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1.2 Interface detection
Attenuation method – Figure 1.3
Attenuation is the reduction in strength of the ultrasonic signal caused by its transmission through a
liquid. Viscous liquids, emulsions and liquids containing solid particles have a greater attenuation
than clear thin liquids. Usually the difference in attenuation between the two liquids is sufficient and
the attenuation method can be used to determine which liquid is in the sensor gap. In this case the
gain of the control unit is set so that the relay is energised only when the liquid with the lower
attenuation is in the gap. For this application Sensor Type 402S is used horizontally. The heavy-duty
sensor type 433S may also be used, perhaps from above on an extension tube. For use in pipes, the
sensor pair 442S should be mounted in line, facing one another, generally horizontally across the
diameter, to detect the interface or presence of liquid. As an example, the interface between oil and
water can be detected using this method.
Figure 1.3 - Attenuation Method
Reflection Method – Figure 1.4
If the attenuation’s are similar and the attenuation method does not work, then the reflection method
must be used.
If an ultrasonic beam is transmitted from one liquid to another at a suitable angle, it does not go
straight through the interface, but is bent, so that it does not reach the receiver crystal. If there is no
interface in the gap, but only one liquid, then the beam travels in a straight line, is received and the
relay energised.
For this application the sensor must be mounted at about 10º from the horizontal, as shown in Figure
1.4. Note that this method gives an alarm only when the reflective surface of the interface itself is at
the sensor.
In order to differentiate between two liquids, interface sensors have a large gap (usually 150mm) and
oscillate at 3.75MHz.
OIL
WATER
Receiver Crystal
Transmitter Crystal
Sensor in Oil. The ultrasonic beam is
attenuated and will not reach the
receiver crystal.
Receiver Crystal
Transmitter Crystal
Sensor in Water. The ultrasonic beam
reaches the receiver crystal.
Oil
Water
Oil
Water
Sensor in lower liquid. The ultrasonic
beam reaches the receiver crystal.
Sensor at interface level. The ultrasonic
beam is reflected/refracted and will not
reach the receiver crystal.
Transmitter
Crystal
Receiver
Crystal
Figure 1.4