Teledyne 335 - Analog control room monitor for personnel safety - Manual (Page 9)

2-3 TELEDYNE ANALYTICAL INSTRUMENTS
Oxygen Alarm Monitor Operational Theory 2
At the top end of the cell, under the retaining screen, is a diffusion
membrane of Teflon whose thickness is very accurately controlled. (See
Figure 2-2.) Beneath the diffusion membrane lies the oxygen sensing
element (cathode) with a surface area almost 2.5cm
2 . The cathode has
many perforations to ensure sufficient wetting of the upper surface with
electrolyte, and it is plated with an inert metal.
The anode structure is below the cathode. It is made of lead and has a
proprietary design used to maximize the amount of metal available for
chemical reaction.
At the rear of the cell, just below the anode structure, is a flexible
membrane designed to accommodate the internal volume changes that
occur throughout the life of the cell. This flexibility preserves the integrity
of the structural elements surrounding the cathode and prevents the associ-
ated changes in electrical activity that would result.
The entire space between the diffusion membrane, above the cathode,
and the flexible rear membrane, beneath the anode, is filled with electro-
lyte. Cathode and anode are submerged in this common pool. They each
have a conductor connecting them to one of the external contact rings on
the contact plate on the bottom of the cell.
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
electrons.)
The electrons released at the surface of the anode flow to the cathode
surface when an external electrical path is provided. The current is propor-
tional to the amount of oxygen reaching the cathode. It is measured and
used to determine the oxygen concentration in the gas mixture.