Principles of operation
166
Instructions for use Babylog 8000 plus SW 5.n
Flow sensor
Two different sensor types are available:
– The Y-piece flow sensor is integrated in the Y-
piece.
– The ISO 15 flow sensor is built in between the
Y-piece and the tube connector.
Both sensor types use the same sensor insert.
Despite this, the sensor properties are not
identical. The sensor type is set in the
Calibration/Configuration
>
Sensor
dialog
window in order to optimize the measurement for
this type of sensor.
Reference conditions
Hot wire manometers primarily measure gas
quantities, not volumes or flows. The volume that a
certain gas quantity takes up depends on the
ambient conditions as expressed in the equation of
state for ideal gases:
– Atmospheric pressure
– Temperature
– Humidity
Babylog 8000 indicates flow and volume
measured values for one of two reference
conditions:
The desired reference condition is set in the dialog
window
Calibration/Configuration
>
Sensor
.
Leakage rate
In an unblocked tube, breathing gas often flows
into the environment from between the trachea
wall and the tube. The flow sensor on the device is
located in the Y-piece, i.e. upstream from the
location of the leakage. During inspiration,
breathing gas is lost after measurement; during
expiration it is lost before measurement. The tidal
volume measured on the inspiratory side is
therefore greater and that on the expiratory side
less than the actual tidal volume. Averaged over
time the difference between the inspiratory flow
and expiratory flow is equal to the leakage flow
because the gas quantity that does not flow back
through the sensor during exhalation must have
escaped through the leak. The device determines
the mean leakage flow from the difference
between inspiratory minute volume MVi and the
expiratory minute volume MVe (displayed as MV).
Standardized as MVi, the result is the leakage rate
displayed in percent:
Leakage rate = 100 % x (MVi - MVe) / MVi
Flow trigger
The device detects spontaneous breathing via flow
measurement. During inspiratory effort the flow
signal, which was previously negative (=
expiratory) or zero, increases. To reliably detect
inspiration and not trigger a mechanical breath in
case of interference signals, the patient must first
inhale a certain volume Vtrig. This volume is set on
a scale of 1 to 10 in the form of the trigger
sensitivity in the
Options
dialog window. On this
scale 1 represents high sensitivity and
10 represents low sensitivity.
NTPD
Ambient temperature 20 °C (68 °F), air
pressure 1013 mbar (or hPa or
cmH
2
O), dry gas
BTPS
Body temperature 37 °C (98.6 °F), air
pressure 1013 mbar (or hPa or cmH
2
O)
plus mean airway pressure 10 mbar (or
hPa or cmH
2
O), water-vapor-saturated
gas