CONVERTING BETWEEN DEPTH AND
PRESSURE WITH A TABLE
You can also use the above table for conversions
between depth and pressure. Interpolating is simple
if using metric depth–each meter adds 0.1 ata. In
U.S./Imperial measure, each three feet adds approximately
0.1 ata.
CALCULATING PARTIAL PRESSURES
To determine the partial pressure of a gas in a mixture,
multiply the gas fraction (the gas percentage
expressed as a fraction of one) by the absolute pressure.
You have seen the formula earlier in this chapter:
Pg = Fg x Ptotal
or summed up as: “The part is a fraction of the whole.”
This equation can be rearranged to find the gas
fraction/percentage if you know the partial pressure and
the total pressure (Fg = Pg / Ptotal) or to find the
absolute pressure if you know the partial pressure and
fraction/percentage of the gas (Ptotal = Pg / Fg).
Some people find it easier to use a graphic memory
picture rather than a mathematical formula or a
mnemonic phrase. The figure below shows the mathematical
relationship of partial pressure, gas fraction,
and total pressure.
Partial
Pressure
Total
Pressure
Gas
Fraction
To use the figure, cover the item you want to know
or solve for. The mathematical expression will be shown
by the two exposed items. If “partial pressure” is covered,
the side-by-side terms indicate that they are to be
multiplied. Similarly, if you want to know what fraction
will provide a desired partial pressure on the dive, cover
“fraction.” The uncovered terms (“partial pressure” over
“total pressure”) tell you to divide the desired partial
pressure by the absolute pressure at depth.
For example: At what depth does the partial pressure
of oxygen in the air (21% O2) reach a partial pressure
of 1.4 atmospheres?
First find the absolute pressure at which the PO2
reaches 1.4 atmospheres. Then convert that absolute
pressure to its equivalent depth.
NAUI Nitrox Diver
24 Gases & Gas Mixtures
Absolute Pressure at Depth
Depth
(fsw)
Depth
(msw)
Pressure
(ata)
0 0 1
10 3 1.3
20 6 1.6
30 9 1.9
40 12 2.2
50 15 2.5
60 18 2.8
70 21 3.1
80 24 3.4
90 27 3.7
100 30 4.0
110 34 4.4
120 37 4.7
130 40 5.0
140 43 5.3
150 46 5.6
FIGURE 2-7: THIS TABLE CAN BE USED TO CONVERT EITHER
FROM DEPTH TO ABSOLUTE PRESSURE OR ABSOLUTE
PRESSURE TO EQUIVALENT DEPTH