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By adjusting the bias vent area, the model was set to provide very low flow rate under
the pressure setting same as in the experiment. An average nose volume of 32 cm 3 [65,
66] is subtracted from the mask capacity since the nose volume is significant comparing
with the capacity of a nasal mask. The breath load is adjusted to 500 ml for average
“restful” breath. The CO2 concentration in exhaled air is given as 5%. Based on these
conditions, the model gives 0.252% of CO2 in inhaled air. Comparing to the average
experimental result 0.17%, the model prediction is reasonably close considering the
small percentage, the sensitivity of the mask volume and the limited number of objects
which does not represent a statistical average.
5.3 Experimental validation of the thermal dynamic part
This section focuses on the thermodynamic validation. There are two subsections; one is
under steady flow and the other under patient breath-added fluctuating flow.
5.3.1 Thermal dynamic model under steady flow
This experimental investigation was conducted to quantify the steady flow situation and
compare the results with the outputs from the thermal dynamic model. The results
include evaporation rate, the chamber water temperature, in-chamber airflow
temperature and airflow temperature at the end of the HADT near the elbow as well as
observation of condensation in HADT. The experiment is conducted under combination
of different ADU pressure settings, heater plate settings, tube heating settings and
ambient conditions. The experiment was carried out under high ambient temperature,
normal room temperature and low temperature to validate the model’s applicability on a
wide environmental variety. The high ambient temperature and low ambient
temperature conditions were achieved by a big environmental control room (Figure
5.12). The high temperature is at about 33°C and low is at about 14°C.
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