Introduction
This thought experiment consists of a series of steps that you will take to understand an important concept for Mission Specialists: partial pressure.

Special equipment monitors the atmospheric pressure and the constantly changing mix of gases on the space station. Atmospheric conditions must be maintained within "normal" ranges if the Astronauts are to remain healthy and able to do their jobs. If there were too much oxygen, or too little nitrogen, or too much carbon dioxide, or even the slightest amount of carbon monoxide in the atmosphere, the Astronauts' lives would be endangered.

Scientists measure atmospheric pressure using millimeters of mercury (mmHg). Even though scientists try to maintain the total atmospheric pressure in the Space Station at, or near, 760 mmHg, which is the atmospheric pressure at sea level, the gaseous content of the air is constantly changing as the astronauts breathe and exhale, conduct scientific experiments, and manage the life support functions of the Space Station. For this reason, scientists need a way to monitor the pressure of each separate, important gas. The pressure of each gas within a mix of gases is called that gas' partial pressure.

The partial pressures of all the gases in the atmosphere add up to the atmosphere's total gas pressure. To learn what partial pressure means and how to state a gas' partial pressure in mmHg, you will conduct a thought experiment.

The Thought Experiment
Equipment
• Your imagination and thinking
• Graph A that you prepared from article, "Atmospheric Pressure: How and why it changes"
• A pencil
• A straight edge or ruler
• Scissors
• Two copies of "Three Pie Charts" article
• Hypoxia table

Procedure
1. On your graph entitled "Atmospheric Pressure vs. Altitude" that you prepared while reading the article "Atmospheric Pressure: How and why it changes," draw two vertical lines. Draw one line starting at 760 mmHg, or sea level, up to 130,000 feet. Draw the second line from the point on the curve at which the Atmospheric Pressure is ½ STP (380 mmHg) up to 130,000 feet.

2. Imagine that each of the two lines you drew in #1 represents a hollow tube of air, 1 cm square.

3. Think: the total pressure of all of the gas molecules in the tube rising from sea level creates the same pressure at sea level as the 760 mm of Mercury of Toricelli's tube. The total pressure of all of the gas molecules in the "tube" rising from the graph at 380mmHg exerts half the pressure.

4. Remember: From the story "How I Discovered Air" how many molecules of air are in a cubic centimeter at sea level, or 760 mmHg? Answer: __ _ (2.688 x 1019)

5. Think: How many molecules of air are in the bottom cubic centimeter of the tube drawn at 380 mmHg? The quantity of molecules is directly related to the pressure of the atmosphere at any given altitude. With the temperature constant, the pressure depends completely upon the number of gas molecules in the entire tube. The more the molecules the higher the pressure. We know that where we find one half the atmospheric pressure of sea level, we will find only one half the molecules in the air. If there are half as many molecules banging around, there is half the pressure.

6. Follow the math: 2.688 x 1019 divided by 2. 26,880,000,000,000,000,00/2 = 13,440,000,000,000,000,000 or 1.344x1019 Note: Scientists use the notation 10x power instead of all those "0's". That is called "scientific notation." It is a shorthand way of writing very large, or small, numbers.

7. Cut out two copies of Pie Graph #1 from the article "Breathing on the Space Station."

8. Imagine that the two pie graphs are containers and that the volume of air in each pie graph is exactly 1 cm3.

9. Place one pie graph-container at the bottom of each of the vertical lines, or "tubes," you have drawn on your graph so that it looks like the center of the graph is "on" the vertical tube.

10. First, consider the pie graph-container at sea level. We know that the total pressure created by all of the gas molecules in this container is exactly 760mmHg, or 100% atmospheric pressure at STP. How much of that pressure is caused by the Nitrogen molecules? By the oxygen molecules?

11. Think: If 100% of the pressure of all the molecules is 760 mmHg, refer to Table #1 to determine the percentage of each gas in the atmosphere.

12. With this information, you can complete this table:

13. Observe: Did you notice the "pp" that was placed in front of the mmHg denoting nitrogen's "share" of the total pressure? The "pp" stands for "partial pressure." Partial pressure, or pp, is a "signal" to all scientists and specialists that that gas' pressure is the pressure of just one of a mix of gases. It's a part of the total pressure created by all of the gases.

14. Create a second table with the same headings as the one in #13 for the gases in the pie chart-container you placed on the vertical "tube" at 380mmHg, or ½ atmospheric pressure. What is the total air pressure in this pie chart at this altitude? What percentage of STP is this?

15. Move this second pie chart-container up the tube to the height which represents exactly 1/3 the atmospheric pressure at sea level, or 253.33 mmHg. This is close to the atmospheric pressure found at the top of Mt. Everest. How many gas molecules would you find in the pie chart-container at this altitude?

16. Consider: You have two containers on your graph, one at sea level, one close to the tip of Mt. Everest. One contains one third the molecules as the other. What does this tell us about all of the molecules in our Earth's atmosphere? (Answer: Two thirds of the molecules in the atmosphere around the Earth are located in the bottom 1/3 of the entire layer of atmosphere surrounding the Earth-below the top of Mt. Everest.) How does this observation emphasize the importance of keeping our atmosphere clean?

17. Bonus Thought: What is the pp (partial pressure) of the oxygen at the top of Mt. Everest? Answer: _______ (.20946 x 253.33 mmHg = O???ppmmHG)

18. Recall the Hypoxia Chart: Would climbers at the top of Mt. Everest have to worry about Hypoxia? (Yes) What steps might they take to avoid hypoxia? (oxygen masks)

Conclusion
Given the percentage of a gas within any mix of gases and the total pressure of all the gases, you can now calculate the gas' partial pressure. On Space Station Alpha the altitude does not change, but the total atmospheric pressure and partial pressure of gases may change, especially if a piece of technology-a part of the Environmental Control and Life Support System (ECLSS)-malfunctions. If you refer to the hypoxia or carbon dioxide poisoning charts, it should be easier for you to appreciate what might happen to the Astronauts if the atmospheric pressure in the Space Station and the partial pressure of oxygen were to suddenly drop as the partial pressure of carbon dioxide suddenly rises.