February 12, 1945

Commander Chalmers L. Gemmill

U. S. Naval Air Training Center

Pensacola, Florida

School of Aviation Medicine

Dear Commander Gemmill:

I believe that the answer to the questions in your letter of 25 January 1945 can be expressed in the following way. Our oxygen meter indicates directly the partial pressure of oxygen, under the conditions in which the oxygen is introduced into the meter. The reading of the partial pressure of oxygen is essentially independent of the presence or absence of other gases.

Inasmuch as the instrument indicates the partial pressure of oxygen in a gas, the reading of a gas mixture containing water vapor should be changed on removal of the water vapor to a value greater by division by the quantity one minus the ratio of the water vapor pressure to the total pressure. If the instrument is kept at body temperature, then the ratio would be as indicated in your letter; if, however, the instrument is at a lower temperature, the correction factor would not be so great.

The answer to your last question about the reference pressure for the instrument is that the instrument always indicates the partial pressure of oxygen in absolute pressure units, either millim[e]ters of mercury or fractions of a normal atmosphere, depending upon how your instrument is calibrated. The fraction of oxygen in the gas under investigation would then be obtained by dividing the reading of the instrument by the total pressure of the gas under the conditions on measurement.

Sincerely yours,

Linus Pauling

LP:fh

cc: to Dr. Arnold O. Beckman