Teledyne 3350 - Microprocessor based control room monitor for personnel safety - Manual (Page 11)

2-1
Alarm Oxygen Monitor Operational Theory 2
Teledyne Analytical Instruments
Operational Theory
2.1 Introduction
The analysis is specific for oxygen, i.e., the measuring cell will not generate
an output current unless oxygen is present in the sample gas. Thus, the instrument
has an absolute zero and no zero gas is required to operate the analyzer.
The measuring cell has the ability to respond accurately to the presence of
oxygen irrespective of flowrate. TAI recommends using ambient air as a span
gas or, if that is not possible, using a known calibration gas of about 80% of the
range of interest value.
The measuring cell (U.S. Patent #3,429,796) is a solid-state maintenance-
free structure that carries a TAI guarantee for performance and usable life. The
cell consumes oxygen from the gas surrounding it and generates a proportional
microampere current. The Control Unit processes the sensor output and trans-
lates it into electrical concentration, range, and alarm outputs, and a percent
oxygen meter readout. It contains a microcontroller that manages all signal
processing, input/output, and display functions for the analyzer.
2.2 Micro-Fuel Cell Sensor
2.2.1 Principles of Operation
The oxygen sensor used in the Model 3350 is a Micro-fuel Cell designed
and manufactured by TAI. It is a sealed, disposable electrochemical transducer.
The active components of the Micro-Fuel Cell are a cathode, an anode,
and the aqueous KOH electrolyte in which they are immersed. The cell converts
the energy from a chemical reaction into an electrical potential that can produce a
current in an external electrical circuit. Its action is similar to that of a battery.
There is, however, an important difference in the operation of a battery as
compared to the Micro-Fuel Cell: In the battery, all reactants are stored within
the cell, whereas in the Micro-Fuel Cell, one of the reactants (oxygen) comes
from outside the device as a constituent of the sample gas being analyzed. The
Micro-Fuel Cell is therefore a hybrid between a battery and a true fuel cell. (All
of the reactants are stored externally in a true fuel cell.)