AQ EXPERT Instruction & Operations Manual
Version 1.13
Page 17 of 63
1. ELECTROCHEMICAL SENSORS
All electrochemical sensors are mounted on bayonet style fittings for easy removal.
The electrochemical sensors are described below:
A. Nitrogen Dioxide sensor (NO
2
)
This is a three electrode sensor that responds to nitrogen dioxide gas. Nitrogen dioxide
is a “sticky” gas and this sensor’s response is usually the slowest of all sensors.
B. Sulfur Dioxide sensor (SO
2
)
It is a three electrode sensor that responds to sulfur dioxide gas. It is equipped with an
inboard filter to remove interference from H
2
S gas and other acidic gases.
C. Nitric Oxide sensor (NO sensor)
This is also a three electrode sensor that responds to nitric oxide. It is equipped with an
inboard filter to remove any interference from other acidic gases.
The nitric oxide sensor requires a constant bias-voltage for proper operation. This is
supplied by the monitor
’s battery. If the battery voltage drops below a certain value, the
monitor will turn off automatically to maintain the sensor bias. If, however, the battery
voltage further drops to near zero, one must wait 24 hours after connecting the battery
charger, for the sensor bias to recover.
D. Oxygen sensor (O
2
sensor)
This sensor measures the oxygen concentration in the sample. It is a two-electrode
electrochemical cell. It has a silver cathode and a lead anode. Oxygen diffuses through
a tiny hole and reacts with the lead anode. The reaction produces an electric current.
The unit software linearizes the current vs. oxygen response. The cell becomes
exhausted when all the lead is consumed.
