Principle Of Operation - Aalborg DFM 26 Operating Manual

Use of the DFM flow transducer in a manner other than that specified in this man-
ual or in writing from Aalborg7, may impair the protection provided by the equip-
ment.
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
transducer. In order to sense the flow in the sensor tube, heat flux is introduced
at two sections 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 flow-
ing 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 instanta-
neous 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 DFM model Mass Flow Meter incorporates a Digital Signal
Processor (DSP) and non-volatile memory that stores all hardware specific vari-
ables and up to 10 different calibration tables. Multi parameter flow meters
(Models: 27/37/47) provide accurate data on three different fluid parameters:
= flow
= pressure
= temperature
The flow rate can be displayed in volumetric flow or mass flow engineering units
for standard or actual (temperature, pressure) conditions. Flow meters can be
programmed locally via the four button keypad and LCD, or remotely, via the RS-
232/RS-485 interface. DFM flow meters support various functions including: flow
totalizer, flow, temperature, pressure alarms, automatic zero adjustment, 2 SPDT
relays output, 0-5 Vdc / 0-10 Vdc / 4-20 mA analog outputs for flow, pressure and
temperature.
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