CareFusion MicroCO Meter Manuel D'utilisation page 53

This will appear momentarily while correction for ambient levels is
executed. During this time the unit must not be exposed to raised
concentrations of CO. The buzzer will then sound and the display will
change to:
Upon hearing the buzzer instructs the subject to inspire fully and hold
their breath for 20 seconds. The display will count down from 20 to 0
as an aid to timing the breath holding period. The green indicator will
then illuminate and the display will change to:
B L O W
At this time the subject should seal their lips around the mouthpiece
and exhale slowly and fully. The 20 second breath holding period is
recommended to allow time for equilibration of alveolar gas.
If, however, the subject is unable to hold their breath for this period,
the unit may still be used before 20 seconds have elapsed.
The unit must not be used, however, for one second after the unit is
first switched on i.e. before the countdown commences. Expired
alveolar gas is then entrapped between sensor and mouthpiece valve.
The display reading will rise to a plateau over the course of a several
seconds. The final value will be held until the unit is turned off and
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