Density altitude and diving

[rjack321]

example. 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 not the density of the air at sea level in key largo. The humidity actually increased the density not decreases.

 

[lowflyer]

This is not the density of the air at sea level in key largo. The humidity actually increased the density not decreases.

Humidity decreases air density thus increasing density altitude. Pilots are most often concerned with density altitude for some important reasons. Increased density altitude means less dense air.

The affect of humidity on air density is much less than temperature and is therefore seldom taken into consideration

 

[NAM001]

Curious question - Why don’t we consider the local density altitude in diving? That is including temperature and pressure in the math?

all the agencies say we need to consider altitude effects when diving above 1000 feet. But that 1000 feet appears to assume standard day conditions. So diving on a warm day or day with lower than standard pressure (ex a low pressure front) would very likely put us over 1000 feet. So roughly 74 deg F is 1000 feet density altitude. 90 deg F is ~2100 feet. 90 deg and a typical local pressure of 29.5 in Hg is ~2800 ft. This suggests pretty much all warm weather diving is an altitude dive.

hOW WOULD YOU MAKE DIVE TABLES FOR THE VARIOUS COMBINATIONS???

I would suggest that the very reason we are discussing this is that it is too complicated to effectively learn in basic dive classes.

 

[boulderjohn]

hOW WOULD YOU MAKE DIVE TABLES FOR THE VARIOUS COMBINATIONS???

I would suggest that the very reason we are discussing this is that it is too complicated to effectively learn in basic dive classes.

This is true, but there is even more to it than that.

As people have known for many decades, perhaps a century, individual divers and their personal physiologies vary in their risk of DCS. The most recent studies emphasize that. Ideally, we would have different tables for different people, but that is an obvious impossibility. So what happened is that testing for dive tables over the decades involved a lot of different people with a lot of different physiologies. If 23% (or whatever) of the population has a PFO, then the odds are that 23% of the people engaged in those tests had PFOs. When everything was done, they created dive tables that were deemed to be relatively safe for that wide variety of people. Some are safer than others, but pretty much everyone who dives should feel included in that spectrum of safety.

With the exception of some work by Buhlmann and maybe some others I don't know about, these tests were done at sea level. They were done under a variety of climatic conditions--air pressure, humidity, density, temperature, etc. None of those were at extremes, of course, but they covered the kind of weather conditions divers would likely encounter. It would be nice to have separate tables for separate weather conditions, but that is not possible. When the tables were done, they included conditions that were deemed to be relatively safe for the variety of weather conditions divers would encounter.

If you truly understand why diving at altitude needs to have adjustments, you will know that the reasons to be concerned vary with the altitude, the depth, and the length of the dive. If you are doing a relatively shallow recreational dive at 2,000 feet elevation, you really don't have to do a lot different from what you would do at sea level. Do the same dive at 6,000 feet, and you should be making adjustments. Do a deco dive at 6,000 feet, and you need to be very aware of the difference. If you are diving above 10,000 feet elevation, remember that the US Navy manual says that you should not so such a dive without special clearance, and that warning is in bright red, bold-faced letters.

 

[dorsal]

John. Thank you for the insight. It’s wasn’t clear looking at the Buhlman research what the conditions were or how PADI or the other agencies arrived at the arbitrary 1000 foot number. I see the “reduction funnel “ is at work quite a bit here.

 

[BRT]

Density altitude doesn't change the air pressure, which is what affects divers.

 

[dorsal]

It does give the local pressure altitude which is the equivalent altitude based on local pressure. So the local sea level pressure is not 1.0 as we are taught, but likely less considering a typical a warm dive day.

 

[BRT]

It does give the local pressure altitude which is the equivalent altitude based on local pressure. So the local sea level pressure is not 1.0 as we are taught, but likely less considering a typical a warm dive day.

A barometer measures the atmospheric pressure. It goes up and down with different things. But not with temperature.

 

[tursiops]

local pressure altitude

You continue to miss the point. Somehow you continue to think that the entire column of air above you has sudden changed just because it is a bit warmer where you are standing? That density altitude thing is NOT real; it is a term used ONLY to let you know to what altitude you would have to go to find the same density that you measured at the surface, because you need to know (as a pilot) what kind of lift you have on your wings. You are confusing this imaginary term, useful to a pilot, with the actual atmospheric (barometric) pressure measured at the surface. The barometric pressure is nothing more than the total weight of all the air in the atmosphere above you. Now tell me how that is affected simply because it is a bit warmer where you are standing at the surface.

 

[broncobowsher]

Pressure, the only thing that matters. Don't care how you get it, air or water above you, in this activity it is all that matters. How dense the material making that pressure means NOTHING! Air, fresh water, The dead sea. All different density, all will get you to the same pressures. It just happens to take a taller column of air to get what fresh water will take to get to the same pressure. The dead sea, an even shorter column of super salty water. Try swimming in a pool of Mercury, well the column of mercury needed is really short

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.