EANx MOD @ altitude

[djensen]

I have a question re: the MOD of EANx at altitude:


The MOD for EAN32 is 110 feet at sea level, but say I'm diving at 5000 feet above sea level. My adjusted altitude depth is 132 feet while diving 110 feet. Does that mean I've shot past my 1.4 ATA goal & am exposing myself to 1.6 ATA at this adjusted altitude depth?


This should be simple but I'm just not wrapping my head around it because, while I understand the increased residual nitrogen load at altitude, we're not ACTUALLY experiencing greater water pressure right? So would oxygen toxicity be based on sea level or altitude-adjusted depths?


I know the conservative answer is very easy. I'm looking for the empirical answer. Thanks!

 

[eponym]

I have a question re: the MOD of EANx at altitude:


The MOD for EAN32 is 110 feet at sea level, but say I'm diving at 5000 feet above sea level. My adjusted altitude depth is 132 feet while diving 110 feet. Does that mean I've shot past my 1.4 ATA goal & am exposing myself to 1.6 ATA at this adjusted altitude depth?

This should be simple but I'm just not wrapping my head around it because, while I understand the increased residual nitrogen load at altitude, we're not ACTUALLY experiencing greater water pressure right? So would oxygen toxicity be based on sea level or altitude-adjusted depths?

I know the conservative answer is very easy. I'm looking for the empirical answer. Thanks!

(I assume you're saying that you use tables and will calculate your ending residual nitrogen group as if you had been to 132 feet, even though you're actually only going to 110 feet, to account for the lower surface pressure at the end of your dive.)

The pressure on you at 110 feet in a 5000 fasl lake is actually less (about 0.2 atm less) than it would be in the ocean, because of reduced air pressure at the lake surface. So the pressure of oxygen in your lungs is also less during your altitude dive than it would be when doing the same dive in the ocean. Partial pressure of oxygen at 110 feet in this lake (breathing 32% nitrox) will be closer to 1.3 atmospheres.

pressure at surface = 0.83 atm
pressure of 110 ffw = 3.23 atm
total pressure = 4.06 atm
fraction of oxygen = .32
pressure of oxygen = .32 * 4.06 = 1.3 atmospheres

 

[iamjblevine]

I believe that MOD = (1.4 / oxygen percentage x 33 ) - surface pressure in feet of water. I'm not at home to check my Nitrox book. I'm not qualified on altitude diving, so take this only as "armchair diving" information.
To further elaborate, MOD is based upon absolute pressures and the adjusted altitude depth is based upon the fractional ratio of pressure at depth and pressure at the surface.​

 

[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

 

[eponym]

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.

 

[windapp]

PO2 is based on the absolute pressure. Here are the tiny errors that can occur and their result:

You didn't zero your depth gauge for the altitude, and it reads from see level: You read fsw from 1 atm, so the results are spot on for PO2.
Your computer switched to ffw. The computer reads deeper, so the results a little more conservative.
You did zero your depth gauge for altitude. You read deeper than if you were zeroed to 1 atm, so the results are more conservative.

Basically, just calculate your MOD the same as you would at sea level, and you will be fine.

The adjusted altitude depth is a trick to use tables at altitude, and should not really considered to be a "depth" especially since the pressure you experience is actually slightly less. Your residual nitrogen load is not higher, but your tolerance for residual nitrogen on the surface is lower. A computer doesn't do this, but instead uses a different set of empirical values in its equations.

 

[boulderjohn]

Partial Pressure = Fraction of gas X pressure in atmospheres (MOD)

So if you want to find MOD (Pressure in atmospheres) for a partial pressure target of 1.4 and EANx 32, you would plug in these values:

1.4 = 0.32 X P = 4.375 ATA. This is your MOD expressed in atmospheres of pressure (ATA). So now you have to convert 4.375 ATA to feet to know your MOD.


At sea level, you would subtract 1 for air pressure and multiply by 33.

4.375-1 = 3.375

3.375 X 33 = 111.375, which is commonly rounded off to 110 feet.


At 5,000 feet altitude, you have to start with the fact that you are in fresh water, not salt water, so the multiplying factor is 34. In addition, your air pressure is 0.83.

4.375 - 0.83 = 3.545

3.545 X 34 = 120.53

 

[windapp]

Partial Pressure = Fraction of gas X pressure in atmospheres (MOD)

So if you want to find MOD (Pressure in atmospheres) for a partial pressure target of 1.4 and EANx 32, you would plug in these values:

1.4 = 0.32 X P = 4.375 ATA. This is your MOD expressed in atmospheres of pressure (ATA). So now you have to convert 4.375 ATA to feet to know your MOD.


At sea level, you would subtract 1 for air pressure and multiply by 33.

4.375-1 = 3/375

3.375 X 33 = 111.375, which is commonly rounded off to 110 feet.


At 5,000 feet altitude, you have to start with the fact that you are in fresh water, not salt water, so the multiplying factor is 34. In addition, your air pressure is 0.83.

4.375 - 0.83 = 3.545

3.545 X 34 = 120.53

Be careful. This assumes that your depth gauge is reading ffw. A depth gauge that reads fsw will read fsw in salt or fresh water.

 

[boulderjohn]

Be careful. This assumes that your depth gauge is reading ffw. A depth gauge that reads fsw will read fsw in salt or fresh water.

It does not assume anything of the sort. The accuracy of your depth gauge is another issue altogether.

 

[windapp]

It does not assume anything of the sort. The accuracy of your depth gauge is another issue altogether.

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.