Dalton’s law states that the total pressure exerted by a mixture of gases is equal to
the sum of the pressures of the different gases making up the mixture, with each
gas acting as if it alone occupied the total volume. The pressure contributed by any
gas in the mixture is proportional to the number of molecules of that gas in the total volume. The pressure of that gas is called its partial pressure (pp), meaning
its part of the whole.
The formula for expressing Dalton’s law is:
include:
Sample Problem 1. A helium-oxygen mixture is to be prepared which will
provide an oxygen partial pressure of 1.2 ata at a depth of 231 fsw. Compute the
oxygen percentage in the mix.
Sample Problem 2. A 30-minute bottom time dive is to be conducted at 264 fsw.
The maximum safe oxygen partial pressure for a dive under normal operating
conditions is 1.3 ata (Table 14-4). Two premixed supplies of HeO2 are available:
84/16 percent and 86/14 percent. Which of these mixtures is safe for the intended
dive?
Sample Problem 3. Gas cylinders aboard a PTC are to be charged with an HeO2
mixture. The mixture should provide a ppO2 of 0.9 ata to the diver using a MK 21
MOD 0 helmet at a saturation depth of 660 fsw. Determine the oxygen percentage
in the charging gas, then compute the oxygen partial pressure of the breathing gas
if the diver makes an excursion from saturation depth to 726 fsw.
