ALPHA OMEGA INSTRUMENTS CORP.
OXY-SEN OXYGEN MONITOR
Instruction Manual
SECTION 2.0
SYSTEM
DESCRIPTION
General Description
The OXY-SEN Oxygen Monitor is a percent oxygen monitor designed to provide accurate and
dependable percent oxygen measurements in a variety of gases. The monitor enclosure is
made from aluminum, and is rated for general purpose service. The monitor is powered from
115 VAC, 208/240 VAC (auto-adjusts to input voltage) or 24 VDC (must be selected at time of
order).
Ambient Temperature Electrochemical Sensor with EES
The OXY-SEN Oxygen Monitor features an extended life oxygen sensor with EES (enhanced
electrolyte system) a weak acid electrolyte system. The weak acid electrolyte system retards
passivation of the sensor anode by allowing the products of oxidation to dissolve in the acid
electrolyte. In effect, the sensor is renewed continuously as the weak acid electrolyte tolerates
over 20 times the lead oxide (PbO) than potassium hydroxide (KOH) based sensors. The result
is a sensor with a greatly extended useful life.
The extended life sensor is a lead-oxygen battery comprised of a lead anode, a gold cathode,
and a weak acid electrolyte. A gold electrode is bonded onto a non-porous Teflon (FEP)
membrane. A small amount of oxygen permeating through the membrane is reduced
electrochemically at the gold electrode. A resistor and a thermistor (for temperature
compensation) are connected between the cathode and anode. As a result, the lead-oxygen
battery is always discharged.
Current flowing through the resistor and thermistor is proportional to the oxygen concentration of
the gas in contact with the Teflon membrane. By measuring the voltage between the resistor
and the thermistor, oxygen concentration can be accurately determined. The two electrode
reactions are depicted below:
Cathode: O
2
+ 4H
+
+ 4e
2H
2
O
Anode: 2Pb + 2H
2
O
2PbO + 4H
+
+ 4e
Overall: O
2
+ 2Pb
2PbO
If PbO generated at the anode remains on the anode, the anode's potential will change and the
output voltage of the sensor would become extremely low. However, in actuality the surface of
the lead anode is continuously renewed because PbO is dissolved back into the electrolyte.
There is a maximum level to the solubility of PbO in the weak acid electrolyte. As a result, when
the electrolyte becomes saturated with PbO, the sensitivity of the sensor will begin to drop
signaling time for replacement of the sensor. When the extended life oxygen sensor is used in
ambient air monitoring applications, carbon dioxide (CO
2
) that is present will not adversely affect
the sensor's performance. Due to the weak acid electrolyte, CO
2
will not react with the
electrolyte to form potassium carbonate as it does with KOH based sensors. As a result, there
is no loss in output due to the presence of CO
2
.
2
