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Omega FMA 4000 User Manual: Principle Of Operation

Digital mass flow meters.
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3.

PRINCIPLE OF OPERATION

The stream of gas entering the Mass Flow transducer is split by shunting a small
portion of the flow through a capillary stainless steel sensor tube. The remainder of
the gas flows through the primary flow conduit. The geometry of the primary con-
duit and the sensor tube are designed to ensure laminar flow in each branch.
According to principles of fluid dynamics the flow rates of a gas in the two laminar
flow conduits are proportional to one another. Therefore, the flow rates measured
in the sensor tube are directly proportional to the total flow through the transducer.
In order to sense the flow in the sensor tube, heat flux is introduced at two sec-
tions of the sensor tube by means of precision wound heater-sensor coils. Heat is
transferred through the thin wall of the sensor tube to the gas flowing inside. As
gas flow takes place heat is carried by the gas stream from the upstream coil to
the downstream coil windings. The resultant temperature dependent resistance
differential is detected by the electronic control circuit. The measured temperature
gradient at the sensor windings is linearly proportional to the instantaneous rate
of flow taking place.
An output signal is generated that is a function of the amount of heat carried by
the gases to indicate mass-molecular based flow rates.
Additionally, the FMA 4000 Mass Flow Meter incorporates a Precision Analog
Microcontroller (ARM7TDMI7 MCU) and non-volatile memory that stores all hard-
ware specific variables and up to 10 different calibration tables. The flow rate can
be displayed in 23 different volumetric or mass flow engineering units. Flow meter
parameters and functions can be programmed remotely via the RS485/RS232
(optional) interface. FMA 4000 flow meters support various functions including:
programmable flow totalizer, low, high or range flow alarm, automatic zero adjust-
ment (activated via local button or communication interface), 2 programmable
SPDT relays output, 0-5 Vdc / 4-20 mA analog outputs (jumper selectable), self
diagnostic alarm, 36 internal and user defined K-factor. Optional local 2x16 LCD
readout with adjustable back light provides flow rate and total volume reading in
currently selected engineering units and diagnostic events indication.
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