Oxygen Facts

The earth's atmosphere consists of approximately 21% oxygen, 78% nitrogen and 1% of other trace gases.  Oxygen under normal conditions is an odourless, colorless, tasteless, non-combustible gas.  Oxygen is very reactive and oxides of most elements are known.  Liquid and solid oxygen are pale blue (see sidebar) and strongly paramagnetic.


 What is the difference between aviation, medical and industrial grade oxygen?

There is a ongoing controversy about whether there is any difference between the grades of oxygen. Oxygen gas is produced from the boiling off of liquid oxygen.  Therefore, it appears that the oxygen grades are the same because the oxygen comes from the same source.  However, the standards are different for each oxygen grade.  There are four different grades of oxygen that are merchandised or sold to users.

  • Aviation grade oxygen;

    • 99.5% minimum oxygen content.

    • Tested for moisture content because of the risk of freezing at high altitudes. Maximum moisture content is guaranteed.

    • Higher cost when compared to the medical and welding grades.

  • Medical grade oxygen;

    • 99% minimum oxygen content.

    • Regulated the same as prescription drugs.

    • Maximum level of CO and CO2 gas is guaranteed.

  • Welding (Industrial) grade oxygen;

    • 99.5% minimum oxygen content.

  • Research grade oxygen;

    • Highest grade of oxygen.


 Why we need oxygen

At each breath we fill our lungs with air. Millions of tiny air sacs (known as "alveoli") in our lungs inflate like tiny balloons. In the minutely thin walls enclosing each sac are microscopic capillaries though which blood is constantly transported, from the lungs to every cell in the body. The oxygen extracted from the air in the lungs is carried by the blood to every part of the body. Because the body has no way to store oxygen over a period of a long time, it leads a breath-to-breath existence.

The human body must have oxygen to convert fuel (the carbohydrates, fats, and proteins in our diet) into heat, energy, and life. The conversion of body fuels into life is similar to the process of combustion; fuel and oxygen is consumed, while heat and energy is generated. This process is known as "metabolism".

The rate of metabolism, which determines the need for and consumption of oxygen, depends on the degree of physical activity or mental stress of the individual. Not all people require the same amount of oxygen. A man walking at a brisk pace will consume about four times as much oxygen as he will while sitting quietly. Under severe exertion or stress, he could possibly be consuming eight times as much oxygen as resting.


 Oxygen Requirements at Altitude

The FAA requires that all pilots flying their aircraft above 12,500 feet for 30 minutes or longer or at 14,000 feet or above during the entire flight must use supplemental oxygen. The amount required is 1 liter of oxygen per minute for every 10,000 feet. For example, at 18,000 feet there should be a flow of 1.8 liters per minute of oxygen available via a standard breathing device. The FAA requires there should be a device so attached to each breathing device that visually shows the flow of oxygen. (Nelson flow meters meet this FAA requirement.) The FAA also regulates that passengers must have supplemental oxygen available over 15,000 feet and that it is recommended that supplemental oxygen be used at night at altitudes over 5,000 feet.


 Effective Performance Time

This is the amount of time during which a pilot is able to effectively or adequately fly his aircraft with an insufficient supply of oxygen. At altitudes below 30,000 feet this time may differ considerably from the time of total consciousness (the time it takes to pass out). Above 35,000 feet the times become shorter and eventually coincides, for all practical purposes, with the time it takes for blood to circulate from the lungs to the head

Average Effective Performance Time for flying

15,000 to 18,000'........30 minutes plus

22,000' .......................5 to 10 minutes

25,000' .........................3 to 5 minutes

28,000'....................2 1/2 to 3 minutes

30,000' .........................1 to 2 minutes

35,000' .....................30 to 60 seconds

40,000'......................15 to 20 seconds

45,000' .......................9 to 15 seconds

Factors that determine Effective Performance Time

  1. Altitude. EPT decreases at high altitudes.
  2. Rate of ascent. In general, the faster the rate, the shorter the EPT.
  3. Physical Activity. Exercise decreases EPT considerably.
  4. Day-to-Day Factors. Physical fitness and other factors (smoking, health, stress) may change your ability to tolerate hypoxia from day to day, thereby changing your EPT. 

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