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

[nohappy]

Dear @RonR,
Thank you for your clear explanation!
1. Just for clarify, when you say "we look to the last ambient pressure reading as surface pressure", do you refer "last ambient pressure" as air pressure at the surface before submerge?

2. I'm confused that - shouldn't gauge type sensor display the correct depth at different altitude? Because its offset (0 pressure) is always at the surface, and absolute pressure sensor has to aware the air atmosphere to calculate correct depth on the other hand? What did I miss here?

 

[Tom N.]

I assume because the difference in atmospheric pressure , ie the "weight" of the atmosphere above you is materially lower at say 6000 feet of altitude compared to sea level.So when you hit the surface at 6000 feet you are at a materially lower pressure compared to sea level. Whereas water is so heavy that 6000 feet less of air in combination with your water depth adds little "weight" , so it doesn't significantly change your depth reading at realistic diving depths. Am I correct ?

 

[RonR]

Dear @RonR,
Thank you for your clear explanation!
1. Just for clarify, when you say "we look to the last ambient pressure reading as surface pressure", do you refer "last ambient pressure" as air pressure at the surface before submerge?

2. I'm confused that - shouldn't gauge type sensor display the correct depth at different altitude? Because its offset (0 pressure) is always at the surface, and absolute pressure sensor has to aware the air atmosphere to calculate correct depth on the other hand? What did I miss here?

1. Yes, we save the last pressure reading. When a dive starts (we use an algorithm that looks at the rate of pressure change over time to determine that) the last saved surface pressure is used for the pressure in deco calculations as well as for determining depth.

2. Absolute pressure sensors are referenced to vacuum. Gauge sensors are referred to atmospheric pressure, and usually don’t allow for negative values. I don’t know what is going on inside anyone else’s computer, but I could imagine that for depth display a computer should just use a standard 1 ATM as surface pressure and display depth based on the pressure over that. So long as the surface pressure value pressure used for deco calculations comes from the altitude settings, and those are set correctly by the user, it wouldn’t matter for decompression. But the actual physical depth- if you measured it with a line- would be deeper than the value displayed. So long as the pressure is accurate this doesn’t make any difference to decompression calculations.

Ron

 

[RonR]

I assume because the difference in atmospheric pressure , ie the "weight" of the atmosphere above you is materially lower at say 6000 feet of altitude compared to sea level.So when you hit the surface at 6000 feet you are at a materially lower pressure compared to sea level. Whereas water is so heavy that 6000 feet less of air in combination with your water depth adds little "weight" , so it doesn't significantly change your depth reading at realistic diving depths. Am I correct ?

See my rule of thumb in a post above. If your computer measures exactly the same absolute pressure, at 6,000 feet elevation you would have about 6' more water over you than you would have if you were at sea level- that is if you ignore the difference between fresh and salt. Looked at another way, at 6' depth you would be at about sea level pressure. None of this makes any difference to decompression calculations, which are based on pressure readings, not depth figures. The important thing is that when you exit the water at 6,000' the ambient air pressure is markedly less, which needs to be taken into account and is why you will have shorter NDL's.

Ron

 

[dmaziuk]

2. I'm confused that - shouldn't gauge type sensor display the correct depth at different altitude? Because its offset (0 pressure) is always at the surface, and absolute pressure sensor has to aware the air atmosphere to calculate correct depth on the other hand? What did I miss here?

That they don't do negative numbers.

 

[boulderjohn]

I assume because the difference in atmospheric pressure , ie the "weight" of the atmosphere above you is materially lower at say 6000 feet of altitude compared to sea level.So when you hit the surface at 6000 feet you are at a materially lower pressure compared to sea level. Whereas water is so heavy that 6000 feet less of air in combination with your water depth adds little "weight" , so it doesn't significantly change your depth reading at realistic diving depths. Am I correct ?

No. This was pretty well covered in the thread already. See post #21 for one explanation.

The reason the computers are not showing a difference is because that computer does not show the correct depth at altitude.

 

[nohappy]

I found that this article from DAN is really helpful. Establishing a Baseline
But I doubt that one paragraph is true -

In a review of dive computers currently on the market, all were found to feature automatic compensation of depth readings for altitude. This is good, as it saves the user from needing to manually perform an adjustment.

I just looked up some manuals from different manufactories, but most of them didn't mention ambient pressure detection on land. (at the surface).
Any comment?

 

[RonR]

They might call it "automatic altitude compensation" or some other catchy phrase. But if a computer describes manually setting an altitude range, it is probably not detecting actual atmospheric pressure. I'm pretty sure there are a number of lower end dive computers that do not detect altitude and rely on the diver setting them manually.

 

[John Bantin]

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.

 

[boulderjohn]

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.

You write as if those 63 miles of change are equal, and they are nowhere close to equal. Most of the change happens very quickly. Fortunately, you can look up the difference of air pressure at altitude easily via the Internet. If you do, you will find that the air pressure at 2,000 meters (6,600 feet) is 80% of the pressure at sea level. If you to the math, you will find that this 20% loss of pressure is much more than that altitude's percentage of 63 miles.

2,000 meters is roughly the altitude of Lake Tahoe, where diving is very common, and a 20% loss of ambient pressure is something that should be considered in diving.

By the way, you will find all of this explained in a number of posts that were already made in this thread.