Monitoring for Oxygen
T.R. Consulting, Inc.
November 2003 Safety Article
Written
and compiled by:
Tony
Rieck
Copyright
2003 T.R. Consulting, Inc.
http://www.trconsultinggroup.com/
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Note: T.R.
Consulting, Inc. presents the information contained in this article as an aid
in understanding of the subject matter. Referenced standards must be read and
thoroughly understood in order to assure compliance with the standard. T.R.
Consulting, Inc. attempts to provide accurate information, but makes no
warranty with regard to either the completeness or accuracy of the information
contained herein.
What Is Oxygen?
Oxygen is a colorless odorless and
tasteless gas that makes up approximately 20.9% of the air that we breathe. There are three commonly referenced forms of
oxygen:
- Elemental oxygen is a single oxygen
molecule. Elemental oxygen is very
reactive and is a component of hundreds of thousands of organic compounds and
will combine with most elements.
- The oxygen we breathe is a pair of
oxygen molecules bonded together and is represented chemically as O2
- Triatomic Oxygen (O3)
is also called ozone. Ozone plays an
important part in the filtering of UV radiation from the sun in the upper
atmosphere, but is considered a toxic air contaminant when emitted into the air
at this level of the atmosphere.
Oxygen Deficiency
The body requires oxygen to live, if the
oxygen concentration decreases, the body reacts in various ways. Death occurs
rapidly when the concentration decreases to 6%.
Oxygen can be reduced in the atmosphere through chemical reduction
reactions, combustion, and displacement with atmospheric contaminants or inert
gases.
Physiological
Effect of Oxygen Deficiency
% Oxygen (by volume)
At sea Level Effects
-------------------- ----------------------
21 Nothing
abnormal
16
- 21 Increased
breathing volume. Accelerated heartbeat.
Impaired attention and thinking. Impaired coordination.
14
- 10 Very faulty
judgment. Very poor muscular coordination. Muscular exertion brings on rapid
fatigue that may cause permanent heart damage. Intermittent respiration.
10
- 6 Nausea. Vomiting.
Inability to perform vigorous movement, or loss of all movement. Unconsciousness, followed by death.
<6
Spasmodic breathing.
Convulsive movements. Death in minutes.
Physiological effects of oxygen
deficiency are not apparent until the concentration decreases to 16%. The
various regulations and standards dealing with respirator use recommend that
percentages ranging from 16-19.5% be considered indicative of an oxygen
deficiency. Such numbers take into account individual physiological responses,
errors in measurements, and other safety considerations. In hazardous response
operations, 19.5% oxygen in air is the figure that decides between
air-purifying and atmosphere-supplying respirators.
Oxygen
Enriched Atmospheres
An
atmosphere is considered to be oxygen enriched when the concentration of oxygen
exceeds 23.5%. Oxygen enriched
atmospheres pose a serious risk of fire and explosion because flammable and
combustible materials will ignite readily and burn violently. This is why you will see signs where oxygen
is in use, such as at hospitals, reading “No Smoking,
Importance
of Oxygen Monitoring
Since
we require oxygen to live and oxygen (or a lack thereof) cannot be detected
with the senses, we need a reliable means of determining the oxygen content of
an atmosphere before we enter it.
Additionally, determining the level of flammability associated with an
atmosphere is also oxygen dependant as combustible gas indicator readings are
unreliable unless appropriate concentrations of oxygen are present (19.5% to
23.5%).
At
rest, a person inhales six to eight liters of air per minute. At the height of exertion, a person may breathe
as much as 75 liters of air per minute.
As one’s activity level increases, so too does one’s body demand more
oxygen. Since work does not typically
involve lying on the couch and eating bon bons while watching one’s favorite
talk show host (being at rest), it is assumed that some exertion will be taking
place. Also, since individuals can be
more sensitive to oxygen depletion and because of the potential ranges of instrumentation
error, regulations require oxygen concentrations in the workplace to exceed the
level at which physiological effects of oxygen depletion are expected to
occur. Thus, even though physiological
effects of oxygen deficiency are not expected to manifest themselves at oxygen
concentrations greater than 16%, a work atmosphere must contain no less than
19.5% oxygen ( a little “breathing” room) to allow for
worker occupancy without the use of air-supplied respirators.
How
Oxygen Monitors Work
A
typical oxygen monitor is an electronic box containing a circuit board, display
screen, battery, pump or fan unit, and an oxygen sensor cell. The fan or pump draws a metered volume of air
over the oxygen sensor cell. The oxygen
sensor cell is a sealed plastic electrochemical transducer containing an anode
and a cathode immersed in an electrolytic solution such as potassium hydroxide
(KOH). The sensing surface of the oxygen
sensor cell is a diffusion membrane (typically Teflon)that
allows the oxygen to diffuse into the cell.
The resultant reaction causes an electrical charge that is proportional
to the amount of oxygen that has diffused into the cell.
Inert
Atmospheres
A
common method for reducing hazards from flammable vapors and gases is the
introduction of an inert gas, such as carbon dioxide or nitrogen, into a
container to displace the oxygen. Since
combustible gas indicator readings are unreliable in oxygen deficient
atmospheres, the only way to be certain that the space has been properly
inerted is to use an oxygen meter. It is
important to remember that inerting is a process that removes oxygen from an atmosphere;
the fuel (usually a flammable vapor or gas) is still present. Once inerted, any task that could allow the
reintroduction of air will result in a potentially explosive situation. Inerted containers should be provided with a
pressure relief valve (to prevent over pressurization) and pressure gage,
pressurized slightly (no more than 1 psig) with the inert gas, and sealed.
* Special
attention should be paid to inerted containers that are stored indoors as any
leakage into the storage room could cause a reduction of oxygen in the
room.
* An
inerted container should never be considered safe for cutting. As soon as the cutting begins, air is again
free to mix with the atmosphere in the container.
* A
rule of thumb for determining the appropriate level of oxygen to consider a
container inert is that the available oxygen should be no more than half of the
oxygen required for the ignition of the stored material. For example, many common petroleum products
can be ignited in atmospheres containing as little as 11% oxygen. Thus, in order to assure a truly inert
atmosphere, an oxygen reading of somewhat less than 6% would be required. Note: this point also demonstrates that fires
and explosions can occur in atmospheres containing far less oxygen than one
requires for breathing.
Oxygen
Monitor Limitations
Like
any other monitoring instrument, the oxygen monitor has a standard calibration
error and there are components that can age and cause inaccurate readings.
*
Calibration error - many manufacturers will instruct the user to calibrate the
instrument to 21% in fresh air. Since
the actual concentration of oxygen at sea level is 20.9% and can vary slightly
due to altitude, a slight error has already been introduced. Additionally, some oxygen monitors will self
calibrate each time that they are turned on.
If they turn on while being worn in an atmosphere containing somewhat
less than normal oxygen, the error will be even greater.
*
Oxygen sensor cell degradation - aging of the oxygen sensor cell will cause the
oxygen meter to take longer to come to a stable reading after initially being
turned on. As the cell ages more, the
instrument will require frequent calibration adjustments and will provide low
readings. Moisture and carbon dioxide
can shorten the life of an oxygen sensor cell.
On most instruments, this cell can be easily replaced by removing a
cover, loosening two screws, removing the old cell, placing the new cell into
the meter, tightening the screws and replacing the cover.
*
Altitude of calibration - the meter should always be calibrated at the altitued and under the proximate conditions that it will be
used. A meter calibrated at 5,000 feet
elevation may alarm, indicating an oxygen deficient atmosphere, at 9,000
feet. Certainly, the meter would be less
accurate.
*
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