Teledyne 300P - Percent oxygen analyzer - Manual (Page 8)

Teledyne Analytical Instruments 2-2
2 2 2 2 2 Operational Theory Model 300P
sion membrane, has a surface area of 2.48 cm 2 . The sample gas enters the
sensor block through an inlet tube between the cathode and the sensor
block cap, diffuses through the Teflon membrane, and any oxygen in the
sample gas is reduced on the surface of the cathode by the following
mechanism:
O
2
+ 2H
2
O + 4e – – – – –
→
4OH – – – – – (cathode)
When the oxygen is reduced at the cathode, lead is simultaneously
oxidized at the anode by the following mechanism.
2Pb
→
2Pb +2 + 4e – – – – – (anode)
The electrons released at the surface of the anode flow to the cathode
surface via an external circuit. This current is proportional to the amount of
oxygen. It is measured and used to determine the oxygen concentration in
the gas mixture.
The overall reaction for the fuel cell is:
2Pb + O
2
→
2PbO
The output of the fuel cell is limited by the amount of oxygen in the
cell at any one time, and the amount of stored anode material. In the
absence of oxygen, there is no current generated.
Warning: The sensor used in the Model 300P uses electrolytes which
contain substances that are extremely harmful if touched,
swallowed, or inhaled. Avoid contact with ANY fluid or powder
in or around the unit. What may appear to be plain water could
contain one of these toxic substances. In case of eye contact,
immediately flush eyes with water for at least 15 minutes. Call
physician. (See Appendix - Material Safety Data Sheet (MSDS)).
2.3 Circuit Description
The 300P circuitry consists of four stages: an input stage, a tempera-
ture compensation/ gain stage, a low-pass filter and an output voltage-to-
current stage. The output of the O
2
cell is a current directly proportional to
the amount of oxygen present. The input stage converts this current to a
voltage signal.
The temperature compensation stage consists of circuitry that ampli-
fies the signal by a factor which is dependent upon the O
2
cell temperature.
This temperature dependent gain is used to compensate for the rise in
output current of the O
2
cell with temperature. A thermistor mounted in
the sensor assembly monitors this temperature and changes resistance (up