8. The usual partial pressure of oxygen that people get at sea level is 0.20 Atm., that is, a fifth of the usual sea level air pressure. People used to 1 Atm. air pressure begin to become "light-headed" at about 0.10 Atm oxygen. As a rule of thumb, the air pressure decreases one inch of mercury each thousand feet of altitude above sea level. At what altitude should airplane cabins be pressurized? Up to about what altitude should you be able to use unpressurized pure oxygen? (Express your answer in feet above Mean Sea Level, or MSL.)
To answer this question, we need to understand the relationship between air pressure, altitude, and the corresponding partial pressure of oxygen.
First, let's start by converting the usual partial pressure of oxygen at sea level from Atm. to mmHg (millimeters of mercury). Since 1 Atm. is equal to 760 mmHg, the usual partial pressure of oxygen at sea level is 0.20 Atm. x 760 mmHg = 152 mmHg.
Next, let's determine the altitude at which people begin to become "light-headed" due to a decrease in oxygen partial pressure. According to the information given, this occurs at around 0.10 Atm. The corresponding partial pressure of oxygen can be calculated as follows: 0.10 Atm. x 760 mmHg = 76 mmHg.
Now, we need to determine the altitude at which the air pressure decreases to 76 mmHg. We know that air pressure decreases by approximately 1 inch of mercury per thousand feet of altitude. Since 1 inch of mercury is equivalent to 25.4 mmHg, the decrease in pressure for each thousand feet is 25.4 mmHg.
To find the altitude corresponding to a decrease of 76 mmHg, we can divide 76 mmHg by 25.4 mmHg/thousand feet: 76 mmHg / 25.4 mmHg/thousand feet = 2.992 feet.
Therefore, airplane cabins should be pressurized up to an altitude of approximately 2.992 thousand feet above Mean Sea Level. This means that the cabin pressure should be maintained equivalent to the pressure at 2.992 thousand feet above sea level.
Lastly, let's determine the maximum altitude at which unpressurized pure oxygen can be used. Since the partial pressure of oxygen needed to prevent becoming "light-headed" is around 76 mmHg, we need to find the altitude where the air pressure decreases to 76 mmHg.
Using the same conversion factor as above (25.4 mmHg/thousand feet), we can calculate the maximum altitude by dividing 76 mmHg by 25.4 mmHg/thousand feet: 76 mmHg / 25.4 mmHg/thousand feet = 2.992 thousand feet.
Therefore, you should be able to use unpressurized pure oxygen up to an altitude of approximately 2.992 thousand feet above Mean Sea Level.