EANx MOD @ altitude

[boulderjohn]

I am not talking about depth gauge accuracy. I depth gauge that is calibrated for fresh water reads ffw, while a depth gauge that is calibrated for salt water reads fsw regardless of the medium you are diving in. You would not calculate an MOD for fresh water when your depth gauge is measuring salt water depth. The majority of manual depth gauges read fsw.

Maybe I am not understanding your point. You are saying it is not about depth gauge accuracy, but ti is rather about the fact that the depth gauge might not be accurate. Is that what you are saying?

The math I described has nothing to do with depth gauges. It is to do with the distance you are under water in absolute terms. A lot of people call that DEPTH. It can be measure in many ways. For example, you could have a long tape measure descending from the surface. You could have any of a number of different depth gauges with different levels of accuracy depending upon the altitude and the saltiness of the water. You could have computers that are adjusted to altitude and salinity--or not.

The correct MOD is the same regardless of what kind of depth gauge you have. If your depth gauge is not giving you the correct depth, then it is up to you to calculate that correction.

 

[windapp]

Maybe I am not understanding your point. You are saying it is not about depth gauge accuracy, but ti is rather about the fact that the depth gauge might not be accurate. Is that what you are saying?

The math I described has nothing to do with depth gauges. It is to do with the distance you are under water in absolute terms. A lot of people call that DEPTH. It can be measure in many ways. For example, you could have a long tape measure descending from the surface. You could have any of a number of different depth gauges with different levels of accuracy depending upon the altitude and the saltiness of the water. You could have computers that are adjusted to altitude and salinity--or not.

The correct MOD is the same regardless of what kind of depth gauge you have. If your depth gauge is not giving you the correct depth, then it is up to you to calculate that correction.

That is the thing. We don't actually measure the distance from the surface (I have never seen anyone dive with a tape measure), but rather we measure the pressure using a depth gauge. The correct MOD should therefore be calculated based on how your depth gauge is calibrated, rather than based on the medium you are diving in. The simplest way to calculate MOD is to use sea water, and don't make any assumptions about atmospheric pressure for the altitude you are diving. The error in this calculation always leads to a more conservative MOD.

 

[boulderjohn]

That is the thing. We don't actually measure the distance from the surface (I have never seen anyone dive with a tape measure), but rather we measure the pressure using a depth gauge. The correct MOD should therefore be calculated based on how your depth gauge is calibrated, rather than based on the medium you are diving in. The simplest way to calculate MOD is to use sea water, and don't make any assumptions about atmospheric pressure for the altitude you are diving. The error in this calculation always leads to a more conservative MOD.

So, what you are saying is not that the calculations I posted were inaccurate but rather that there is no point in paying any attention to the actual MOD, and divers should go instead with the sea level calculation because it will be more conservative at depth. Is that what you are saying?

 

[windapp]

So, what you are saying is not that the calculations I posted were inaccurate but rather that there is no point in paying any attention to the actual MOD, and divers should go instead with the sea level calculation because it will be more conservative at depth. Is that what you are saying?

And you don't have to worry about whether the depth gauge is calibrated for salt or fresh water, and you don't have to worry about the exact ambient atmospheric pressure but yes, your calculation is not inaccurate. I did not realize that my comment could be interpreted that way. Sorry.

 

[gekodivebali]

Calculations in imperial are so much more complicated... 10m=1bar, isn't that beautiful?

 

[darushin]

Yes you can at 5000 feet dive it to the 132ft you adjusted to. This is because the 132 adjusted is simply telling you that a non adjusted depth gauge at 5000ft reading 132ft had the same pressure as one reading 110ft at sea level. Now if you have a computer or an adjustable gauge then the rule doesn't apply.
Daru

Going to have to disagree with that. The "132" is a theoretical depth, used to track nitrogen loading when planning repetetive dives.

At a measured depth of 110 feet in sea water, a correctly calibrated gauge will see (1 + 110/33) 4.33 atmospheres and read "110".

In a fresh water lake at 5000 fasl, with the same gauge, going to 4.33 atmospheres and getting the reading "110" requires going down to roughly 118 feet.

I didnt actually do the calculations but used the numbers provided by the original poster. But yes you are correct he can take the nitrox mix to proper adjusted depth.

 

[djensen]

Fantastic discussion, guys! Thanks.