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3.0 INTRODUCTION
3.1 GENERAL DESCRIPTION
The Edgetech Instruments Inc. OxyTrans 2-wire 4 to 20 mA Loop Powered
Oxygen Transmitter consists of two sub-systems:
1. The Analyzer Unit, which is an electro-chemical fuel sensor designed
for measuring oxygen.
2. The Electronics Unit, which provides sensor signal conditioning, digital
display, and electrical analog outputs.
The gas to be measured is transported to the sensor membrane of the Micro
Fuel sensor. This sensor is an electro-chemical and galvanic cell that transforms
the oxygen in the measured gas into an electrical current.
The Electronics Unit measures this electrical signal, providing visual and
electrical information to the operator, directly in either parts-per-million of oxygen
(ppm O2), or as a percentage of the total measured gas (%O2).
3.2 MICRO FUEL SENSOR
The oxygen sensor is a sealed plastic disposable electrochemical sensor. The
active components are a cathode and an anode, which are immersed in a 15%
aqueous KOH electrolyte solution. When oxygen in the sample gas is sensed by
the membrane, the cell converts the resulting chemical reaction into an electrical
current change. Its action is similar to that of a battery, with one important
difference. In a battery, all reactants are stored within the cell. In the oxygen
sensor, one of the reactants, oxygen, comes from outside the device as a
constituent of the sample gas being analyzed.
3.3 THE EFFECT OF PRESSURE
In order to determine the amount of oxygen present in the gas sample, it is
necessary that the gas diffuse into the cell under constant pressure, by Dalton’s
Law. If the total pressure increases during a measurement, the rate that oxygen
reaches the cathode through the diffusing membrane will also increase. The
electron transfer, and therefore the resulting measured output current, will
increase even though the oxygen concentration in the sample has not changed.
Therefore, it is important that the sample pressure at the fuel cell remain
constant during measurements.
