5
Installation and Operating Manual
IP277, Rev DA
Mobrey MCU200 Series
February 2020
Figure 2. Sensor in clear water and sludge
When the gap is filled with liquid, the piezoelectric receiver crystal converts the ultrasonic wave
into an electrical signal, which is transmitted back to the control unit using a second coaxial
cable. Usually the two coaxial cables to the sensor are in one overall sheath.
The control unit circuitry is a feedback amplifier, which oscillates when the sensor is wet and is
quiescent when the sensor is dry. The “oscillating” and “non-oscillating” sensor states
determine the output relay states of the control unit.
For sludge blanket or interface detection the sensor “oscillates” in a clear liquid and is “non-oscil-
lating” in the sludge or at the interface. An amplifier gain adjustment determines the sludge
density for the change between these two states. See
“Interface detection” on page 12
.
2.3
Switching levels and orientation
Mobrey gap sensors are mounted with the sensor faces vertical. This is to avoid a build-up of
solids on the sensor faces on either side of the gap. In this orientation, the switching level is
half-way up the sensor face. If the sensor is mounted from the side of the tank, the switching
level is normally on the centreline of the cylindrical body.
2.4
Installation of the sensor
The sensor must be handled with care - it is a measuring instrument. Before installation, check
that sensor, cable and control unit have not been damaged in transit. If necessary, drill and tap a
hole with a suitable thread. It is advisable to use a boss or similar on thin walls. The sensor has a
tapered thread. Use PTFE tape or similar to seal the thread. Mark the sensor hexagon to identify
the gap orientation of the sensor, if appropriate. Take care not to damage the sensor cable
during tightening.
The cable should be laid on cable trays and separated from any high voltage or mains cables.
The normal cable termination is a plastic gland (to fit the MCU200 control box drilled hole) and
crimped terminal pins to suit the MCU200 terminals.
Sensor in sludge
The ultrasonic beam is attenuated and
will not reach the receiver crystal.
Sensor in clear water
The ultrasonic beam reaches the
receiver crystal with a strong signal.
Air
Sludge
MCU200
Series
Controller
Air
Water
MCU200
Series
Controller
Receiver
crystal
Transmitter
crystal
