Oxygen Systems

Oxygen is essential to support human life and the body's requirements are met by the free oxygen that comprises about one fifth of the air we breathe.  At sea level the partial pressure of this atmospheric oxygen is 200 millibars (mbs) or one fifth of the total atmospheric pressure of 1 bar. Inside the lung there is a different mixture of gases with increased levels of carbon dioxide and water vapour.  The gases in the lungs are in equilibrium with those in the blood stream. They are saturated with water vapour and contain the excreted carbon dioxide at the expense of the oxygen content. As a result the normal partial pressure of the oxygen in the lungs is reduced to 130 mbs at sea level and the oxygen transfer mechanism can accept partial pressures down to 80 mbs oxygen. The breathing process is controlled by the level of carbon dioxide within the lung and the bloodstream will continuously remove sufficient oxygen from the air inhaled with each breath provided the partial pressure of the oxygen within the lung is greater than 80 mbs.

As you climb above sea level the total pressure, and hence the partial pressures reduce until at 10,000 ft the partial pressure of the oxygen in the lung reaches 80 mbs. This is the minimum that a healthy person can tolerate and accordingly, if the climb is continued above this height, the first symptoms of lack of oxygen, or hypoxia, appear. Between 10,000 and 15,000 ft the ability to perform skilled tasks such as aircraft control and navigation are impaired while between 15,000 and 20,000 ft there is a marked deterioration of performance, even of simple tasks, together with a loss of critical judgment and willpower. Thinking is slowed while muscular incoordination and clumsiness result. Above 20,000 ft the symptoms become severe, rapidly leading to unconsciousness. The great danger of hypoxia is that it is insidious and the individual is not aware of the gradual deterioration in his performance which, together with the sense of general well-being that is created, means that the onset may not be detected. Furthermore, as the breathing process is controlled by carbon dioxide levels, deliberate over-breathing will wash out carbon dioxide from the lungs and disturb the oxygen transfer mechanism. This is known as hyperventilation and can accelerate the onset of hypoxia as well as wasting oxygen.

The onset of hypoxia can be delayed by increasing the proportion of oxygen in the inspired air with the result that the partial pressure of oxygen in the lung is increased. Assuming that the pilot is breathing 100% oxygen when the climb is commenced, then the partial pressure of oxygen in his lungs will not fall below 130 mbs until 34,000 ft is reached. Climbing above this altitude, even when breathing 100% oxygen, will result in a reducing partial pressure of oxygen in the lung and breathing 100% oxygen at a height of 40,000 ft is equivalent to breathing air at 10,000 ft. Above 40,000 ft, hypoxia can only be prevented by employing pressure breathing. In practice, oxygen systems are never 100% efficient and 35,000 ft is a sensible limit.

Types of Oxygen System

There are two types of oxygen systems available to pilots; demand systems and continuous flow systems. Each will be described separately together with their advantages and disadvantages.

Demand Systems

In a simple demand system, a mask is worn which is connected to a regulator by a wide bore, corrugated tube. On inhaling, the pressure drop sensed in the regulator opens an oxygen demand valve allowing oxygen to flow to the mask. As soon as inhalation ceases, the demand valve closes shutting off the oxygen supply. Examples of this type of regulator supply air in sub-aqua breathing equipment and firemans breathing apparatus.

For aircraft use, demand regulators are fitted with air dilution which ensures economy at the lower levels. These diluter demand regulators can be recognised by having a lever marked 'Normal'/'100%' which controls the air inlet valve. The air inlet valve is also controlled by a barometric capsule which ensures the correct air/oxygen mixture for any given altitude, thus maintaining adequate oxygen partial pressure in the lung and hence adequate blood saturation. When 'Normal' is selected at sea level the oxygen demand valve hardly opens and the user breathes mostly ambient air; by 30,000 ft the air inlet is fully closed and the user receives 100% oxygen. In gliders the lever should be wire locked in the 'Normal' position as there is no risk of cockpit fumes or smoke, and accidental selection of '100%' will rapidly empty the cylinder. The A-12A regulator is of the diluter demand type and can be safely used up to 35,000 ft provided the user has a well-fitting mask. The A-14 regulator is of similar design but provides a positive over pressure at heights above 35,000 ft. This type of regulator may be used up to 45,000 ft providing the user has undergone aviation medicine indoctrination.

