Carbon Monoxide Alarm User Manual

Page 38
Appendix B How to determine where
your alarms should be set
1. Oxygen alarms
Two oxygen alarm set points have been provided; one for
low concentrations associated with oxygen deficiencies,
and one for high concentrations associated with oxygen
enrichment.
Oxygen deficiency is the leading cause of worker fatality
during confined space entry. All confined spaces must be
tested for oxygen deficiency before entry. Normal fresh
air contains 20.9 percent O2. Any environment in which
the oxygen concentration is less than 19.5 percent has
been determined by OSHA to be oxygen deficient. The
normal PhD
2
low-alarm setting for oxygen deficiency is
19.5 percent.
Common causes of this hazard are bacterial action,
displacement of oxygen by other gases, oxidation
(rusting), consumption (burning), or absorption by
materials such as wet activated carbon.
The PhD
2
will also alarm for an excess of oxygen. Too
much oxygen in an environment can result in an increased
flammability hazard. The new OSHA standard for
confined space entry (29 CFR 1910.146) requires that
oxygen concentrations not exceed 23.5 percent. The
normal setting for the high oxygen alarm is 23.5 percent.
2. Combustible gas alarms
As an environment becomes contaminated with
combustible gases or vapors, concentrations can climb
until they eventually reach ignitable or explosive levels.
The minimum amount of a combustible gas or vapor in air
which will explosively burn if a source of ignition is present
is the Lower Explosive Limit (LEL) concentration. PhD
2
combustible gas readings are given in percent LEL, with a
range of zero to one-hundred percent explosive. The
PhD
2
combustible gas sensor is non-specific and
responds to all combustible gases and vapors.
Combustible sensors contain two coils of fine wire coated
with a ceramic material to form beads. These two beads
are strung onto the opposite arms of a balanced
Wheatstone Bridge circuit. The "active" bead is
additionally coated with a platinum or palladium based
material that allows catalyzed combustion to occur on the
surface of the bead. The platinum catalyst is not
consumed in the combustion reaction, it simply enables it
to occur. It is not necessary for the combustible vapor to
be present in LEL concentrations in order for this reaction
to occur. Even trace amounts of combustible gas present
in the air surrounding the sensor will be catalytically
burned on the surface of the bead.
The "reference" bead lacks the platinum outer coating but
in other respects exactly resembles the active bead. A
voltage is applied across the active and reference
elements, causing them to heat. If combustible vapors
are present, the active bead will be heated by the reaction
to a higher temperature. The temperature of the untreated
reference bead is unaffected by the presence of gas. The
difference between the temperatures of the two beads will
be proportional to the amount of combustible gas present.
Since the two beads are strung on the opposite arms of a
Wheatstone Bridge electrical circuit, the difference in
temperature between the beads is perceived by the
instrument as a change in electrical resistance.
It is important to note that catalytic "hot bead" type
combustible sensors
require the presence of oxygen (at least 8 - 10 percent by
volume) in order to detect accurately. A combustible
sensor in a 100 percent pure combustible gas or vapor
environment will produce a reading of zero percent LEL.
The amount of heat produced by the combustion of a
particular gas on the active bead will reflect the "Heat of
Combustion" for that gas. Heats of combustion may vary
from one combustible gas to another. For this reason
readings may vary between equivalent concentrations of
different combustible gases.
A combustible gas and vapor reading instrument may be
calibrated to any number of different gases or vapors. If
an instrument is only going to be used for a single type of
gas over and over again, it is usually best to calibrate the
instrument to that particular hazard. If the instrument is
calibrated to a particular gas it will be accurate for that
gas. This is what is illustrated in the following chart.
ACTUAL LEL CONCENTRATION
METER RESPONSE
RELATIVE LEL
CALIBRATION STANDARD
Note that in a properly calibrated instrument, a
concentration of 50 percent LEL produces a meter
response (reading) of 50 percent LEL.
The following graph illustrates what may be seen when a
combustible reading instrument is used to monitor gases
other than the one to which it was calibrated. The chart
shows the "relative response curves" of the instrument to
several different gases.