Datex-Ohmeda Anesthesia Delivery Unit S/5, Technical Reference Manual

Page: 234 / 494

Datex-Ohmeda ADU
B-8
Document No. 8502854
Vaporizer section
Theory of the vaporizer
The electronically controlled vaporizer is integrated with the gas flow controls in the A-FGC1 module.
When the vaporizer is used, the gas flow from the flow controls is split into a bypass flow and a liquid
chamber flow i.e. Cassette flow. The agent vaporizes freely in the liquid chamber without any active
heating or pressurization that means that the vaporizer can be categorized as a combination of
conventional variable bypass and measured flow vaporizers.
The operator sets the anaesthetic concentration with the Agent wheel and the cassette flow con-
troller controls the concentration in the fresh gas outlet.
The essential principle used in the A-FGC1 vaporizer is that the saturated vapor pressure of the
anaesthetic agent is dependent on the temperature of the liquid agent. By continuously measuring
the temperature and the cassette pressure inside the liquid chamber of the Aladin cassette, the
anaesthetic agent concentration in the liquid chamber is always known.
The agent concentration in the fresh gas outlet of the A-FGC1 module is a function of the agent
concentration in the cassette and the ratio of the cassette outflow and the bypass flow measurement
unit which are measured continuously. The target parameter for the agent controller is the set flow
ratio (setr) which is calculated from the required agent concentration in the fresh gas outlet (setaa)
and the current agent concentration inside the cassette (pvapor/pcassette).
The set flow ratio is also compensated with the ambient pressure which means that the vaporizer
output concentration increases as the ambient pressure decreases. The effect is similar to traditional
vaporizers, thus a compensation factor directly proportional to the ratio of sea level pressure
(psealevel) and current atmospheric pressure (pambient) is used in the calculations. This yields that
the set flow ratio is calculated in the following way:
where
The controlled parameter is the measured flow ratio (measr), which is the ratio of the measured liquid
channel (F
liqch
) and bypass flow (F
bypass
)
:
The concentration in the fresh gas outlet is controlled by the position of the agent proportional valve,
which is set continuously according to the information from the agent controller.
setr
compaa
100
P
vapor
P
cassette
-------------------
è ø
æ ö
compaa –
⋅
----------------------------------------------------------------- =
compaa
P
sealevel
P
ambient
------------------- setaa
⋅
=
measr
F
liqch
F
bypass
----------------- =