float (dead sea), or somewhere in the middle. An airplane cares about the density so it can float on a cushion of air. If it can't float, it can't fly. You need dense enough air for a plane to fly.

Careful there... the lift on a plane's wings is dependent on the density of air, but also on the speed of the plane. Plane's don't "float" on air... if the plane stops moving, it definitely falls. Because planes are denser than air.

Unlike blimps, which do float... because they are giant helium balloons.
 
Another way to think about it is that your aircraft things -- density and pressure altitudes -- are point quantities, which is what the airplane cares about. The atmospheric/barometric pressure is an integrated quantity...all the atmosphere above the diver.

Oh goodness, that's a clear and simple way to contrast the difference.

On behalf of the OP, thank you.
 

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