Operational Theory
PEM-9004
Teledyne Analytical Instruments
6
2.3 Specific Sensor Network
The sensors employed in the PEM-9004 operate according to
Faraday's Law of Electrolysis: In an electrochemical cell, the rate at which
gas is produced at one pole is directly proportional to the quantity of
electric current flowing through the electrodes. Conversely, the current
produced by the cell is directly proportional to the consumption of gas at
the pole.
The PEM-9004 incorporates a network of up to five individual sensors
for analyzing oxygen, carbon monoxide, and optionally, sulphur dioxide,
and nitrogen oxides. The individual sensors are arranged on a sensor block
located underneath the front panel of the unit. Each sensor produces an
output current proportional to the consumption rate of the particular gas for
which it is sensitive. Typically, the sensor output is in the microampere
range. Cross-sensitivity between sensors is measured and corrected for,
using an internal algorithm. The sensor output is fast and stable and only
mildly affected by changes in relative humidity or temperature.
2.4 Combustion Efficiency
The PEM-9004 measures oxygen, combustion gas temperature,
draft pressure and ambient air temperature, and it contains an internal
database of the thermodynamic properties of several common fuels. It
uses these to calculate combustion efficiency as follows:
2.4.1 Calculating the CO
2
Composition of Exhaust Gas
It first calculates the CO
2
composition of the exhaust gas on a dry
basis using data extracted from the selected fuel. The following equation
is used to calculate the CO
2
concentration in volume %.
Where: CO
2MAX
is the fuel specific theoretical maximum CO
2
concentration in volume %
O
2
is the measured oxygen concentration in volume %
20.9 is the oxygen content in air (volume %).
⎟
⎠
⎞
⎜
⎝
⎛ −
×
=
9
.
20
O
1
CO
CO
2
2
2
MAX