Datex-Ohmeda Aespire 7900, User'S Reference Manual

Page: 111 / 118

6 Specifications and Theory of Operation
1009-0633-000
6-25
Oxygen
O
2
monitor theory of
operation
The O
2
monitor measures and displays O
2
concentration in the patient circuit.
The O
2
sensor assembly contains an oxygen sensor that produces a voltage
proportional to the oxygen partial pressure (concentration) at its detecting
surface.
The O
2
sensor is an electrochemical device (galvanic cell). Oxygen diffuses
through a membrane into the device and oxidizes a base metal electrode. This
oxidation produces an electrical current proportional to the partial pressure of
the oxygen at the electrode’s sensing surface. The base metal electrode
gradually wears out from the oxidation process.
The voltage from the sensor cartridge is affected by the temperature of the
monitored gas mixture. A thermistor in the sensor’s housing automatically
compensates for temperature changes in the sensor.
The O
2
monitor uses signal processing and analyzing circuitry to convert the
sensor signal into a corresponding % oxygen value. The system displays this
value and compares it to saved alarm limits. If the value falls outside the
limits, the monitor produces the appropriate alarms.
Display range: 5 to 110% O
2
Display resolution: 1% increments
Sensor type: Galvanic fuel cell
Measurement range: 0 to 100% O
2
Measurement accuracy: ± 3% of full scale
Cell response time: 35 seconds, 10 - 90%
Response time of cell and adapter as measured using the
test method described in ISO 7767 (1988-12-15), clause
50.9
Low O
2
alarm range: 18% to 100%
High O
2
alarm setting: 21% to 100%
Note: Low O
2
limit may not be set above the high O
2
limit,
nor may the high O
2
limit be set below the low O
2
limit.
Expected cell life: Four months of shelf life (23 ° C room air) and one year of
normal operation