Analytical Industries AII-3000 M Series Portable Oxygen Analyzer - Manual (Page 11)

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 heated breathing circuit and the tem-
perature 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 moving an analyzer calibrated at sea level into the moun-
tains causes the analyzer to display an decrease in the oxygen reading dis-
played. When if fact, the decrease in the reading displayed is not related to a
change in the oxygen percentage but to the decrease in partial pressure
(corresponding to the increase in total pressure) at altitude.
Calibrate at the temperature and pressure (altitude) at which the ana-
lyzer will be operated.
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Effect of Humidity
The analyzer is not affected by non-condensing relative humidity (RH). How-
ever, the use of a humidifier to introduce water vapor and increase the mois-
ture level of the gas mixture does affect the oxygen concentration and the
resultant reading displayed by the analyzer. The addition of water vapor in-
creases the total pressure thereby diluting or decreasing the oxygen concentra-
tion of the gas mixture resulting in a lower oxygen reading.
Effect of Condensation
Excessive condensation collecting on the sensing area or the electrical connec-
tions at the rear of the sensors can adversely impact the performance of elec-
trochemical sensors. Condensation blocks the diffusion path of oxygen into the
sensor and can reduce the oxygen reading to 00.0 if the condensation covers
the entire sensing area. Condensation on the electrical connections at the rear
of the sensor can affect oxygen readings. Remedy either situation by shaking
out the condensation and allowing the sensor to air dry.
Erroneously characterized in many instances as a sensor failure, excessive con-
densation is remedied by gently wiping away the condensation with a soft cloth
or simply allowing the sensor to air dry.
Effect of Electromagnetic Radiation
Tested over a 26 MHz to 1000 MHz electromagnetic field, the analyzer is sus-
ceptible at all frequencies tested except those between 930 and 990 MHz.
Never operate the analyzer near equipment capable of emitting high
levels of electromagnetic radiation. Do not continue to operate the ana-
lyzer if the reading becomes unstable.
5.3 Calibration
Calibrating the analyzer or monitor during normal operation involves the same
precautions and procedures as those described in Sections 4.7 Start-up Calibra-
tion with the same cautions to review Sections 3 Safety Warnings and 5.2 Ap-
plication Considerations.
5.4 Sampling
Assuming the START-UP instructions are followed and the tests are completed
successfully the devices default to the SAMPLING mode.
Never operate the analyzer if the reading is unstable or if a malfunction
is suspected. If calibration is required as indicated herein, do not pro-
ceed until the analyzer is calibration successfully.
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