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combustible gas sensors. For a more complete list
of known sensor poisons see Biosystems Standard
Warranty in Appendix E.
After any detector exposure to a suspected or known
poison/inhibitor source, combustible sensor accuracy
should be verified immediately by exposure to
calibration gas of known percent LEL concentration.
Note: If the combustible sensor in the PhD Lite
suffers a loss of sensitivity, it tends to be lost first
with regards to methane.
As described above, combustible gas sensors may
become desensitized if exposed to certain substances.
In some cases a desensitized combustible sensor may
still respond accurately to propane and other
hydrocarbons while showing a dangerously reduced
response to methane.
Biosystems’ “Propane Equivalent” calibration gas
mixtures have been developed to eliminate this
potentially dangerous source of calibration error.
Biosystems’ “Propane Equivalent” mixtures are based
on methane, so any loss of sensitivity to methane is
detected (and can be corrected) immediately.
Using Biosystems brand calibration gas and
regularly verifying accuracy ensures that proper
sensitivity is maintained for the life of the sensor.
4.2.3 Effects of high concentrations of
combustible gas on the combustible sensor
The accuracy of combustible sensors may also be
affected by exposure to high concentrations of
combustible gas. To minimize the chance for damage
or loss of sensitivity to the combustible sensor, the PhD
Lite is designed to "alarm latch" whenever the
concentration of combustible gas exceeds 100 percent
LEL. Under these conditions an “X” will appear in place
of the combustible gas reading to indicate that an over-
limit condition has occurred, and “LEL OVERRANGE”
will be displayed at the bottom of the LCD.
During an “LEL OVERRRANGE” condition, the power to
the LEL sensor is interrupted and the audible and visible
alarms are activated until the instrument is manually
reset by turning it off.
A combustible sensor
overrange alarm indicates a potentially explosive
atmosphere. Failure to leave the area immediately
may result in serious injury or death!
In the event of a combustible
sensor overrange alarm, the PhD Lite must be
turned off, brought to an area that is known to be
safe and then turned on again to reset the alarm.
Make sure that the PhD Lite is
located in fresh air before turning the instrument
back on after a combustible sensor alarm latch
condition has occurred. Fresh air calibration
adjustments may only be made when the PhD Lite is
located in air that is known to be fresh. After a
combustible sensor alarm-latch condition occurs,
the accuracy of the combustible gas sensor must be
verified by exposure to known percentage LEL
concentration test gas before further use.
Note: The combustible sensor used in the PhD Lite
requires a minimum of 10% oxygen by volume in
order to generate accurate combustible gas
readings. Combustible sensor accuracy may be
diminished if the instrument is used in oxygen-
deficient atmospheres.
A rapid up-scale reading
followed by a declining or erratic reading may
indicate a hazardous combustible gas concentration
that exceeds the PhD Lite’s zero to 100 percent LEL
detection range. Failure to leave the area
immediately may result in serious injury or death!
4.2.4 Effects of contaminants on toxic gas
sensors
Biosystems “substance-specific” electrochemical “smart
sensors” used to measure many toxic gases have been
carefully designed to minimize the effects of common
interfering gases. “Substance-specific” sensors are
designed to respond only to the gases that they are
supposed to measure. The higher the specificity of the
sensor, the less likely the sensor will react to other
gases, which may be incidentally present in the
environment. For instance, a “substance-specific”
carbon monoxide sensor is deliberately designed not to
respond to other gases that may be present at the same
time, such as hydrogen sulfide and methane.
Although great care has been taken to reduce cross-
sensitivity, some interfering gases may still have an
effect on toxic sensor readings. In some cases the
interfering effect may be positive and result in readings
that are higher than actual. In other cases the
interference may be negative and produce readings that
are lower than actual or even result in negative gas
readings.
4.3 Single sensors capable of monitoring for
two different gases
The OSHA standard for permit-required confined space
entry (29 CFR 1910.146) explicitly requires the use of a
direct-reading, substance-specific sensor whenever a
particular toxic hazard is likely to be present. For
example, if hydrogen sulfide is likely to be present, one
of the toxic sensors selected should be specifically
designed for the direct detection of H
2
S.
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