Instrumentation & Measurement Magazine 23-4 - 84

Table 4 - Comparison of the results obtained with the HYT 271 and with BS sensors.
trise(s)

τrise(s)

1/erise(s)

tfall(s)

τfall(s)

1/efall(s)

Brewer Sensor

0.446 ± 0.019

0.224 ± 0.013

0.155 ± 0.013

0.489 ± 0.013

0.222 ± 0.007

0.168 ± 0.007

HYT 271

1.056 ± 0.098

0.530 ± 0.007

0.260 ± 0.008

1.736 ± 0.343

0.650 ± 0.088

0.371 ± 0.008

in which the average 1/erise (32 ms) appears suitable to record of the variation of trise, τrise e 1/erise and of the tfall, τfall e 1/efall dereal time variation during mechanical ventilation.
pending on the sensor position and on the flux, they are all
Humidity sensor dynamic characterization was performed fol- plotted in Fig. 4.
lowing the same protocol described in [12]. Briefly, using a
The values obtained highlight that the fastest response
customized pneumatic system combined with a bubbling hu- times at 36.8% (155 ms for rising H and 168 ms for falling
midifier, increasing or decreasing steps of RH between two H) could be obtained by placing the sensor perpendicular in
values of RH were created. Sensors were thus exposed to these front of the air flux. Results from this condition are summaRH steps to evaluate their response times. The system, schema- rized in Table 4 and illustrated in Fig. 5, where the responses of
tized in Fig. 2, is composed
of: a compressor (a), which
creates a pressure-controlled airflow; a bubbler
(b), to increase the humidity;
two regulators (VXD232)
(c) and (d), that can be alternately switched on or off
(opening time around 10
ms) with a customized LabVIEW program, to select the
desired path for air flow;
reference sensor (e) and
Brewer sensor (f) positioned
in a tube section at the end
of the system with an anemometer TESTO-405-V1 (g)
for measuring air flow velocity. If the regulator (d) is
opened and the regulator
(c) is closed then there is dry
air, vice versa the air is humidified. The step obtained
by inverting the position of
the valves is of around 40%
RH. Different flow values
from 15 to 45 l/min usually
used in MV were analyzed.
Furthermore, three orientations of the sensor with
respect to the flow direction
were chosen to assess sensor
response times under the
different variable combinations. For each trial, 10 steps
were selected, respectively,
for the evaluation of both
rise (trise, τrise e 1/erise) and fall
(tfall, τfall e 1/efall) times.
In order to provide an
explanatory representation
Fig. 5. Comparison between the step response of HYT271 and of BS sensor.
84	

IEEE Instrumentation & Measurement Magazine	

June 2020



Instrumentation & Measurement Magazine 23-4

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