Carbon Monoxide Alarm User Manual

Page 18
readings. In the event of a protective over-limit alarm
latch condition the instrument must be turned off,
taken to an area where the air is fresh, and turned
back on to resume normal operation. Verify accuracy
by exposing the sensor to known concentration test
gas before further use.
The combustible sensor used in
the PhD
2
design requires the presence of oxygen in
order to detect combustible gas. The accuracy of the
combustible sensor may be affected 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 PhD5’s zero to 100 percent LEL detection
range.
Avertissement: Toute lecture rapide et positive,
suivie d'une baisse subite au erratique de la valeur,
peut indiquer une concentration de gaz hors gamme
de détection qui peut être dangereuse.
3.2.3 Effects of contaminants on toxic gas
sensors
Some toxic sensors are sensitive to more than one
contaminant. Others may have a cumulative loss of
sensitivity as a function of high exposure to the
contaminant they were designed to measure.
Cross sensitivity of PhD
2
toxic sensors to common
interfering gases is listed in Appendix E.
3.3 Fresh air "zero" calibration
The fresh air "zero" must be done in fresh,
uncontaminated air. In this procedure the instrument
automatically adjusts its oxygen, combustible gas, and
toxic gas readings to match the concentrations present in
fresh air (20.9 percent O2, 0.0 percent LEL, 0.0 PPM
toxic gas).
Since fresh air contains 20.9 percent oxygen, this is the
only calibration needed for the oxygen sensor. Toxic and
combustible gas sensors must also undergo span
calibration to insure accuracy.
If the PhD
2
cannot be taken to an area where the air is
fresh, or if it is not certain whether or not the air is
uncontaminated, special procedures are required. These
procedures are discussed at greater length in Appendix
C.
3.3.1. Fresh air "zero" calibration sequence
The fresh air zero procedure may only be done while the
instrument is being operated in either the Technician or
Basic operating mode.
Version 4.7 and higher PhD
2
micro-processor software
allows the instrument to be fresh air zeroed directly
through the MODE button using the procedures
discussed in Section 3.3.1.1. Version 4.6 and lower
instruments may only be fresh air zeroed using the
hidden push-buttons and procedures discussed in
Section 3.3.1.2. (Version 4.7 and higher instruments
may be fresh air zeroed using either method.)
3.3.1.1 Fresh air "zero" through MODE
button
(1) Turn the instrument on.
(2) Verify that the instrument is in the Basic or
Technician operating mode. Switch modes (if
necessary) by simultaneously holding down the "+"
and"-" keys. (Each time that the operating mode is
changed, the meter screen will briefly indicate the
current operating mode.)
(3) Wait at least three minutes after turning the
instrument on to allow sensor readings to stabilize
fully before initiating the fresh air zero procedure.
(4) Make sure the instrument is located in an area where
the air is known to be fresh.
(5) While in the normal gas reading screen press the
MODE button three times within two seconds. This
will "wake up" the instrument from normal operation,
and put it into the fresh air zero mode. A screen will
display the message "Fresh Air Cal Adjustment."
This screen will be followed by another displaying the
message "Mode = Adjust." This screen also includes
a timer which counts down the number of seconds
which remain in which to choose to make the
adjustment.
↓
(6) Pressing the MODE button within five seconds
causes the fresh air adjustment to be made. An
information screen is briefly displayed while the
adjustments are being made, and another when the
adjustments have been completed.
↓
(7) If the MODE button is not pushed within five seconds
a fresh air zero adjustment will not be made. A
message screen indicating that the zero values have
not been changed will be briefly displayed, and the
instrument will be returned to normal operation.