Density altitude and diving

[dorsal]

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.

 

[boulderjohn]

Do you dive using tables? Most people use computers these days, and almost all of them automatically adjust for altitude. They do that by reading barometric pressure. That means that if you are using a computer that automatically adjusts for altitude, then you are indeed considering local air pressure.

 

[dorsal]

Hey @bolderjohn

the question is more academic than anything. Much of what I’m finding has limited science, and is more qualitative or myth in how the 1k number and its implementation came around. Based on how long it’s been taught, it appears to be a limitation of tables more than anything.

I’m an engineer and pilot (fly for fun) and this is something that has been curious for some time.

 

[tursiops]

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.

Perhaps we don't consider it because it really doesn't matter. The local effects of temperature (as an airplane would feel in the lift on its wings) are not the issue for a diver. We are concerned with the weight of the entire column of air above us.

 

[yle]

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.

Pressure, of course, is the key issue, right? And pressure, temperature, density are all connected... change one, and at least one of the other two must also change.

But I think what you'll find is that, assuming relatively calm conditions, that at a given altitude the density and temperature will vary together (inversely, of course) but the pressure stays fairly constant.

If you're approaching this from the pilot perspective, then density of the air is important (for flying), and why you have to be concerned with temperature (because density changes with temp.) But for divers it's the pressure that's important... and at a given altitude that stays fairly constant (which is why pitot tubes are able to do what they do.)

 

[dorsal]

@yle 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.

@tursiops - 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 not sure where you are going with the “weight of the air column” as it’s the pressure we care about.

 

[tursiops]

I’m not sure where you are going with the “weight of the air column” as it’s the pressure we care about.

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.

 

[broncobowsher]

Barometric pressure (what they talk about on the evening weather) has a slight factor.
Pressure is what matters, that is why there is an altitude class.
Density doesn't matter, air or water, different density but you can make a dry dive in a recompression chamber just as if you were in water.

 

[dorsal]

the original question is about the “Diving in altitudes higher than 300 metres/1000 feet above sea level is altitude diving” cert agencies want us to consider before taking into account altitude while diving.

It seems there is confusion on what density altitude is and how it relates to pressure altitude. @broncobowsher is doing a great job trying to explain that it’s local pressure we care about.

from Wikipedia (last sentence is key)

The density altitude is the altitude relative to standard atmospheric conditions at which the air density would be equal to the indicated air density at the place of observation. In other words, the density altitude is the air density given as a height above mean sea level. The density altitude can also be considered to be the pressure altitude adjusted for a non-standard temperature.
​

As @bolderjohn points out some computers try to account for this with measured ambient pressure.

but using a computer is not the question.

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.

So back to the question rephrased, and in relation “Diving in altitudes higher than 300 metres/1000 feet above sea level is altitude diving “ (PADI). Why wouldn’t we consider the local pressure altitude which is the actual elevation adjusted for local pressure, temperature and humidity? It seems we are forgetting an important bit to not include “pressure altitude”.

 

[tursiops]

You are missing the main point: the only thing we care about is barometric pressure at the surface of the water we are diving in, because that is what affects our DCS calculations. You are apparently a pilot, thus your obsession with density altitude.....which is related to the lift on an airplane wing at whatever altitude the plane is f.lying. If the air is hot, or the barometric pressure is low, you have less density and so your wing has less lift. Pressure altitude and density altitude are related -- but not the same thing. But both Pressure altitude and Density Altitude are not actual altitudes....they are statements of what would the altitude be (in a standard atmosphere) where you find that density, or pressure.

So you measure the temperature at your dive site. Maybe it is 30 deg C, a really hot day. But what matters is the barometric pressure...i.e. the weight of all the atmosphere above you. The variation in atmospheric/barometric pressure with altitude in a standard atmosphere is about 3% per 1000 ft, or 1% per 100m. The 1000 ft limit above which it is altitude diving is effectively saying greater than 3% is too big to ignore.

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.