Analytical Industries AII-2000 M Oxygen Analyzer - Manual (Page 12)

5.2 Application Considerations
Effect of Anesthetic Agents
The AII 2000 Series Oxygen Analyzers and Monitors utilize an electrochemical
galvanic fuel cell type sensor, model AII-11-60, that has been characterized by
its gas permeable sensing membrane that allows the gas to be analyzed to
diffuse into the sensor where oxygen can be reacted. The displayed oxygen
concentration of all sensors of this design decreases in the presence of anes-
thesia gases. EN 12598:1999/ISO 7767:1997 (E) established standards for the
maximum error allowable over a given duration. The anesthetic agents listed
(Halothane, Enflurane, Isoflurane, Sevoflurane and Desflurance) were vapor-
ized into a gas stream of 30% oxygen / 70% nitrous oxide.
Gas Test Level Decrease in O
2
Reading
Helium 50%, Balance O
2
0%
Nitrous Oxide 80%, Balance O
2
0%
Carbon Dioxide 10%, Balance O
2
0%
Halothane 4% <-1.5%
Enflurane 5% <-1.5%
Isoflurane 5% <-1.5%
Sevoflurane 5% <-1.5%
Desflurane 15% <-1.5%
The errors listed were observed after a two (2) hour exposure period. The
table above summarizes the performance of the AII 2000 Series electronics
and AII-11-60 Oxygen Sensor. The above performances all meet or exceed the
requirements established by EN 12598:1999/ISO 7767:1997 (E).
Do not operate any device in the presence of flammable anesthetic
agents such as Diethal Ether or Cyclpropane.
Note: The AII-11-60 Oxygen Sensor has been specifically designed and tested
to be compatible with nitrous oxide. For optimum results, mount the oxygen
sensor with the sensing area facing down toward the floor and be flushed or
calibrated with 100% oxygen every eight (8) hours.
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.
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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 connected to a ventilator circuit, the six (6) second T90
response time of the AII-11-60 Oxygen Sensor causes the analyzer to display
an increase in the oxygen reading displayed when in fact the alternating
breathing pressure cycles generated by the ventilator is increasing the total
pressure.
The increase in the reading displayed is not related to a change in the oxygen
percentage but to the increase in partial pressure (corresponding to the in-
crease in total pressure).
Calibrate at the temperature and pressure (altitude) at which the ana-
lyzer will be operated.
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.
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