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

[Bob DBF]

Depth doesn't change, a hundred feet at thousand feet elevation is still a hundred feet. Only off-gassing fudge factor changes.

Yes. My mind was thinking about the differences between measuring instruments, at 130' a bourdon tube depth gauge will be different than a computer or capillary depth gauge. The water is the same depth, but the measuring instruments will show a difference.


Bob

 

[dmaziuk]

Yes. My mind was thinking about the differences between measuring instruments, at 130' a bourdon tube depth gauge will be different than a computer or capillary depth gauge. The water is the same depth, but the measuring instruments will show a difference.

I learned something today: apparently "most pressure measuring equipment give the pressure of a system in terms of gauge pressure as opposed to absolute pressure". Apparently it's "more practical" that way.

 

[rsingler]

To add to the confusion, there is at least one Mfr (Scubapro, IIRC) that automatically shifts from salt to fresh water above a certain altitude, because there are no high altitude sea water lakes. Apparently the OP's computer is manual in that regard, as is Shearwater, I believe.

 

[boulderjohn]

Yes, and OP asked about a Leonardo. Which I happen to own.

You have me stumped as to your point here. He said that the two computers were giving the same depth despite one being set to altitude and the other not being set to altitude. The first people responding said (incorrectly) that there is not much difference in pressure at depth (and thus depth measurement) at altitude. I pointed out that two Shearwaters showed a significant difference in depth when set at different altitudes, indicating that there is more of a difference than people are thinking. Exactly what is wrong with that?

Later on I did the math to show it.

 

[boulderjohn]

Apparently the OP's computer is manual in that regard, as is Shearwater, I believe.

Yes, the Shearwater's is manual, but an earlier post indicated that there is no salt water/fresh water adjustment on that computer.

 

[dmaziuk]

The first people responded said (incorrectly) that there is not much difference in pressure at depth (and thus depth measurement) at altitude. I pointed out that two Shearwaters showed a significant difference in depth when set at different altitudes, indicating that there is more of a difference than people are thinking. Exactly what is wrong with that?

Nothing. Except that Leonardo, unlike Shearwaters, does not seem to be programmed to show absolute pressure. It seems to be showing gauge pressure regardless of the altitude setting.

An interesting question is why it matters: if you are saturated at 3,000 feet, all your on- and off-gassing from diving should be relative to atmospheric pressure at 3,000 feet. If you're at sea level, it should be relative to 1 atm. (Edit: the rate of change would be different, same way it goes up closer to the surface, but I wonder by how much.) Only gauge pressure should matter for deco calculation. I guess this must be why some people apparently believed altitude does not affect decompression.

 

[Bob DBF]

To add to the confusion, there is at least one Mfr (Scubapro, IIRC) that automatically shifts from salt to fresh water above a certain altitude, because there are no high altitude sea water lakes. Apparently the OP's computer is manual in that regard, as is Shearwater, I believe.

Oceanic also shifts to fresh at altitude.


Bob

 

[boulderjohn]

Nothing. Except that Leonardo, unlike Shearwaters, does not seem to be programmed to show absolute pressure. It seems to be showing gauge pressure regardless of the altitude setting.

An interesting question is why it matters: if you are saturated at 3,000 feet, all your on- and off-gassing from diving should be relative to atmospheric pressure at 3,000 feet. If you're at sea level, it should be relative to 1 atm. (Edit: the rate of change would be different, same way it goes up closer to the surface, but I wonder by how much.) Only gauge pressure should matter for deco calculation. I guess this must be why some people apparently believed altitude does not affect decompression.

Pages 3-6 should help answer your questions.

http://deepadventurescuba.net/assets/uploads/files/altitudecombined2.pdf

 

[dmaziuk]

Yes, thank you. The point is that when programming the computer, you essentially change GF Hi, or bubble magic equivalent, to reduce your surfacing M-value because near the surface is where the altitude actually matters. This is independent of the "depth" reading from your pressure sensor. When the more sophisticated devices automatically read atmospheric pressure and do the adjustment for you, it's convenient -- arguably as convenient as when they calculate GTR for you, so you don't have to remember about gas planning.

If I were programming a basic-level device like Leonardo I could be tempted to simply make "altitude setting" and "safety factor" two different interface knobs for the exact same conservatism setting. And I believe some Oceanic (?) manuals come clean about doing exactly that.

 

[RonR]

<snip>
So, I don't see how 2 identical computers with different altitude settings could give the same depth. If I were to give the computer the benefit of the doubt, I would say that for its decompression readings, it is going off of the ATAs it is measuring but is not translating it to the proper depth for you.

It depends on the computer. For computers like the OP’s that require manual setting for altitude, this indicates that the device is looking up a table of pressure ranges it would use for decompression calculations, rather than measuring the ambient pressure directly. Probably they are using a gauge type sensor. So yes, in that case you would expect that if the altitude settings were different the depth display would be affected, if they are handling it correctly. But maybe as you suggest they ignore altitude settings for depth display and use that setting only for decompression calculations. So long as pressure readings are accurate it would not matter for deco. But the depth display would be off with altitude.

In the Cobalt, and quite few other computers that don't require manual altitude setting, we 1) use an absolute pressure sensor rather than a gauge sensor; and 2) store the ambient pressure reading every few minutes, even in sleep. When water is detected, followed by an increase in pressure, we look to the last ambient pressure reading as surface pressure and use that for both depth reference and for decompression calculations. For those computers the depth reading should not be altered by altitude, though fresh water will be a little deeper to have the same pressure value.

Rough rule of thumb, 30 mBar = .3 m or 1’ of depth.
So going to 1000’ / 300m will drop atmospheric pressure about 40 mBar, or a little over 1’ / .3m equivalent.
Going to 5000’ / 1500m will drop atmospheric pressure about 170 mBar, or 5.5’ / 1.7m equivalent.

But whatever the depth reading displayed, pressure is pressure, and what matters is the difference between the pressure profile a diver experiences on a dive and the surface ambient pressure they are exposed to on exit.

__________
It bears repeating, since the question comes up frequently in these forums, that dive computers do decompression calculations based on pressure readings. It makes no difference what depth display that pressure is converted to. The depth display is there for the diver’s reference. Changing from fresh water calibration to salt and back only makes a difference to the “tape measure” depth value the pressure is converted to and displayed as, it does not alter the decompression calculations.

-Ron