Carbon Monoxide Alarm User Manual

Page 17
Chapter 3 Calibration
The PhD
2
detector has been designed for easy
calibration. Adjustments are made by using simple push-
button controls located on the instrument keypad.
The accuracy of the PhD2 should
be checked periodically with known concentration
calibration gas. Failure to check accuracy can lead
to inaccurate and potentially dangerous readings.
For a detailed discussion of calibration frequency,
see Appendix G.
Verification of accuracy is a two step procedure. In the
first step the PhD
2
is taken to an area where the
atmosphere is fresh and the readings are checked. If the
readings differ from those expected in fresh air a "zero"
adjustment must be made. The second step is to make
sure the sensors are accurate by exposing them to known
concentration test gas and noting the response. This is
known as a functional or “bump” test. If the readings
during the bump test are considered accurate, the
instrument may be used without further adjustment.
Span adjustments are made by pressing buttons on the
instrument keypad. Use of the push buttons in span
adjustment procedures is reserved for authorized
personnel.
3.1 Functional (bump) test
A simple functional (bump) test is all that is needed to
verify accuracy. To do a functional (bump) test, turn the
PhD
2
on, and wait at least three minutes to allow the
readings to fully stabilize. Make sure the instrument is
located in fresh air. Check to see that the readings match
the concentrations present in fresh air (20.9 % O2, 0 %
LEL, 0 PPM toxic). If necessary, fresh air zero the
instrument using the procedures discussed in Section 3.3
of this manual. Slip on the calibration adaptor as
discussed in Section 3.4, and flow calibration gas to the
sensors. Wait for the readings to stabilize. (Forty-five
seconds to one minute is usually sufficient.) Note the
readings. Oxygen readings are considered to be accurate
when the display is within ±0.5% of the expected
concentration as given on the calibration gas cylinder.
LEL and toxic readings are considered accurate when
they are between 90% and 120% of the expected value as
given on the calibration gas cylinder. If readings are
accurate, there is no need to adjust your gas detector. If
the readings are inaccurate, the instrument must be be
adjusted using the "span" calibration procedures
discussed in Section 3.4 before further use.
Note: All-in-one calibration mixtures are particularly
useful in functional test procedures. Appendix D
lists currently available mixtures.
3.2 Effect of contaminants on PhD
2
sensors
The atmosphere in which the PhD
2
monitor is being used
can have an effect on the sensors. Sensors may be
poisoned or suffer degraded performance if exposed to
certain substances.
There are three basic types of sensors that may be
installed in the PhD
2
detector; oxygen, combustible gas
(LEL), and electrochemical toxic. Each type of sensor
uses a slightly different detection principle. The kinds of
conditions that affect the accuracy of the sensors varies
from one type of sensor to the next.
The accuracy of the PhD2 should
be checked immediately following any known
exposure to contaminants by testing with known
concentration test gas before further use. Failure to
check accuracy can lead to inaccurate and potentially
dangerous readings.
3.2.1 Effects of contaminants on oxygen
sensors
Oxygen sensors may be affected by prolonged exposure
to "acid" gases such as carbon dioxide. The oxygen
sensors used in Biosystems instruments are not
recommended for continuous use in atmospheres which
contain more than 25 % CO
2
.
3.2.2 Effects of contaminants on
combustible sensors
Combustible sensors may be affected by exposure to
silicone containing substances (found in many lubricants
and hydraulic fluids), the tetra-ethyl-lead in "leaded"
gasoline, halogenated hydrocarbons (Freons

, or
solvents such as trichloroethylene and methylene
chloride). High concentrations of hydrogen sulfide may
also damage the 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
2
is
designed to "alarm latch" whenever the concentration of
combustible gas exceeds 100 percent LEL. Under these
conditions the combustible alarm sounds continuously,
the display shows an “X” for combustible gas, and power
to the combustible sensor is cut to avoid damage due to
over-heating. The alarms will continue to sound until the
instrument is reset. The instrument is reset by pressing
and holding down the MODE button until the instrument
turns off, then taking the instrument to a location where
the air is fresh and turning the instrument back on.
Normal combustible sensor operation is resumed at the
time the instrument is reset.
CAUTION: Never reset the instrument by fresh air
zeroing! Attempting to reset the instrument by doing
a fresh air “zero” adjustment will result in inaccurate