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1. Sensor Installation
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 for the sensor dry. The “oscillating” or “non-
oscillating” sensor states dictate the output relay states of the
MCU200.
For sludge blanket or interface detection the sensor
“oscillates” in a clear liquid, and is “non-oscillating” in the
sludge or at the interface. The amplifier gain adjustment
determines the sludge density for the change between these
two states. See section 3.4.
For Mobrey Hisens sensors, (type numbers HL, HD etc) the
metal body of the sensor provides the ultrasonic coupling
between the piezoelectric crystals. This coupling is reduced
when the sensor is under a liquid, so that for Hisens sensors
“oscillating” state is dry, in air, and the “non oscillating” state
is wet, submerged in a liquid.
1.2 Switching levels and orientation
Mobrey gap sensors should normally be mounted with the gap
vertical, to avoid build up of solids on the sensor faces on
either side of the gap. In this condition the switching level
will be half way up the face: if the sensor is mounted from
the side of the tank this is normally on the centreline of the
cylindrical body.
Occasionally such sensors are mounted with the sensor faces
horizontal, either to avoid air bubbles passing through the gap
or for convenience of installation.
In this case the switching level will be at the sensor face at
the top of the gap.
Fig.2. Hi-Sens transducer
1.3 Installation of 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. 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.
Fig 1.
1.4 Extension cables
Extension Cables up to 50 metres long can be fitted to most
Mobrey ultrasonic sensors in the factory to special order but a
better site arrangement is to have a separate extension cable.
When double coaxial cable needs to be extended, two sets of
coaxial plugs and sockets will be needed, one set for transmit
and one receive.Care must be taken that the connectors are
not earthed or shorted together in any way, to prevent cross-
talk or pick-up. The coaxial connections must be made in a
waterproof junction box. Terminal blocks should not be used.
The extension cable needs to be of 50ohm characteristic
impedance. Suitable dual coaxial extension cables can be
purchased from KDG Mobrey (Part No. K178).
For extensions over 50 metres it is recommended two runs of
single coaxial low loss cable is used, with the transmit and
return cable runs separated by 0.15 metres to minimise cross-
talk.
If several sensor cables are being run together then all the
transmit cables (those connected to E2) should be grouped
together and all receive cables (those connected to 1E)
grouped together maintaining the separation specified above.
The Hi-Sens sensor consists of two ultrasonic transducers
mounted on the inside of a cylinder. When the sensor is not
submerged in the liquid, the signal from one transducer
resonates round the cylinder like a bell ringing. If the liquid
rises up around the sensor, this ringing is damped and the
signal received by the second transducer is significantly
attenuated. The ringing or oscillation of the sensor is
detected by the control unit. Switching occurs when the
liquid is about half way up the cylinder of the Hi-Sens body.
NOTE: For satisfactory operations the Hi-Sens must not be
positioned in a vessel or tube less than 100mm internal
diameter.
1.1 General description
Each Mobrey ultrasonic sensor contains two piezoelectric
crystals. A high frequency signal (1 MHz or 3.7 MHz)
generated by the control unit is transmitted to one
piezoelectric crystal by coaxial cable. This crystal converts the
electrical signal into an ultrasonic oscillation.
The sensor design allows the ultrasonic oscillation to pass from
the transmitter crystal to the receiver piezoelectric crystal.
Most Mobrey sensors (300 or 400 series) are “gap” type
sensors, where the two piezoelectric crystals are separated by a
gap. When the gap is in liquid the signal reaches the receiver,
because of the low ultrasonic attenuation of the liquid. When
the gap is filled with air, no ultrasonic signal can pass from
transmitter to receiver.
See figure 1.