Teledyne 3350 - Microprocessor based control room monitor for personnel safety - Manual (Page 13)

2-3
Alarm Oxygen Monitor Operational Theory 2
Teledyne Analytical Instruments
2.2.3 Electrochemical Reactions
The sample gas diffuses through the Teflon membrane. Any oxygen in the
sample gas is reduced on the surface of the cathode by the following HALF
REACTION:
O
2
+ 2H
2
O + 4e – →
4OH – (cathode)
(Four electrons combine with one oxygen molecule—in the presence of
water from the electrolyte—to produce four hydroxyl ions.)
When the oxygen is reduced at the cathode, lead is simultaneously oxidized
at the anode by the following HALF REACTION:
2(Pb + 2OH –
) →
2(Pb +2 + H
2
O) + 4e – (anode)
(Two electrons are transferred for each atom of lead that is oxidized. TWO
ANODE REACTIONS balance one cathode reaction to transfer four elec-
trons.)
The electrons released at the surface of the anode flow to the cathode
surface when an external electrical path is provided. The current is proportional
to the amount of oxygen reaching the cathode. It is measured and used to
determine the oxygen concentration in the gas mixture.
The overall reaction for the fuel cell is the SUM of the half reactions above,
or:
2Pb + O
2
→
2PbO
(These reactions will hold as long as no gaseous components capable of
oxidizing lead are present in the sample. The only likely components are the
halogens—iodine, bromine, chlorine and fluorine.)
The output of the fuel cell is limited by (1) the amount of oxygen in the cell
at the time and (2) the amount of stored anode material.
In the absence of oxygen, no current is generated.
2.2.4 The Effect of Pressure
In order to state the amount of oxygen present in the sample as a percent-
age of the gas mixture, it is necessary that the sample diffuse into the cell under
constant pressure.
If the pressure changes, the rate that oxygen reaches the cathode through
the diffusing membrane will also increase. The electron transfer, and therefore the
external current, will increase, even though the proportion of oxygen has not
changed.
Fortunately, Dalton's Law confirms that every gas in a mixture contributes
the same pressure to the mixture that it would exert if it were alone in the same