CareFusion MicroCO Meter Manuel D'utilisation page 58

CAUTION: opening the control knob more than indicated will
give erroneous reading and could damage the cell.
The unit will then beep 3 times, store the new calibration value, and
display the following:
The gas supply should then be turned off.
If the signal from the CO sensor is too low a new calibration value will
not be stored and the display will show:
The most likely cause for this is an expired cell but may also be caused
by depressing the calibration switch with no gas applied. Ensure that
the concentration of calibration gas is correct (20ppm), that the
connections to the gas cylinder are secure, and that the gas cylinder is
not empty, and then repeat the calibration procedure.
If the above message is repeated, the CO meter must be returned to
Micro Direct Inc, for sensor replacement. Sensor life ranges from 2 to
5 years and depends upon both the amount of exposure to CO and
other gases, particularly solvents such as alcohol and cleaning fluids.
56