A Low-Cost Capacitive Sensor For Water Level Monitoring in Large-Scale Storage Tanks
A Low-Cost Capacitive Sensor For Water Level Monitoring in Large-Scale Storage Tanks
A Low-Cost Capacitive Sensor For Water Level Monitoring in Large-Scale Storage Tanks
Abstract—Water-level sensors are indispensable for Capacitive-type liquid-level sensors are also frequently
monitoring the level of water in storage tanks, which are employed for measuring the capacitance developed between
used in drinking water distribution networks. In this the electrodes immersed in the desired liquid, which is then
paper, a long-range capacitive-type water-level sensor is used for calculating the corresponding level of the liquid in the
presented. The proposed sensor is constructed using tank. Printed Circuit Board (PCB) electrodes were designed in
widely-available multilayer tubes, which are used for [1] for developing a capacitive water-level sensor in a
building drinking water systems. Thus, both the monitoring system that was designed to measure the rise of
manufacturing cost of the sensor and the cost of the the water level for avoiding floods, without been affected by
associated electronic circuits, which are used for the chemical composition of the water. That sensor was tested
interfacing the sensor to a digital data-acquisition unit, are in the range of 0-30 cm, exhibiting good linearity. In [5], the
low. The performance of the proposed sensor has been circuit design for floating and grounded capacitive sensors
evaluated in a water storage tank of a city-scale water fabricated on a PCB, is presented. Operational amplifiers are
distribution network. The experimental results indicate used, which results in linear behavior and an acceptable
that the accuracy of the proposed experimental prototype accuracy. Cylindrical capacitive electrodes were developed in
sensor is equivalent to that of a commercially available [6] for a liquid-level sensor. A linearization network for the
ultrasound water level sensor, while, additionally, its capacitance measurement circuit was also used, providing
manufacturing cost is significantly lower. results with good linearity in the scale of 0 - 25 cm.
A capacitive-type liquid level sensor was presented in [7],
Keywords— Sensor; Capacitive; Water level; Data-acquisition consisting of two non-inductive, two-layer windings mounted
on coaxial cylinders immersed in the liquid, which was
I. INTRODUCTION contained in a metallic storage tank. Results are demonstrated
Sensors are used for monitoring the level of liquids in for a maximum liquid level of 60 cm with excellent linearity.
storage reservoirs, containers and tanks. Multiple alternative However these electrodes exhibit the disadvantage that it is
liquid level measurement techniques have been applied, such difficult to be transported to remote locations due to their non-
as magnetic, radar, ultrasonic etc., since varying the range, flexible structure. The design of long-range capacitive sensors
installation conditions and liquid type impose different has been studied in [8], but it has only been limited to
specifications for the sensor [1]. The ultrasound and Time computer simulations. The water level capacitive sensors
Domain Reflectometry (TDR) sensors are frequently reported in [9-13] have been designed to operate over a
employed for measuring the level of liquids in storage tanks.
relatively small range (i.e. below 1 m). The capacitive-type
However, these sensors exhibit various disadvantages. The
sensor, which is presented in [14], can operate only in metal
ultrasound sensor measurements are affected by the bubbles in
liquid surface, which can result in the scattering of the sound tanks, since in that case the tank shell acts as one of the
waves in the surrounding air, causing errors in measurements. capacitive-sensor electrodes. Although capacitive sensors
TDR sensors comprise a very accurate measurement solution; exhibit good linearity, the use of an Artificial Neural Network
on the other hand, the cost of monitoring pulse-duration is proposed in [15] in order to increase the linear range of
changes with high resolution increases significantly the cost of capacitive sensors and provide self-calibration features to the
the overall data-acquisition system [2], [3]. In [4], a measurement system.
long-range sensor is presented, using an optical fibre As cities are expanding, together with the increase of water
Fabry-Perot interferometer. The results indicated an excellent and energy prices, sustainability of water-supply systems is
resolution with small error in water measurements. However, vital. Thus, an important application of level sensors is for
specific instruments are required and a complicated measuring the level of water, which is temporarily stored in
installation process for applying this measuring technique, large-scale tanks contained within the water distribution
which may not be feasible in case of tanks installed in an networks of communities, cities etc. In these systems, the
urban environment. water level information acquired by a large number of water
storage tanks is typically transmitted to a central monitoring
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where ε o is the electric permittivity of vacuum, ε r is the
relative dielectric constant of the insulator between the
capacitor electrodes, d1 and d 2 are the internal and external,
respectively, diameters of the multi-layer tube and h is the
length of the capacitor electrodes.
The aluminum layers of the tubes are not in direct contact with
the water, which enhances the long-term durability of the
sensor by avoiding the impact of metallic corrosion on the
sensing capacitor electrodes.
