AVIATION Oxygen -- page 6 of 6
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Flow Meters

A device visually indicating the flow of oxygen must be used with each breathing device. Typically what is supplied is an indicator in line between the breathing device and the hose connector. This in line type of indicator is operated by the flow of oxygen. As soon as there is a flow of oxygen, the red indication is replaced with a green reading. The change from red to green only shows that there is a flow of oxygen (which is the only thing the FAA requires). The green indication does not tell you that the system is working properly. As a matter of fact, the green indication on some of the red/green indicators will operate with a flow of oxygen required for less than 5,000 feet. You could actually be flying at 25,000 feet with the flow indicator showing green, but actually only have enough oxygen for 5,000 feet. In our opinion the typical red/green indicator is practically worthless and potentially very dangerous. The Nelson flow meters replace the red/green indicator.

To reduce the possibilities of hypoxia, the Nelson flow meters provide a fail safe means of visually observing the actual oxygen flow to the breathing device as well as providing a means to adjust the flow of oxygen as required.

Types of Oxygen Systems

There are several types of oxygen systems commonly found in general aviation aircraft: 1. Constant flow. 2. Altitude adjustable. 3. Altitude compensating. Each type has advantages and disadvantages.

Constant Flow Systems.

The most common and lowest cost system found in general aviation is the constant flow type. The basic system includes three parts: the cylinder(s), regulator, and manifold system.

The cylinder is common to all systems. It can be made from steel, aluminum, or composites. The tank pressure is usually less than 2,200 pounds per square inch (psi). The regulators which step down the pressure from 2200 psi to 20-75 psi can be attached separately from the cylinder(s) or directly screwed onto the cylinder. Most regulators are of the diaphragm type. They typically hold a constant output pressure between 20 and 75 pounds, depending on the manufacturer, from either a full cylinder to one that is almost empty. A manifold system is built into the regulator for portable systems. For built-in systems there is a manifold system installed in the aircraft. The manifold system operates at the 20-75 pound pressure, and not the 2,000+ pounds cylinder pressure.

The constant flow type provides the same output pressure or flow regardless of altitude. There is virtually no maintenance required. It is low in cost and well as low in weight. The regulator output is typically 2.5 to 3.0 liters per minute at a regulated line pressure of 25 to 75 pounds. The output is controlled by a small orifice in the regulator itself or most commonly done by the connector going into the manifold system. The connector orifice can be a hole as small as .012 inches in diameter. Cessna aircraft with factory supplied built in oxygen systems use a constant flow type of system manufactured by Puritan. Most portable systems are also of the constant flow type.

We strongly recommend the manifold output pressure of all constant flow systems, Cessna built in systems in particular, be checked for factory recommended output pressure at least during the annual. This may or may not be a required check during the annual, but it should be. We found one customer's Cessna 210 that had a line pressure in excess of 200 pounds. The correct pressure should be about 70 pounds for the Cessna's built in Puritan system. Excess manifold pressure from the oxygen regulator can cause the hose going to a flow control device to burst.

The disadvantage of the constant flow system is that there is a waste of oxygen at lower oxygen altitudes. The system typically provides the pilot a flow of 2.5 liters per minute. This is the correct amount of oxygen at 25,000 feet. However, if the aircraft were only at 15,000 feet, only 1.5 liters per minute are required. There is a waste of 1 liter per minute of oxygen. The excess oxygen used has no serious medical effect other than drying out your nose quickly. Obviously, however there is an economic disadvantage.

Using a Nelson flow meter with a constant flow oxygen system eliminates the non required high flow rate of oxygen. The savings can be over 100%. By setting the flow to what you actually need, two things are accomplished. The saving of oxygen is accomplished thus extending the use of oxygen in your aircraft or lowering oxygen costs. You also have improved the safety at oxygen altitudes by knowing that the system is working properly. If the floating ball is at the correct altitude setting, then everything is working properly.

With the Nelson flow meter used in a constant flow oxygen system you can have your cake and eat it too. The economical constant flow system with the addition of a low cost Nelson flow meter will provide you with a system that is reliable, safe, and economical.

Altitude Adjustable Systems.An altitude adjustable oxygen system is similar to the constant flow system except there is an adjustable control to set the necessary flow. This adjustment is accomplished by turning a control knob so a reading on a gauge, calibrated in altitude, is the same as the aircraft's altimeter setting. There is a significant saving in oxygen, since you are not wasting the excess flow of oxygen. Not many built in systems use this type. However some portable systems have this feature. The military surplus A8A regulators of the altitude adjustable type are commonly used in many sailplanes.

One disadvantage to this type of system, other than it costs more, is that there is no positive indication of flow to the individual breathing devices. You cannot adjust individually the flow of oxygen to each of the breathing devices. Not all people require the same amount of oxygen (for example the smoker). The red/green indicator is commonly used to show flow. As previously mentioned, this doesn't tell you that the system is working properly.
The Nelson flow meters can effectively be used with this type of system. The flow meter can be set wide open.and the resulting flow from the altitude adjusting system can be observed in the flow meter. What is recommended is to have at least one Nelson flow meter installed so the pilot can monitor the resulting flow from the altitude adjustments made on the flow adjustment control on the regulator.

What most users do is to turn the altitude adjustment to the service ceiling of the aircraft and leave it there all the time. Do the adjusting on each of the Nelson flow meters. That way you can individually adjust each breathing station as required.

Altitude Compensating Systems.

The altitude compensating system is similar to the altitude adjustable systems except that the adjustment is done automatically instead of manually setting the flow rate to an altitude gauge. Beechcraft and Mooney use this type of system. Also, some portable systems have this feature. The systems work quite well in the automatic mode. There are again disadvantages to this type of system. Some systems do not turn on or provide any oxygen until the system is at 8 to 10,000 feet. If you want oxygen at a lower altitude, you are out of luck. Like the altitude adjustable system, you cannot individually adjust the flow of oxygen since all of the outlets are controlled by the automatic system. If there is a person on board who requires extra oxygen, you cannot provide additional oxygen for that person. In addition, usually there is no actual flow meter available to indicate if the automatic flow control device is working properly. The use of a Nelson Flow Meter in the pilots station will tell you if the automatic system is working properly.

The information given herein is deemed accurate and reliable, but there is no guarantee given or
expressed for its accuracy.

All of the information given is the courtesy of NELSON AIRCRAFT CO.

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Updated: 2/9/98
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