RKI Instruments GasWatch 1 - Manual (Page 6)

2 • Description GasWatch Instruction Manual (Two-Gas)
Description
This section describes components of the GasWatch.
Sensors
The two-gas version of the GasWatch includes two of the following sensors (see Table 1).
The sensors are above the display screen. Labels indicate the target gas of each sensor
(CO, H
2
S, or O
2
). A dust cover protects each sensor from harmful objects in the
monitoring environment. The sensor and dust cover are secured by the sensor retainer.
Carbon monoxide and hydrogen sulfide sensor
The CO and H
2
S sensors are electrochemical cells that consist of three precious metal
electrodes in an acid electrolyte. A gas permeable membrane covers the cell and allows
gas in the monitoring environment to diffuse into the electrolyte at a rate proportional to
its partial pressure. The gas reacts in the cell and produces a current proportional to the
concentration of the target gas. The current is amplified by the GasWatch’s circuitry,
converted to a measurement of gas concentration, and displayed on the display screen.
Oxygen sensor
The O
2
sensor is an electrochemical cell that consists of gold and lead electrodes in an
alkaline electrolyte. A fluorocarbon membrane covers the cell and allows gas in the
monitoring environment to diffuse into the electrolyte at a rate proportional to the partial
pressure of oxygen. The oxygen reacts in the cell and produces a current proportional to
the concentration of oxygen. The current develops a voltage across a temperature-
compensating thermistor/resistor network. The voltage is measured by the GasWatch’s
circuitry, converted to a measurement of gas concentration, and displayed on the display
screen.
Display Screen
During normal operation the display screen displays the current concentration of one of
the target gases and the time of day. To display the second target gas, press the DISP
button.
You can also display and/or adjust peak readings, time-weighted averages, alarm points,
date, and time. See “Operation” on page 5 for more information. The GasWatch includes a
back light feature so you can see the display screen in dimly-lit environments.
Alarm Light
The alarm light is above and to the right of the display screen. The alarm light is a red LED
that alerts you to gas, low battery, and sensor failure alarms.