Analytical Industries Palm O2 D Oxygen Analyzer - Manual (Page 7)

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5 Operation
5.1 Principle of Operation
The Palm O2 Oxygen Analyzer utilizes an electrochemical galvanic fuel cell type
oxygen sensor of the type that is extensively used to measure oxygen concen-
trations from 0% to 100% in gas streams. Oxygen, the fuel for this electro-
chemical transducer, diffusing into the sensor through a gas permeable mem-
brane reacts chemically at the sensing electrode to produce an electrical current
output proportional to the oxygen concentration in the gas phase. The sensor
has an absolute zero meaning that when no oxygen is present to be chemically
reacted the LCD displays 00.0 oxygen.
The sensor’s signal output is linear over the entire range, remains virtually con-
stant over the specified useful life and drops off sharply at the end. The sensor
itself requires no maintenance and is simply replaced at the end of its useful life
like a battery. Inasmuch as the sensor is a transducer in its own right, its ex-
pected life is not affected by whether the analyzer is ON or OFF.
The relationship between the sensor’s signal and changes with the oxygen
concentration is both proportional and linear, thus allowing single point
calibration. Other factors that can affect the signal output are described
in Section 5.2 Application Considerations and Section 3 Safety Warnings
which should be read before use.
Historically, the expected life of galvanic fuel type sensors has been specified as
“in air (20.9% O
2
) at 25°C and 760mm Hg”. The actual life of any galvanic fuel
type sensor is inversely affected by changes in the average oxygen concentra-
tion, temperature and pressure it is exposed to during its useful life. For exam-
ple, the AII-11-75-PO2D and AII-11-75-PO2RD sensors have a 32 month ex-
pected life in air (20.9% oxygen) at 25°C and ambient pressure, however, in a
100% oxygen atmosphere the expected life is 12.6 months [60mo/
(100%/20.9%)].
The Palm O2 Oxygen Analyzer is battery powered by (2) AA alkaline batteries
and controlled by a state-of-the-art microprocessor. The batteries provide
enough power to operate the analyzer continuously for approximately 13,000
hours. Both devices utilize a membrane type keypad for users to communicate
commands to the microprocessor. The digital electronics provide features such
as system diagnostics and warning indicators that enhance both safety and
effectiveness. The design criteria, quality program and performance features
ensure reliable and accurate oxygen measurements.
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5.2 Application Considerations
Effect of Temperature
All membrane clad electrochemical sensors are temperature dependent due to
the expansion and contraction of the Teflon sensing membrane. As result more
or less of the sample gas including oxygen to be reacted diffuses into the sen-
sor. The oxygen sensor’s electrical current signal output varies linearly with
oxygen concentration. The signal also varies with changes in ambient tempera-
ture. The temperature coefficient is typically 2.54% of the signal or reading per
degree C change in temperature.
The temperature dependent current signal output is compensated by using a
resistor-thermistor network. With a proper resistor-thermistor network, the
signal can be compensated to within +5% of the oxygen reading over the 5-
45°C temperature range. This is the worse case situation when going from one
extreme of the operating temperature range to the other. The error will be
eliminated when the thermistor in the temperature compensation network and
the electrolyte inside the sensor reach thermal equilibrium in approximately 45-
60 minutes.
Erroneous oxygen readings can result if the gases flowing over the
sensing area of the sensor are not at ambient temperature. This occurs
because the sensor is exposed to different temperatures. The sensing
area of the sensor is o-ring sealed in the sample gas and the tempera-
ture compensation network at the rear of the sensor is exposed to ambient
temperature.
Effect of Pressure
Electrochemical sensors actually measure the partial pressure, not the percent-
age, of oxygen in the gas stream they are exposed to. These sensors are accu-
rate at any pressure provided the pressure is constant and the analyzer has
been calibrated at the same pressure as the sample gas measured.
For example, when connected to a gas stream where the pressure varies, oxy-
gen sensor causes the analyzer to display fluctuating oxygen readings. The
fluctuations in the readings displayed are not related to a change in the oxygen
percentage but to the change in partial pressure.
Calibrate at the temperature and pressure (altitude) at which the ana-
lyzer will be operated.