1. Sensor Installation
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.
Sensor in air
The ultrasonic signal will not
reach receiver crystal
Sensor in liquid
The ultrasonic signal
reaches receiver crystal
Control unit
Relay output is
energised-
NORMAL
STATE
Control unit
Relay output is
de-energised-
ALARM STATE
Signal is
received
Transmitter
crystal
Receiver
crystal
No signal is
received
Fig. 1.
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.
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.
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 a 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
Delta 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.
Fig 2 Suitable extension cables
50mtr
50-100m
over 100m
RG174
URM76
Consult
RG178
RG58
factory
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