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4.2.5 Mask mixing calculation subsystem
The mask mixing subsystem shown in Figure 4.9 is created from Eq. (2.46). As
mentioned before, the provided breath-load takes flow in inhalation phase as negative
when the air is taken away from the mask by breathing. Oppositely, the exhalation
phase is considered positive since exhalation adds air into the mask. Inputs are listed in
Table 4.9 below.
Table 4.9 Inputs to the mask mixing calculation subsystem
Input port number Input Unit
1 The patient’s breath load g/s
2 Mask capacity m 3
3 Exhaled air level in airflow from lump 30 In decimal
Figure 4.9 - Mask Mixing Calculation Subsystem
The upper part of the subsystem gives the mixing algorithm for exhalation phase. For
calculating the mixing procedure during the exhalation phase, the integration should
only take in the exhalation part and the integration starts at each cycle’s beginning of
exhalation phase. These are controlled by switch block and integration block. The
switch block restrains the integration within exhalation phase and the integration block
with a falling external-reset-setting resets the integration at the beginning of each
exhalation.
The lower part of the subsystem gives the mixing algorithm for inhalation phase. The
level of exhaled air re-entering the mask from lump 30 is added in from input port 3. So
the integration from the lower part includes the influence of the reverse flow’s re-
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