GE B20 User Manual page 70

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NOTE: The message 'Zero error' displays in case the zeroing fails.
NOTE: The message 'Calibration failed' displays, if you do not start
feeding gas within one minute after the automatic zeroing is
complete, or if the calibration fails due to a large gain adjustment.
NOTE: If zeroing or calibration failed, select Recalibrate to restart the
calibration procedure.
Checking temperature, NIBP and invasive blood pressure
calibration
Checking temperature, NIBP and invasive blood pressures
calibration should be performed at least once a year by qualified
service personnel as a part of the Planned Maintenance. for more
information, see the "Technical Reference Manual."