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1.3 Solids Density Detection
The Electropulse system can be used in conjunction with a 433S type sensor to provide blanket level
detection in settling tanks, facilitating the control of automatic de-sludging. In addition the
Electropulse system can be used to detect the solids density of a slurry in a pipe line. In this case the
Mobrey pipe section is used instead of the sensor.
Figure 1.5 shows the operation of the sensor. In a clear liquid the ultrasonic signal is carried across
the gap so the sensor oscillates and the control unit gives a ‘Normal’ indication. When a sludge is
present this scatters the signal as shown in Figure 1.5(b) The signal is attenuated, and the control
unit gives an alarm indication.
(a) Clear liquid, signal transmitted (b) Solids attenuate the signal
Figure 1.5 - Sludge
density sensor,
operation
1.4 Head Amplifier Units
The head amplifier is mounted on the end of, or near to the sensor. It converts the low level high
frequency signal of the sensor into a series of low frequency current pulses that are sent to the control
unit. These pulses are virtually immune to electrical interference.
This means that the length of the cable between the head amplifier and the control room can be 1km
or more. There are five versions available, which fall into two categories:
i) Normal acting head amplifiers
When the sensor is in its normal state (i.e. Dry for Hi-Sens and Wet for Gap Sensors) the signal from the
transmitting transducer is received by the second transducer, and fed back to the head amplifier
causing it to transmit current pulses to the control unit. If an alarm state occurs the sensor no longer
oscillates, and the head amplifier stops transmitting the pulses. The control unit indicates the alarm
condition.
Thus in the normal state, the whole system is active. If a fault occurs in the electronics then the
pulses cease and an alarm state is indicated immediately. In this way the whole circuit is continually
monitored. This is in addition to the fault checking facilities of the control unit.
There are four versions of the normally acting head amplifier. Usually it is possible to mount the head
amplifier on the sensor, but it may be necessary for reasons of temperature or space, to use a head
amplifier which is mounted adjacent to the sensor, and is connected to it by a short length of coaxial
cable. Both of these types are available either in an industrial case, or a heavy-duty case that is
suitable for open deck mounting in marine applications.
The sensor mounted marine head amplifier may contain a double circuit board for use with dual Hi-
Sens or gap sensors.