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

[nohappy]

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

It says nothing, only mention NDL

 

[dmaziuk]

It says nothing, only mention NDL

It's in #6 Technical Specificatons: if set to feet, 1 foot (0.3 m) is the resolution you get. In metric it's supposed to read to 0.1 m. So either way your 0.1-0.3 m is barely within its ability to display, even if it can actually detect the difference.

 

[caruso]

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

I see your confusion and I understand completely.

To simplify the explanation- when a diver surfaces after a dive and they've got some N02 aborbed in their tissues, that N02 is going to be more likely to bubble out when they are exposed to thinner atmospheric air as compared to sea level air which for the purposes of the N02 dissolved in the body, is quite significant. But once you're under the water, the pressure of the water is SO much greater than the atmospheric difference of say, sea level vs 10,000 ft that the pressure effect on the dive computer won't translate to a huge depth difference underwater. As the other poster said, maybe it's a foot or less and most dive computers aren't even sensitive enough to display that minor depth difference which in reality corresponds to a rather large pressure difference when you're talking about dissolved N02 in a diver's blood stream.

So the computer adds a safety margin (ndl limits) because the diver will be exposed to a relatively large pressure gradient difference upon surfacing.

Hope that makes more sense

 

[dmaziuk]

On second thought, scratch that: it's displaying the reading from its pressure sensor. You can set your altitude to pink polka dots, the sensor will still read what is.

 

[dberry]

@caruso Not to pick nits, but I sure hope I never have NO2 in my blood after a dive. (obviously a typo and he meant N2).

 

[dmaziuk]

It's what happens when your compressor sucks in the exhaust of your riced-up supercharger.

 

[caruso]

@caruso Not to pick nits, but I sure hope I never have NO2 in my blood after a dive. (obviously a typo and he meant N2).

Been using N02 for years never realized the error and no one ever pointed it out before. And I was a bio major in college. I've forgotten more than I've ever learned if that's even possible.

Another factor in this whole computer calibration/altitude diving situation that hasn't even been considered that plays even more of a role in errant depth calculations is that many dive computers do not have the capability of being calibrated for freshwater versus salt and most or all altitude dives are freshwater for obvious reasons.

 

[boulderjohn]

A few years ago, I noticed that there were times that my dive buddy and I had very different depth readings on our Shearwater computers--up to 5 feet different. We were diving at nearly 5,000 feet. Discussions with Shearwater showed a flaw in the software, which they then corrected. The computers read altitude automatically, so if the computer was on when you entered the water, it would know you were ta altitude. If the computer turned on automatically after you began your descent, it would assume you were at sea level.

As I said, the depth differences were up to 5 feet.

 

[boulderjohn]

I don't know if that computer has an adjustment for salt water or fresh. If you are diving at altitude, you are diving in fresh water, and that requires 34 feet of water to equal 1 ATA, not 33.

 

[boulderjohn]

Between he decreased atmospheric pressure and the switch to fresh water, diving at altitude can have a significant effect on depth. When we dive at Santa Rosa, New Mexico (about 4.600 feet), we use different MOD tables for nitrox and trimix diving. For example, the MOD for 32% nitrox is usually rounded to 110 feet in salt water diving, but we use 120 feet at that altitude. (I can do the math for you if you wish.)