Aalborg DFC Digital - Manual (Page 9)

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
conduit 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 lami-
nar flow conduits are proportional to one another. Therefore, the flow rates meas-
ured in the sensor tube are directly proportional to the total flow through the trans-
ducer.
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 gradient at
the sensor windings is linearly proportional to the instantaneous rate of flow tak-
ing 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, DFC model Mass Flow Controllers incorporate a microprocessor and
non-volatile memory that stores all calibration factors and directly controls a pro-
portionating solenoid valve. The digital closed loop control system of the DFC con-
tinuously compares the mass flow output with the selected flow rate. Deviations
from the set point are corrected by compensating valve adjustments, thus main-
taining the desired flow parameters with a high degree of accuracy.
Free PC Software with Gas Blending and Programmable Flow functions.
4. SPECIFICATIONS
FLOW MEDIUM: Please note that DFC Mass Flow Controllers are designed to
work with clean gases only. Never try to meter or control flow rates of liquids
with any DFC.
CALIBRATIONS: Performed at standard conditions [14.7 psia (1.01 bars) and 70 F
F (21.1
F
C)] unless otherwise requested or stated.
ENVIRONMENTAL (PER IEC 664): Installation Level II; Pollution Degree II.
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