Dräger Cato Instructions For Use Manual page 130

Descriptions
Principles of measurement
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.
Mixtures of different anaesthetic vapours may
considerably impair the accuracy with which the
concentration is measured!
Disturbance variables in sampled gas measurement
When analysing the measured values, the temperature,
humidity and pressure conditions during measurement
must be taken into account.
Whereas calibration takes place with dry gas under
NTPD conditions (Normal Temperature 20 °C,
Pressure 1013 hPa, Dry - relative humidity 0 %), the
measurements during patient monitoring are taken by
sampling the gas under BTPS conditions (Body
Temperature 37 °C, ambient Pressure, Saturated -
relative humidity 100 %).
The gas sampling and measurement process creates a
negative pressure of about 100 to 200 mbar compared to
ambient pressure at the concentration measurement site
(depending on sample flow, condensation and water
separator). The partial pressure measured at the sensor
is corrected to the current ambient pressure with the aid
of the pressure measured in the measuring cuvette.
Effect of temperature:
The gas temperature at the sensor is measured and its
effect on concentration measurement is compensated.
Effect of humidity:
The gas sampled during expiration has a temperature of
37 °C and a relative humidity of about 100 %.
130
It contains about 47 mmHg water vapour. Up to the
water trap, the gas cools down to approximately ambient
temperature. The water vapour content is reduced to e.g.
17 mmHg at approx. 20 °C. The difference condenses in
the sampling hose and is separated in the water trap.
Consequently, the volume at sea level is reduced by
30 : 760 = 4 %, thereby increasing the measured relative
gas concentration by 4%. This error is not corrected in
the PM 8050 because it is small compared to the
specified accuracy of the sensors.
Example:
Gas Concentration at the
Y-piece
O 30%
2
N O 57%
2
Isoflurane 2%
CO 5%
2
Water 6%
vapour
Displayed value
31%
59%
2%
5%
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