Compact Airway modules
Document No. 800 1009-1
17
2.2.2
TPX measuring unit
The TPX unit is a non dispersive infrared analyzer, measuring absorption of the gas sample at seven
infrared wavelengths, which are selected using optical narrow band filters. The IR lamp is a 4 W
filament, surrounded by thermal isolation. There is a hole in the isolation, passing the radiation to a
conical measuring chamber with 4 mm length.
From the sample chamber, radiation goes into seven tubular light guides with reflective inner
surface. At the other end of each light guide, there is a thermopile detector with an optical filter in
front of it.
The Temp sensor measures TPX units’ temperature and it is used for temparature compensation.
The TPX unit includes TPX board located at the end of the unit. Its function is to connect the 7
thermopile signals and the temperature sensor signal to the CPU board.
Figure 9
TPX measuring unit
2.2.3
OM measuring unit
The oxygen measurement is based on paramagnetic susceptibility. The gas and the reference gas,
which usually is room air, are conducted into a gap in an electromagnet with a strong magnetic
field switched on and off at a frequency of approximately 165 Hz.
An alternating differential pressure is generated between the sample and reference inputs due to
forces acting to the oxygen molecules in a magnetic field gradient.
The pressure is measured with a sensitive differential transducer, rectified with a synchronous
detector and amplified to produce a DC voltage proportional to the oxygen partial pressure
difference of the two gases.
