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Rule of thumb for estimating the system response
over time:
T = VC / * FG
where:
T
= Time constant of the system in minutes
VC
= System volume in litres
(breathing system, ventilation hoses,
residual volume of the lungs)
* FG = Fresh gas flow in L/min
Example
for estimating the time constant of the system
(comprising breathing system and hoses):
VC = 6 [L]
* FG = 3 [L/min]
T
= 2 [min]
In this example, the change of concentration in the breath-
ing system would have reached roughly 60% of the change
in fresh gas concentration after approx. 2 minutes.
Principles of measurement
O2 measurement (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 electrochemi-
cal 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.
Flow measurement
Measuring principle and signal processing:
The sensor is based on the principle of a constant-
temperature hot-wire anemometer. The breathing gas
flows round a very thin, electrically heated platinum wire
in a measuring tube. The wire is heated to a constant
temperature of 180 °C which is controlled by a control
circuit. Heat is dissipated when gas flows past this wire.
The larger the volume of gas flowing past per unit time,
the more heat will be dissipated.
The heating current required to maintain a constant wire
temperature is taken as an indicator for the gas stream.
Gas type compensation:
The effect of the various types of gas contained in the
breathing gas is compensated by a second heated
platinum wire. The heat dissipated by the second wire
in the stationary gas column in the measuring tube is
determined during a period in which there is no gas flow
(i.e. during inspiration when the sensor is positioned on
the expiration side). The gas composition is determined
on the basis of the specifically different thermal
conductivity of the types of gas present in the breathing
gas.
Linearization is performed with the aid of internal
calibration tables for the gas mixtures O
Plastic housing
100% O
Basic
Measurement of CO
electrolyte
CO
pump entrains a small amount of breathing gas through a
measuring cuvette irradiated with infrared light. Different
filters make it possible to select a frequency band in
Pb anode
which only one of the gases to be identified is absorbed.
All gases can be measured quasi continuously by
changing filters rapidly.
The absorption reflects the gas concentration in the
cuvette. The gas concentrations in the breathing gas
can be calculated by simultaneously measuring the
temperature and absolute pressure in the cuvette.
Relationship between fresh gas flow and time constants
.
2
and anaesthetic agent
2
and anaesthetic agent absorb infrared light. A
2
Descriptions
Principles of measurement
/N
O, air and
2
2
129
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