Changing my computer's altitude setting didn't affect depth measurement?

[RonR]

33 ft of seawater pressure = approx the same as 63 miles of air pressure + the water pressure. When you went to altitude, what percentage of that 63 miles did you achieve? The depth reading will reflect that but it is minuscule in the water environment. The altitude setting only really adjusts for the off-gassing at the surface and the fact that the pressure change will be greater than it might be a sea-level. Therefore, for safety, it reduces the no-stop time so that you are less loaded with nitrogen back at the surface.

Water is not compressible, but the atmosphere is- so most of that 63 miles is near vacuum. So you can’t look at this in terms of a % of the 63 miles, unlike water depth, the atmospheric pressure change with altitude is not linear. Close to the earth’s surface the change in pressure is much greater for each added foot of altitude.

As others have pointed out as well, going to 5,000’ / 1500m will drop the atmospheric pressure about 170 mBar, equivalent to 5.5’ / 1.7m of water depth- you would be that deep underwater before reaching sea level pressure. That’s not exactly trivial if you are trying to map something, but it is irrelevant for decompression calculations. What matters for deco is only the difference between the pressures the diver experiences on the dive and the ambient pressure they will see on exiting.

-Ron

 

[John Bantin]

The key word was "Miniscule".

 

[boulderjohn]

The depth reading will reflect that but it is minuscule in the water environment.

The key word was "Miniscule".

I see. Since the meaning of "miniscule" is in the mind of the speaker, you could be right no matter what the difference. So if it is 10 feet off (see post #21 for the math) on a dive, that fits your definition of "miniscule." Do I understand that correctly?

 

[John Bantin]

I see. Since the meaning of "miniscule" is in the mind of the speaker, you could be right no matter what the difference. So if it is 10 feet off (see post #21 for the math) on a dive, that fits your definition of "miniscule." Do I understand that correctly?

Ten feet is a lot in hydrostatic pressure but not much in air pressure so the depth reading at 3 metres with air pressure at sea level is not very different to the depth reading at three metres in a lake at 3,000 metres of altitude - but the pressure gradient on surfacing is greater. That is why you will see no difference in the depth reading display of a computer that can only show tenths of a metre but the no-stop times will be shorter when set for altitude.

 

[boulderjohn]

Ten feet is a lot in hydrostatic pressure but not much in air pressure so the depth reading at 3 metres with air pressure at sea level is not very different to the depth reading at three metres in a lake at 3,000 metres of altitude - but the pressure gradient on surfacing is greater. That is why you will see no difference in the depth reading display of a computer that can only show tenths of a metre but the no-stop times will be shorter when set for altitude.

We don't seem to be communicating. We are clearly not talking about the same thing.

 

[John Bantin]

We don't seem to be communicating. We are clearly not talking about the same thing.

I was attempting to answer the OP.

 

[dmaziuk]

OP's photo shows max depth of 10.9 metres, 24 C temperature, and he states the altitude setting was "3": 2,400 to 3,700 metres. According to some online calculator, air pressure at 3,700 m is 0.64 atm.

At 10.9 metres the pressure sensor should read, rounding up, 1 + 1 + 0.1 absolute at sea level, or 0.64 + 1 + 0.1 at "alt 3". The difference between 2.1 and 1.74 is 0.36 atm. A third of an atm is 3+ metres of depth: well within Leonardo's display accuracy of 0.1 m in metric per the fine manual. At 2,400 m the pressure should be 0.75 so you're still looking at ~ 3 metres difference in depth display.

Personally I think that using altitude in calculating surfacing M-value is, of course, a given, but if any computer calculates display depth from absolute pressure, I would like to know. Because 10.9 m down from the surface at sea level is still 10.9 m down at 2,400 m up. If that computer's programmers don't get that, I have to wonder what else they didn't get when they wrote the software that calculates my deco schedule. So that's a computer I'm not buying.