Carbon Monoxide Alarm User Manual

Page 27
Sensor Type Bias Voltage
CO Unbiased 0.0 mV
H2S Unbiased 0.0 mV
SO2 Unbiased 0.0 mV
Cl2 Unbiased 0.0 mV
HCN Unbiased 0.0 mV
NO2 Unbiased 0.0 mV
HCl Biased - 300 mV
NH3 Biased - 250 mV
NO Biased - 350 mV
TOX 1 Biased - 300 mV
Note: The "TOX 1" broad range electrochemical sensor
is designed for use in situations where it is necessary to
use a single sensor to monitor for multiple atmospheric
hazards. When specific contaminants are known to be
present, the safest course is usually to select substance
specific sensors, and calibrate them directly to the
contaminants to be measured. Use of broad range
sensors should be restricted to situations where this
approach is not possible.
Use the following procedure to reset the bias voltage:
(1) Make sure the detector is turned off.
(2) The potentiometer used to adjust the bias voltage is
on the back of the main circuit board. Remove the
five screws holding the two halves of the instrument
case together in order to open the case and access
this pot.
(The bias voltage potentiometer is located near the
board's center, and is the one and only pot on the back of
the board.)
(3) Remove the old toxic sensor in the "Toxic 1" channel.
The following figure shows the sensor compartment with
the "Toxic 1" toxic sensor removed, exposing the sensor
pin connector sockets.
SHORTING WIRE
CENTER PIN
TOX 1
(4) Put a shorting wire across the two bias pin sockets
as shown in the above figure. (A resister lead or
paper clip may be used for this purpose.)
(5) Using a voltmeter (must be capable of reading 0 -
350 mVDC + 1 mV resolution) place the positive lead
on the shorting wire, and the negative lead in the
center pin socket.
(6) Using a small slot-head screwdriver, adjust the bias
voltage pot until the voltmeter reads the required
value.
(7) Remove the shorting wire.
(8) Install the new sensor in the Toxic 1 socket. If the
new sensor is biased it should be allowed to stabilize
for at least twenty-four hours before calibration.
Unbiased sensors should be allowed to stabilize for
at least one hour prior to calibration. (It is not
necessary for the instrument to be turned on during
the stabilization period.)
(9) Carefully, making sure the water resistant gasket is
correctly in place, replace the five screws holding the
two halves of the instrument case together.
(10) Turn the instrument on and enter the sensor
configuration made as discussed in Section 5.3.1.
Select the correct "Toxic 1" sensor type from the
menu, and update the new values.
(11) Calibrate the new sensor before use!
5.3.4 Changing the precision of the toxic
sensor read-out
It is possible to change the precision, that is, the
placement of the decimal point, in the read-out for a toxic
sensor. The toxic gas read-out may be either in full parts-
per-million (PPM) increments, or in tenths of parts-per-
million (0.1 PPM) increments. If 0.1 PPM increments are
selected the toxic channel screen shows the indicator
"X.X" (If full PPM increments are selected,
the screen does not display a precision indication.)
The Toxic 1 channel configured for a chlorine (Cl2)
sensor, with a readout precision of 0.1 PPM will show the
following screen:
Pressing the “Alarm” button changes the readout
precision. Pressing the Cal button advances to the
second (Toxic 2) channel.
5.3.5 Adding an instrument identification
number
Pressing the Cal button again advances to the Serial
Number / Text Only readout latch screen.