A recent introduction is the electronic demand regulator marketed by Mountain High.   This is a compact, efficient regulator that is designed to be used with a cannula.   On breathing in, the regulator supplies a pulse (bolus) of oxygen that depends on height at the start of inspiration.   This bolus of oxygen is then drawn deep into the lungs where it achieves maximum effect.   The system works well and is particularly economical on oxygen.

Continuous Flow Systems

Continuous flow oxygen systems rely on a simple regulator that delivers oxygen at a pre-determined flow rate irrespective of altitude or the needs of the user. The AAV AIR/24 regulator has two flow rates, 'Normal' (two litres per minute) and 'High', (four litres per minute), while other types of constant flow regulators may have a flow control valve marked in thousands of feet. In order for the system to be efficient, it is necessary to devise some means of storing oxygen delivered by the regulator during the period that the user is exhaling. This is usually accomplished by providing a rubber re-breathing bag to which the oxygen flow is piped. A non-return valve at the mouth of the bag closes when the user exhales. On inhaling, the valve opens allowing the oxygen stored in the bag during the time that the user was exhaling, to be inhaled. A further refinement is a simple flow meter which fits in the oxygen supply pipe and shows that oxygen is flowing to the mask. A constant flow system can be safely used up to 20,000 ft on 'Normal' flow and up to 30,000 ft on 'High' flow. Their use above this height is not recommended.

Oxygen Masks

Whichever oxygen system is in use, it is essential that a well fitting and compatible mask is used. Failure of a demand valve to open is usually due to leaks around the mask. Masks are not interchangeable without modification, a demand mask must be used with a demand system and vice versa. Aviation masks are designed to operate in the freezing conditions that prevail at altitudes requiring the use of oxygen. Medical masks, which can normally be recognised by their clear plastic material, will fail at altitude due to ice formation in the throat of the mask. Microphones may be fitted to some aviation masks to facilitate intercommunication. When not in use, masks and associated tubes should be kept in a cool, dark place; ultimately rubber components will age and crack, and should then be replaced.

Cannulas

An alternative to a constant flow mask is the cannula, of which there are two types available.  The simple cannula pipes a constant flow of oxygen into the nostrils.  This is extremely wasteful as it not possible to save the oxygen delivered while exhaling.   The preferred cannula is the AEROX oxy-saver cannula.  This has small oxygen reservoirs which store a small amount of oxygen on exhaling and which is subsequently delivered when inhaling.   Cannulas have the big advantage that normal activities such as using a microphone or eating and drinking can be carried out while the cannula is in use.   Cannulas should not be used above 18,000ft as the amount of oxygen that can be inhaled is insufficient to ensure adequate oxygenation above that height.   None the less, they are a useful alternative to the mask when operating in the 12,000 to 18,000ft height band.   Cannulas should not be used with diluter demand regulators other than the Mountain High electronic regulator.

Choice of System

There is no doubt that a diluter demand system is to be preferred for installation in an aircraft. It makes more effective use of the limited oxygen quantity and a 630 litre cylinder should last at least 5 hours, the actual duration depending upon the height. When fitted with a mechanical or electrical flow indicator, positive indication is given of the flow of oxygen to the pilot. In conjunction with a well-fitting demand mask it can be used safely up to 35,000 ft. However, the equipment is expensive to buy initially and overhaul costs, if needed, are high.

The continuous flow system is a reliable, effective and cheaper alternative provided it's limitations are well understood. The economizer bag and flow indicator, if fitted, require careful monitoring to ensure that oxygen is flowing during flight. For a given cylinder capacity, the duration is less than when using a diluter demand system; on 'Normal' flow, a 630 litre cylinder will last some 5 hours which reduces to approximately 2 1/2 hours on 'High' flow. The maximum safe altitude is 30,000 ft and it is not recommended that this is exceeded.

General Precautions

To obtain safe service from your oxygen system:

• Never dismantle components without understanding what you are about.
  
  
• Never use any oil or grease, even the slightest contamination must be removed, whisky is a convenient and safe solvent!
  
  
• Never exhaust the oxygen cylinder as on descent it will fill with moist air and give you trouble.