54
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Introduction to Industrial Automation
capacitance C is directly dependent on the electrodes areas (A), their distance apart (D) and the
dielectric constant (K) of the material between the electrodes, defined as:
C = KA/D
When the tank is empty, then the capacitance (air dielectric) is C(K
1
). When the level switch
is covered by material that plays the role of a dielectric substance filling the gap between the elec-
trodes, the capacitance gets multiplied by the dielectric constant of this material (K
2
) and specifi-
cally
varies to C´(K
2
) = e C(K
1
), where e is a constant expressing the difference of two dielectric
constants. Its value is directly dependent on the material to be detected and indicatively has the
value 2–3 approximately for corn and 70–80 for water.
The variation in capacitance is subsequently translated into a switching output by an oscillat-
ing circuit, the frequency of which is dependent on the value of capacitance. The oscillation or
stabilization of the circuit corresponds to the two states of the SPDT contact output and hence
to the existence or absence of a material in the tank. The rod-type electrodes of a capacitive level
switch are strong against the buried situation, and therefore are suitable not only for fine powder
but also for bulk solid materials. Since the temperature, moisture content, humidity, and density
of the process material can change its dielectric constant, the capacitive level switches are equipped
with a sensitivity regulating mechanism for calibration. If more than one capacitive level sensor
needs to be mounted in the same tank, a minimum distance between them should be provided
by the manufacturer in order to avoid interference of their electromagnetic fields. Capacitive level
switches can be used for level detection in silos, tanks, and bunkers in all areas of industry for
conductive or non-conductive liquids, as well as for bulk solid material with a dielectric constant
greater than that of air.
Ultrasonic level sensors: Ultrasonic level switches and transducers work both on the same basic
principle of generating and receiving after a target reflection of ultrasonic waves, but are differ-
ent in their output type. Ultrasonic level switches have a digital output signal of a SPDT contact
type, while ultrasonic level transducers are capable of non-contact measuring of the level through
a microprocessor-based circuit and producing an analog output signal, usually 4–20 mA. Some
ultrasonic level transducers offer both switch and current outputs, which means they can be used
either as a level switch or as a level meter. Ultrasonic level sensors can detect liquids, sludge, and
solid materials.
An ultrasonic wave is a high-frequency acoustic wave that cannot be heard by someone. In
general, people can hear an acoustic wave or sound if it is within the range 20 Hz to 20 KHz. The
transmitter of an ultrasonic level sensor emits acoustic waves usually within the range 30 KHz to
200 KHz. The emitted ultrasonic waves hit the liquid, sludge, or solid surface and are reflected
back to the sensor, as shown in Figure 2.42a. The level is then calculated from the time lag
between the emission and the reflection of the ultrasonic wave, and is converted to a digital or
analog signal accordingly. Ultrasonic level switches and sensors are sensitive to temperature, pres-
sure, and humidity conditions, and for this reason they are equipped with compensation units for
reducing measuring errors. On the other hand, ultrasonic level sensors have some basic advantages
over other technologies of level detection. For example, they can detect various materials that are
quite far away, even more than 15 m. Also, the ultrasonic waves are not affected by the color of the
target surface and its possible changes.
The ultrasonic switch shown in Figure 2.42b uses a slightly different method to detect the
presence or absence of a liquid at a designated point. It contains two piezoelectric crystals, one
transmitting a high-frequency (about 2 MHz) sound and one receiving the previous sound, which
Hardware Components for Automation and Process Control
◾
55
are mounted opposite each other at a small distance of a few millimeters. The ultrasonic switch
uses the different behavior of sound transmission in air and liquid to detect the liquid presence.
When there is no liquid in the gap between the two crystals, the receiver accepts a weak signal,
due to the sound transmission in air which presents attenuation. When liquid is present, the sound
retains almost all of its signal strength and the receiver accepts a strong signal. Subsequently, the
electronics detect this difference and switch an SPDT contact output accordingly.
Radar-type level sensor: Since it is difficult for the lag time-based method described in the previ-
ous section to give very accurate measurements for such small time intervals, the frequency modu-
lated continuous wave method is used in radar-type level sensors. A radar signal that is emitted via
an antenna toward the liquid surface is a microwave signal with a continuously varying frequency.
When the reflected signal returns to the receiver, it is compared with the outgoing signal. Since
the transmitter continuously changes the frequency of the emitted signal, there will be a differ-
ence in the frequency between the transmitted and the reflected signals. The distance of the level
from the sensor location (taking into account the dead zone) is then calculated by measuring the
proportional frequency difference, as illustrated in Figure 2.43.
Ultrasonic
sensor
Ultrasonic
switch
Switch output
Switch output
Transmitting
crystal
Receiving
crystal
Emitted and
returning waves
Tank
(a)
(b)
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