Servo Controlled Oxygen Option - GE Giraffe Service Manual

1.8 Servo Controlled Oxygen Option

The Giraffe Servo Control Oxygen System consists of an oxygen sensing circuit, Servo Oxygen circuit board, and
an oxygen delivery system.
The sensing circuit is located beneath the bed and consists of a pair of fuel cell oxygen sensors, a three-way
solenoid calibration valve, and a calibration fan. In normal operation the calibration valve is closed and allows
the Giraffe fan to circulate gas from the infant compartment across the sensors.
The unit must be calibrated at least every 24 hours when servo oxygen is in use. After 24 hours have elapsed
the system prompts the user to perform calibration. Once the operator initiates calibration, the calibration
valve opens and the calibration fan is turned on. This draws ambient air across the sensors until a stable
reading is obtained. This 21% oxygen reference value is then used to calibrate the measuring algorithm. After
calibration 100% oxygen is briefly delivered to the system to ensure there are no occlusions. When calibration
is complete the unit will resume controlling oxygen based on the last set point.
The system must have two sensors present to operate. One sensor is always used for control and the other is
used for a redundant check and display. The sensors generate a voltage of about 40 millivolts at 21% oxygen
concentration and about 200 millivolts at 100% oxygen concentration. The voltage is directly proportional
to the concentration of oxygen. Humidity and temperature sensors located in the sensor plug are used for
voltage compensation. A fan mounted to the sensor-housing door is activated when the temperature reaches
50 degrees C. This fan circulates air to keep the sensors below the maximum allowable operating temperature,
about 55 degrees C.
The Servo O2 board is located in the Giraffe controller. The microcontroller and integrated EPROM on the board
perform the following:
• Convert sensor output from analog to digital
• Activates oxygen alarm conditions.
• Two-way communications via 485 bus with the Giraffe control board.
• Controls the calibration valve to select calibration mode.
• Controls the two supply valves to maintain the desired oxygen set point.
• Opens the safety relay, which removes power to the two-way valve and the supply valves in case of a
system failure.
The oxygen delivery system consists of two solenoid supply valves, and a regulator assembly. The preset
regulator regulates the oxygen supply to 50 psi (345 kPa). Two supply valves, controlled by the Servo Oxygen
board, control flow to the infant compartment. Both valves are opened until the measured level gets close to
the desired set point then one valve is closed. One valve is then cycled on and off as needed to maintain the
desired oxygen levels in the infant compartment. The valve selected is alternated so both valves cycle about
the same number of times. There are 2 fuses between the Servo O2 board and the supply valves that prevent
high current from the board entering the valve housing should a short occur in the supply valves.
© 2001 by Datex-Ohmeda, Inc.. All rights reserved.
Chapter 1: Functional Description
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