Burkert Type 8709 - Manual (Page 12)

12
System Description
w
p
x
out
y
x
x
d
= w-x
Control
electronics
Q sensor Actuating element
(proportional valve)
Components of the Liquid Flow Controller Fig. 1:
Operating mode
The set-point default value (w) is implemented electrically via a standard signal or a field bus.
The actual value (x) measured by the sensor is compared with the set-point value in the controller.
The controller supplies the actuating element with a pulse-width modulated voltage signal as an actuating
variable. The pulse-duty factor of the voltage signal is varied according to the established control deviation.
The actual value is also supplied externally as x
out
via an analogue electrical interface or a field bus and is available
to the user for monitoring purposes or other evaluations (e.g. consumption determination by integration).
function of the lfM, liquid flow Meter 5.2.2.
The LFM Types 8708 and 8709 lack the proportional valve component, compared to the corresponding LFC
Types 8718 and 8719.
The devices can therefore only measure the flow-rate and not correct it independently.
The properties of the other components, in particular the sensor described below, are identical to those of the
LFC Types.
function of the sensor 5.2.3.
The flow-rate is measured according to the differential pressure method.
A measuring orifice in the main duct generates a pressure drop in a flow-rate from p
1
to p
2
; this pressure drop is
measured by the differential pressure sensor (see Fig. 2: Flow-rate measurement; Differential pressure method).
The differential pressure sensor, constructed from two individual pressure measuring cells, supplies a precise and
temperature-compensated measuring signal which is used to calculate the flow-rate.
Irrespective of the control status, the flow-rate value measured by the sensor is sent via a standard analogue signal
output or digitally via the field bus.
To maintain a dynamic or quiet actual value output signal, the damping of the output signal can be
changed with the "Mass Flow Communicator" software (see chapter
10.1.3. Mass Flow Communicator
(PC software)) .
english
Type 8718, 8719 / 8708, 8709