Teledyne 335 - Analog control room monitor for personnel safety User Manual
Page 10
TELEDYNE ANALYTICAL INSTRUMENTS
2 Operational Theory
Model 335
2-4
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
portion (parts-per-million {ppm} or percent {%}) 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 propor-
tion of oxygen has not changed.
From Dalton's Law, the partial pressure of each gas in a mixture is the
same pressure that it would exert if it were alone given the same amount
and confined to the same volume. This means that as long as the total
pressure of the sample remains constant, the mixture can change, but the
diffusion of the oxygen will be affected only by the concentration of the
oxygen.
For this reason, the sample system supplying sample gas to the cell is
designed to keep the pressure on the diffusion membrane constant.
2.2.5 Calibration Characteristics
Given that the total pressure of the sample gas at the surface of the
MFC input is constant, a convenient characteristic of the cell is that the
current produced in an external circuit of constant impedance is directly
proportional to the rate at which oxygen molecules reach the cathode, and
this rate is directly proportional to the concentration of oxygen in the
gaseous mixture. In other words it has a linear characteristic curve, as
shown in Figure 2-3 (using arbitrary units). Measuring circuits do not have
to compensate for nonlinearities.