Burkert Type 8712 - Manual (Page 8)

8
Descriptionofthesystem
Type 8006, 8702
5.1.2. General operation of the mass flow
controller (mfc)
The MFC comprises:
• a sensor for measuring the mass flow rate,
• a control electronics,
• a control valve with low-friction and high sensitivity.
5.2. operation of an mfm or mfc
sensor
• The integrated flow-rate sensors use the thermal measurement
principle (anemometric and calorimetric) to measure the mass
flow-rate. The main components are a heating resistor and a
temperature probe. The gas which passes through the device
modifies the temperature difference measured between
both resistors.
• The thermal measurement principle allows the MFC to control the
required mass flow-rate completely independently of the pressure
and temperature fluctuations in the application concerned.
To obtain an output signal for the effective, dynamic
or uniform value, the damping of the output signal can
be changed with the "Mass Flow Communicator" software
(see section „10.1.3“).
On the MFMs and MFCs types 8712 and 8702, the
integrated sensor technology requires filters to be fitted
upstream of the product when highly soiled fluids are present.
5.3. Detailed operation of an mfc
w
x
out
y
x
x
d
= w-x
Control
electronics
Sensor Actuating element
(solenoid valve)
Gas
inlet Gas
outlet
Fig. 1: Operating principle for the Mass Flow Controller
The control electronics compare the mass flow-rate (x) measured by
the integrated flow sensor with the mass flow-rate set-point value
(w) supplied to the MFC. The control electronics then calculate the
actuating variable (y) to be supplied to the solenoid valve to control
its opening. The flow-rate is either maintained at a constant value,
or modified to a predefined profile.
The control operates independently of fluctuations in pressure or
increases in the flow resistance which may be caused by soiling of the
filter. The rapidly responding solenoid valve and the sensor dynamics
define the overall response time.
The measured value for the mass flow-rate is also transmitted (xout) to
a remote device via an analogue output or a digital output (field bus).
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