Function Of The Lfm, Liquid Flow Meter; Function Of The Sensor - Burkert 8718 Operating Instructions Manual

Fig. 1: Components of the Liquid Flow Controller
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
is available to the user for monitoring purposes or other evaluations (e.g. consumption determination by
integration).
5.2.2.

Function of the LFM, Liquid Flow Meter

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.
5.2.3.

Function of the sensor

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
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 Communi-
cator (PC software)).
12
w
x = w-x
d
Control
x
electronics
p
Q sensor
english
y
Actuating element
(proportional valve)
via an analogue electrical interface or a field bus and
out
Type 8718, 8719 / 8708, 8709
System Description
x
out
to p ; this pressure
1 2