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O
saturation
2
at 660 nm red
at 920 nm infrared
The diagram above shows an example of the light
absorbed by the blood at 660 nm (red) and 920 nm
(infrared).
At 660 nm, the absorption and corresponding pulse
amplitude decrease with increasing O
rise at 920 nm. Since the absorption coefficients of
HbO
and Hb are known for both wavelengths, the
2
system can calculate how much of these two haemo-
globins is present. The quotient obtained by dividing the
oxygenated haemoglobin (HbO
oxygenated haemoglobin (Hb + HbO
functional saturation:
% SpO
(func) = 100 x
2
and refers to the haemoglobin capable of transporting
oxygen.
Dyshaemoglobins, HbCO and MetHb are normally
negligible, but may affect the accuracy of the measure-
ment.
Temperature measurement
Measuring principle
Temperature-dependent change in resistance of an
NTC resistor (NTC = negative temperature coefficient)
with linearization circuit.
100% O
saturation
2
Time (s)
Time (s)
saturation, but
2
) by the reduced and
2
) is known as the
2
HbO
2
HbO
+ Hb
2
Functional oxygen saturation (SpO
Pressure measurement
Measuring principle
Piezoresistive change of resistance in a membrane.
Determination of PEEP and plateau pressure
PEEP (positive end-expiratory pressure) is the airway
pressure at the end of expiration.
The plateau pressure is the airway pressure measured
16 milliseconds before expiration begins.
O
measurement
2
Measuring principle of the galvanic cell
The O
sensor is based on the principle of a galvanic
2
cell.
Oxygen molecules from the gas mixture to be measured
diffuse through a plastic membrane into the electro-
chemical cell and are reduced on precious metal
electrodes.
Plastic membrane
Au cathode 1
O
2
Au cathode 2
ϑ
-
Thermistors
A base electrode is oxidized at the same time. It is
depleted by the oxidation process and essentially
determines the service life of the sensor. The current
flowing through the cell is proportional to the partial
oxygen pressure in the gas mixture to be measured.
At constant pressure and constant temperature of the
gas mixture to be measured, the measured value is
directly proportional to the partial oxygen pressure.
Description
)
2
Plastic housing
Basic
electrolyte
Pb anode
175
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