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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 conduc-
tivity 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
100% O
.
2
To calibrate the flow sensor:
Remove the sensor.
To do so, remove the expiration cone and pull
out the flow sensor complete with connecting cable
(breathing system of the Cicero EM).
Shake the sensor briefly in the ambient air to purge it
of any residual breathing system gases. Tightly seal
the sensor openings (on at least one side).
In »Standby« mode, call up the configuration menu.
Then, using the rotary control, select and confirm the
»Flow Sensor« option in the screen menu under
»Calibrate«.
Measurement of CO
/N
O, air and
Measuring principle
2
2
CO
and anaesthetic agent absorb infrared light. A pump
2
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
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.
Cross-sensitivity of the anaesthetic gas measurement:
Measurement of the anaesthetic agent can be falsified by
vapours of organic substances (such as those contained
in cleaning agents or disinfectants) in the air round about,
the sampling hose or the T-piece. Elevated values for
anaesthetic agent will be displayed, particularly when
using halothane, if the patient's breathing air contains
alcohol.
The Cicero EM is equipped with an automatic anaesthetic
agent recognition system. Mixtures will be identified and
the main component recognised and quantitatively
evaluated. Accuracy may be restricted in this case.
Flow measurement
Measurement of CO
and anaesthetic agent
2
and anaesthetic agent
2
Description
177
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