Nitrox Math
This paper is intended to be a review of the math needed for Nitrox.
Hint: you don’t need to memorize these formulas, just learn how to
use them.
There is no such thing as a “closed book test” when you are either planning
your dive or recording your dive in a log book.
*** ATM ***
ATM is the water depth in atmospheres. If ATM is used in place
of feet, then all the other calculations are easier. It is my suggestion
to always convert feet to ATM, then do the calculations, or get the answer
in ATM, then convert to feet.
The following is a chart to show ATM v. feet:
1 atm = sea level
2 atm = 33 feet in salt water
3 atm = 66 feet in salt water
4 atm = 99 feet in salt water
5 atm = 132 feet in salt water
ATM = (Depth Feet in salt water / 33) + 1
In converting from feet to ATM, divide by 33 then add one.
Adding one is because sea level is 1 ATM.
-----Example: 56 feet in salt water is (56 / 33) + 1
= 2.7 ATM
Depth Feet in salt water = (ATM – 1) * 33
In converting ATM to feet, subtract one first, then multiply by 33.
Subtracting one first is because seal level is 1 ATM
-----Example: 3.5 ATM is (3.5 – 1) * 33 = 82.5 Feet
*** FO2 ***
FO2 is the percentage of oxygen in your tank.
If you have air in your tank, then the FO2 is 21%. If you have
a mix of 36% oxygen in your tank, then the FO2 is 36%
*** PO2 ***
PO2 is the partial pressure of oxygen, at a given depth.
PO2 = ATM * FO2
Partial Pressure of Oxygen equals the Depth in Salt Water (in ATM) times
the Percentage of Oxygen in your tank.
-----Example: at a depth of 3.5 ATM, with 36% oxygen is 3.5
* .36 = 1.26 PO2
*** MOD ***
MOD is the Maxium Operatng Depth you want to dive, based on the
information of the FO2 (percentage of oxygen in your tank) and the PO2 (Partial
Pressure of Oxygen). In calculating the MOD, you have to pick a PO2.
The PO2 you choose is up to how concertive you wish to dive. Some agencies
train to plan on a 1.3 or 1.4 Most agree that a 1.6
is the most you should ever use.
MOD = PO2 / FO2
-----Example: with PO2 of 1.4 and 36% in your tank: 1.4 / .36
= 3.89ATM
(3.89 ATM – 1 is 2.89 ATM * 33 = 95.3 feet)
Basic algebra allows us to rearrange the above expression:
PO2 = MOD * FO2
-----Example: if you select a MOD of 3.0 ATM, and with 32% in your
tank,
3.0 * .32 = .96 PO2 (under 1.4, very concertatve)
FO2 = PO2 / MOD
-----Example: if you select a PO2 of 1.4 and dive a max of 4.5ATM,
1.4 / 4.5 = .311 or 31.1%
*** EAD ***
Equivalent Air Depth. If you have a set of nitrox dive tables
with the right FO2, then just use that table. But if you only have
a set of air dive tables, then you need to convert your maxium depth of your
dive (MOD) to an Equivanent Air Depth (EAD), then use your air tables with
the EAD of a nitrox dive to plan multiple dives.
EAD = (1-FO2 ) / .79 * MOD
Equivalent Air Depth is the percentage of nitrogen in your tank(1-
FO2) divided by the percentage of nitrogen in air, multiplied by MOD.
-----Example: with a F02 of 32% and a MOD of 4.0 ATM (99 feet)
(1-.32) / .79 * 4.0 = 3.44 in ATM or 80.6 feet
*** PLANNING ***
First example:
The only nitrox is available is 32%. You are diving a reef, and want
to know how deep you can go. You select a PO2 of 1.4
MOD=PO2 /F02
1.4 / .32 = 4.375 in ATM
4.375 – 1 is 3.375 * 33 = 111.4 feet
You now want to see what a PO2 of 1.6 is
MOD=PO2/FO2
1.6 / . 32 = 5 in ATM
5 – 1 is 4 * 33 = 132 feet
Second example:
The dive you are planning is 97 feet deep. You want to determine the
“best mix” of oxygen with a selected PO2 of 1.4
F02 = PO2 / MOD
118 / 33 = 3.57 then +1 = 3.94
1.4 / 3.94 = .355 or 35%
*** DIVE LOG ***
Example:
Your dive was a maximum of 102 feet deep. You had a 32% mix.
[need to log this FO2=32%]
ATM = Depth Feet / 33 + 1
102 / 33 = 3.1 + 1 = 4.1 atm [need to log this MOD=102 feet
or 4.1ATM]
PO2 = MOD * FO2
4.1 * .32 = 1.31 PO2 [need to log this PO2=1.31]
EAD = (1-FO2) / .79 * MOD
(1 - .32 ) / .79 * 4.1
(.68 / .79 ) * 4.1 = 3.52 in ATM
3.53 in ATM – 1 = 2.53 * 33 = 83 feet
[need to log this EAD=83.5 feet or 3.53ATM]
Wrong answer: (.68 / .79) * 102 feet = 87.8 feet
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