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: 99 JAN 07
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