Figure 6.3 Flow Diagram
6-14
restriction that provides flow rate information
back to the flow control circuit. This orifice is the
pneumatic equivalent of a current sense resistor.
The differential pressure across the orifice is pro
-
portional to the flow rate, or simply, an increase in
the flow rate increases the differential pressure as
measured across the calibrated restriction.
Three hoses from the sensing orifice connect back
to the flow circuitry. Two of the hoses provide
differential pressure information, while the third
hose provides atmospheric pressure information.
The atmospheric pressure information is used to
compensate for usage at various altitudes. Lastly,
the argon gas is filtered prior to delivery to the
ABC™ handpiece accessory.
6.6.2 Mass Flow Rate Regulation
Referring to the HV/Flow Control Schematic
(Figure C-4b), V
GAS is an analog voltage for the
mass flow rate and has a range of .5V to 6.2V. To
enable the circuit, AR_EN must be
SET causing
U13-7 to go low.
Signal control needs to be defined at this time.
As mentioned, the System 7500™ has two (2)
microcontrollers on the display assembly. Each
of the microcontrollers have control and monitor
-
ing responsibilities for argon flow. The Control
Microcontroller sets the flow rate with Vgas (.5V
- 6.2V) and enables the circuit with a
SET on
AR_EN. The Control Microcontroller can also
monitor a predicted flow at F
MEA. The Monitor
Microcontroller opens the solenoid valve with
a SET on GASEN and verifies the flow rate by
monitoring the current through the flow control
valve.
The absolute pressure transducer (X2) measures
the atmospheric pressure, plus the back pressure
within the argon flow tubes when argon gas is
flowing. This signal is referred to as PABS (TP4)
and is calibrated for a specific voltage depending
on the altitude where calibration occurs. Once
calibrated, then PABS will automatically adjust
to any change in altitude where the system may
be placed into service. At sea level, PABS will be
approximately 5.22V and at an altitude of one
mile, PABS will measure about 4.13V.
The differential pressure transducer (X1) has two
hoses, or two input ports for argon hose connec
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