Model AFC mass flow controllers are designed to indicate flow rates and control set flow rates of gases.
Each
of these units incorporates an advanced straight tube sensor in
conjunction with flow passage elements constructed of stainless steel.
LED readouts of command modules are supplied
with 0 to 100 percent calibrations. Zero and span adjustments are
conviently accessible from outside of the transmitters.
Principles of Operation
Metered
gases are divided into two laminar flow paths, one through the primary
flow conduit, and the other through a capillary sensor tube. Both flow
conduits are designed to ensure laminar flows and therefore the ratio of
their flow rates is constant.
Two precision temperature sensing
windings on the sensor tube are heated, and when flow takes place, gas
carries heat from the upstream to the downstream windings. The resultant
temperature differential is proportional to the change in resistance of
the sensor windings.
A Wheatstone bridge design is used to
monitor the temperature dependent resistance gradient on the sensor
windings which is linearly proportional to the instantaneous rate of
flow.
Output signals of 0 to 5Vdc or 4 to 20mA are generated indicating mass molecular based flow rates of the metered gas.
In
AFC mass flow controllers the combined gas streams flow through a
proportionating electromagnetic valve with an appropriately selected
orifice. The closed loop control circuit continuously monitors the mass
flow output and maintains it at the set flow rate.
Flow rates are unaffected by temperature and pressure variations within stated limitations.
AFC
mass flow controllers include an electromagnetic control valve that
allows the flow to be set to any desired flow rate within the range of
the particular model. The valve is normally closed as a safety feature
to ensure that gas flow is shut off in case of a power outage.
AFC
mass flow controllers are designed to meter and control flow rates of
gases, and are available with flow ranges from 10 sccm to 100 slpm [N2].
Gases are connected by means of 1/4”, 3/8”, or optional 1/8”compression
fittings. These controllers may be used as benchtop units or mounted by
means of screws in the base.
Transducer power supply ports are fuse and polarity protected.
Leak Integrity
1 x 10-9 smL/sec of helium max to outside environment.
Mass Flow Systems
Complete
Mass Flow Systems include Command Modules, transducers and cables.
Command modules contain appropriate power supplies, digital panel meters
with 3-1/2“ digit LED readouts and high precision
potentiometers.External RS-232 or RS-485 are optional.
Switches
in the front panels of Command Modules select LOCAL or REMOTE reference
signals, analog outputs are accessible through convenient 9 pin
D-connectors.
