Endress+Hauser silometer FMC 671 Z - Functional Description

1.4 Functional description
The capacitance or pressure measured by the sensor is converted into a frequency
signal by the electronic insert located in its head. The Silometer FMC 671 Z/676 Z
supplies the power and receives a level-proportional frequency signal over a two-core
cable. The signal is then processed to provide the following functions:
Silometer functions
• Level measurement
By calibrating at two levels, »empty« and »full«, level measurement can be made in
the units entered during calibration.
• Volume measurement
For non-linear volume/level relationships, volume can be calculated from level via
the vessel characteristic which describes the shape of the vessel. This function is
described under »Linearization« in Chapter 5.
• Level limit switching
Channel 2 of the Silometer can be used as a level limit switch in conjunction with a
vibration or capacitance/impedance probe.
• Level measurement with automatic calibration correction
If the dielectric constant or density of the liquid changes or is unknown, the
Silometer can be configured to provide a corrected level measurement on channel 1
by using a level switching probe at channel 2.
• Analogue output
The signal resulting from the calibration and linearisation provides a standard
0/4...20 mA and 0/2...10 V output, proportional to level or volume. Any portion of the
measuring range can be taken to provide a scaled output, see Chapter 6.
• Relay control
The two relays can be assigned to either measuring channel to provide level control
by switching pumps on and off, see Chapter 7.
• Rackbus signal
The measured values and complete configuration can be read by the ZA 672
Modbus Gateway (and ZA 67… computer interface) for integration into process
control systems and visualisation on personal computers.
• Fail-safe operation
If a fault condition is detected, e.g. a break in sensor - transmitter cable, the
analogue signal switches to –10 % or +110 % level or holds the last measured
value. In addition, each relay can be individually set to switch on or off as required.
frequency
analog
digital
current
voltage
sens. [Hz/pF]offset [Hz]
offset [pF] sens. [pF/cm]
range upper limit
V2
range lower limit
V1
frequency
electronic-
insert
electronic-
insert
capacit./
pressure
volume
V2
V1
level
volume
H2
H1
level
cap./press.
level
zero point shift
BA063E08
Fig. 1.7
Signal processing in the
Silometer FMC 671 Z/676 Z
for single channel operation
Silometer FMC 671 Z/676 Z Chapter 1: Introduction
9