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

[nohappy]

Hey Guys,
Out of curiosity only, I wore two Cressi Leonardo computer and dived at sea level with same setting. However, I set one of the computer with "alt 3" on purpose and another one remain "alt 0" (default). I assumed that the one with "alt 3" would show deeper depth when I dived, but it didn't. Does the altitude setting only change decompression algorithm not the depth measurement?

 

[scubadada]

Correct, it decreases the NDL, does not affect the depth.

 

[jgttrey]

Depth is depth, regardless of altitude.

The altitude setting is so the computer can make proper assumptions about pressure at the surface, which is important for deco calculations.

 

[nohappy]

Sorry I didn't make my confusion clear. I thought that our computer calculate depth by ambient pressure. If it thought that I'm at higher altitude, air atmosphere would be lower, hence it should have shown deeper depth given same ambient pressure?
Or does a computer detect the air pressure at the surface as offset automatically? And if it can detect air pressure, why would I need to set altitude configuration?

 

[caruso]

Sorry I didn't make my confusion clear. I thought that our computer calculate depth by ambient pressure. If it thought that I'm at higher altitude, air atmosphere would be lower, hence it should have shown deeper depth given same ambient pressure?
Or does a computer detect the air pressure at the surface as offset automatically? And if it can detect air pressure, why would I need to set altitude configuration?

Yes the computer calculates depth by ambient pressure. The computer doesn't (for all practical purposes) sense the lower atmospheric pressure at altitude, once it's manually calibrated it will adjust your no deco time accordingly. The somewhat thinner, less dense air at altitude has little to no effect on how the computer calculates depth because once you descend you're going to be at 1 ATM at close to 33' regardless of what's happening above the water. In other words, the change in altitude doesn't affect water pressure to a significant degree.

 

[nohappy]

Yes the computer calculates depth by ambient pressure. The computer doesn't (for all practical purposes) sense the lower atmospheric pressure at altitude, once it's manually calibrated it will adjust your no deco time accordingly. The somewhat thinner, less dense air at altitude has little to no effect on how the computer calculates depth because once you descend you're going to be at 1 ATM at close to 33' regardless of what's happening above the water. In other words, the change in altitude doesn't affect water pressure to a significant degree.

I set alt3(2400m above sea level), I thought it'll at least has 0.1-0.3m difference?

 

[Bob DBF]

Sorry I didn't make my confusion clear. I thought that our computer calculate depth by ambient pressure.

Pressure at depth is one factor, the ambient pressure at altitude is another factor, and this is plugged into the computer algorithm to determine your relation to NDL. In fact you are overiding the altitude input when setting the computer to a more conservative setting, making it think it is at a higher altitude when it is not.



Bob

 

[caruso]

I set alt3(2400m above sea level), I thought it'll at least has 0.1-0.3m difference?

Getting a bit technical here and perhaps the real experts will weigh in but I'd think that since you're manually changing the setting on the computer- and you are therefore "telling" the computer that you are diving at altitude, the computer would either a) "know" enough to adjust the displayed depth settings to compensate or b) the computer is not programmed to compensate for that minor depth difference and that would be ok, because we're talking an insignificant difference of probably less than a foot.

 

[dmaziuk]

I set alt3(2400m above sea level), I thought it'll at least has 0.1-0.3m difference?

Yes, and what does the manual say about its depth measurement accuracy?

 

[nohappy]

If the air atmosphere decreases affect depth measurement insignificantly, why can it affect NDL significantly?