DeVilbiss DeVO/MC44-90 - Theory of Operation

SECTION 2
THEORY OF OPERATION
The microcomputer controlled DeVO/MC44 is
similar to the DeVO/44 and MC29 in many ways. It
uses a pressure-depressurization system which
produces high concentrations of oxygen in a very
efficient manner. The four-way and purge valves
are also pneumatically operated similar to the
DeVO/44 and MC29.
Observation of the pressure-depressurization
cycle is one way to isolate problems. When used
in conjunction with the indicator lights located on
the MC board, it can be of significant help when
trouble-shooting the unit. Cycle operation can be
observed by installing a set of gauges to the test
points located on the manifold.
Variations in the pressure-depressurization cycle
can be observed in relation to the activating and
de-activating of the four-way and purge valves. A
complete cycle takes approximately 22 seconds.
This is an approximate time because all cycles are
dependent on the liter flow and altitude. The
cycle and the alarm system are controlled and
monitored by the MC board (microcomputer
control board). This is accomplished by the use of
a pressure transducer on the MC board. The trans-
ducer is connected to the accumulator tank by a
1/16” (1.6 mm) diameter hose. It constantly senses
oxygen pressure in the accumulator.
A cycle change takes place only when the
pressure in the accumulator tank is approximately
25.5 PSI (176 KPa). At this point a signal is sent to
the four-way and purge valves. This signal is
received by pilot valves located on the four-way
and purge. These pilot valves allow compressed
air from the pilot accumulator to enter the four-
way and purge so they can be activated.
The four-way valve will remain in the activated
position until the right sieve bed builds up 25.5
PSI (175.8 KPa) of oxygen in the accumulator
tank. This takes approximately 11 seconds. When
this pressure is reached the four-way is de-
activated causing the left bed to pressurize. Note
that when activated, the four-way allows
pressurization of the right sieve bed. When de-
activated, the left bed is being pressurized.
Although the purge valve is activated at the same
time as the four-way, it only remains in this
activated position for approximately 1.0 seconds.
It is during this time that high purity oxygen from
the pressurized bed is allowed to flow into the
bed that is just beginning its pressurization cycle.
During pressurization a small amount of oxygen
from the pressurized bed is constantly flowing
into the depressurized bed through the orifice
located in the manifold. This oxygen flow helps
purge the nitrogen from the depressurized bed
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which is being exhausted through the bottom of
the bed. The air being exhausted passes through
the four-way and out of the exhaust port of the
purge valve. When the four-way shifts, com-
pressed air is directed into the other bed from the
bottom. At the same time, the purge valve opens
and allows high purity oxygen to enter the same
bed from the top. This additional pressure allows
the bed to start its cycle at higher pressures and
concentrations. When the purge valve closes, the
air in the previously pressurized bed is exhausted.
This exhaust can be heard each time the purge
closes.
At the beginning of the pressurization cycle the
needie on the gauge will suddenly increase to
approximately 15 PSI (103 KPa). This is caused by
the purge opening. When the purge closes, the
needle will hesitate slightly and then continue to
rise until maximum pressure is reached. When
maximum accumulator pressure is reached, the
four-way shifts and the gauge will show a sudden
drop in pressure to approximately 15 PSI (103
KPa). This drop is caused by the purge opening
and releasing some of the oxygen to the other
bed. When the purge closes, the needle will
hesitate slightly and then continue to drop
steadily and level off at approximately 2 PSI (13.8
KPa). This low pressure reading is caused by the
continuous flow of oxygen from the pressurized
to depressurized bed through the manifold
orifice. When maximum accumulator pressure is
reached, the four-way and purge valves shift and
anew cycle begins.