Respiratory Flow Measurement - Dräger Medical Fabius Tiro Technical & Service Manual

Fabius Tiro
Figure 36 O2 Sensor
14 Respiratory Flow
Measurement
48
Dräger Medical AG & Co. KGaA
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The O2 to be measured diffuses through the Teflon membrane, reacts at the
gold cathodes (negative polarity) and forms lead oxide and water at the lead
anode (positive polarity). During this chemical process, a voltage is gener-
ated which is proportional to the O2 partial pressure.
The internal resistance of the cell is determined by the surface area of the
gold cathodes, the O2 diffusion velocity, the distance between the gold cath-
odes and the lead anode. This resistance is approximately 700 ohms.
The chemical process is temperature-sensitive. Therefore temperature-sensi-
tive resistors are connected in parallel with the O2 sensor. These resistors
and the internal resistance of the O2 sensor correct the measuring voltage.
Because two cathodes used in the O2 sensor cell, two different voltages are
generated. These voltages are compared with each other. If their difference
exceeds a certain value, the machine prompts the operator to check the cell.
If the O2 sensor fails, the control box will indicate an error on the graphics
display.
The flow sensor functions according to the constant temperature hot-wire
anemometer principle. Respiratory gas flows past a thin platinum wire. This
platinum wire (A) is located in a measuring tube and is electrically heated.
The platinum wire is held at a constant temperature. Gas flow removes heat
from the hot wire. The higher the gas flow rate, the greater the heat removal.
The amount of electrical current needed to maintain a constant platinum wire
temperature is thus proportional to the gas flow rate.
6020.002
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Function Description
Revision B Released