A signal-conditioning circuit for the proposed sensor has
Fig. 2. The equivalent circuit of the proposed sensor for excitation also been designed, producing water-level measurements in
frequencies in the range of tens to hundreds of kHz.
digital format, which are then interfaced to a data-acquisition
unit for further processing. Due to the specifications of the
and inductive characteristics are negligible. Also, its electrical target application under study, the data-acquisition unit is
characteristics are affected by the existence of dissolved based on the ALIX 3d2 system board, providing an I2C port
solutes [17]. These properties has been exploited during the available for communication with the proposed measurement
design of the proposed water-level sensor by selecting the system. Techniques such as the wireless transmission or the
excitation frequency of the capacitive sensor, which has been employment of industrial connection interfaces (e.g. the
developed, to be within the above range such that the water 4-20mA protocol etc.) are frequently adopted for transmitting
interacting with the sensor parts behaves as a conductor. the sensor measurements to the data-acquisition unit. The
main disadvantage of such industrial protocols is that they
In the proposed sensor, a capacitor is formed by the two raise the cost of the overall data-acquisition system, while
sections of the sensor, which are inside and outside the water additionally impose specific restrictions to the design of the
level, respectively [Fig. 1(b)]. The length of these sections signal-conditioning circuit. For designing the signal-
changes during the sensor operation according to the level of conditioning circuit, special consideration was given to the
the water in the storage tank. In the section of the sensor that fact that in practical applications the water level sensor is
is above the water level, a capacitor is formed between the installed inside a water tank, which is typically at a long
aluminum electrodes, with the two intermediate polyethylene distance away from the data-acquisition unit. Various time-
layers acting as a dielectric (i.e. C1 in Fig. 2). The section of varying conditions in the environment of the water storage
tanks, which affect the performance of the sensor, have also
the sensor which is below the water level behaves as follows: been considered for designing the signal-conditioning circuit,
a capacitor is formed between the two electrodes of the tubes, such as the changes of ambient temperature and water
where the two polyethylene layers comprise the capacitor conductance (e.g. due to salinity, temperature etc.). This is
dielectric (i.e. C2 in Fig. 2), another capacitor is formed indispensable in order to be able to maximize the accuracy
between the water entering the inner tube and the aluminum performance of the proposed water level sensor.
layer of that tube with the polyethylene layer of the tube
acting as a dielectric and, finally, a third capacitor is formed For measuring the capacitance of the water level sensor, a
between the aluminum layer of the outer tube and the water at circuit with operational amplifiers was developed and
prototyped. The signal-conditioning circuit sensitivity and
its outside surface (i.e. C3 and C4 in Fig. 2). The water in the
measuring range can be easily set by adjusting the value of
storage tank behaves as a conductor, connecting electrically some passive components. A block diagram of the signal
the individual capacitors, according to the equivalent circuit conditioning circuit is illustrated in Fig. 3. The capacitive
diagram of the proposed sensor, which is depicted in Fig. 2. sensor is connected to a charge amplifier, which is excited by
The water resistance has been assumed negligible. The total a 32 kHz square wave. As analyzed above, using this
capacitors formed above and below the water level, frequency provides the advantage of the water performing as a
respectively, are electrically connected in parallel. The values resistor. Since the feedback components RF and CF in Fig. 3
of all capacitors are modified as the water level in the storage are set by the circuit designer at constant values, the response
tank changes. At low values of water level, the capacitance C1 of the charge amplifier depends on the water-level-determined
in Fig. 2 predominates the total sensor capacitance. As the capacitance of the sensor. Thus, the output of the charge
amplifier is also a square wave, having amplitude proportional
water level rises, the values of C2 - C4 are increased to the total capacitance of the water-level sensor. Then, a full-
accordingly, while C1 is reduced. The values of the individual wave rectifier is used to rectify the generated square-wave, as
cylindrical capacitors formed in the proposed sensor well as a low-pass filter which produces a DC voltage which
configuration are calculated using the following equation [12]: is proportional to the total capacitance of the sensor. The
circuits of the charge-amplifier, full-wave rectifier and low-
2π ⋅ ε 0 ⋅ ε r pass filter are implemented using the AD8515 operational
Cx = ⋅h (1)
⎛d ⎞ amplifier. The output of the low-pass filter is interfaced to the
ln ⎜ 2 ⎟ AD7745 integrated circuit, which is capable to convert either
⎝ d1 ⎠ capacitances or analog signals into a 24-bit digital format.
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adequate for the water-level monitoring applications under
study. In order to provide a long-distance transmission
capability to the signal-conditioning system of the proposed
sensor, the P82B96 I2C bus extension chips are connected at
the ends of the communication link connecting the signal-
processing circuits of the proposed sensor with the ALIX 3d2
system board, in order to increase the transmission distance up
Fig. 3. A block diagram of the signal-conditioning circuit which was to 30 m. The signal-conditioning circuits are power-supplied
developed for interfacing the water-level measurements to an ALIX 3d2 by the ALIX 3d2 system board, as well as an external power
system board. source through a standard UTP cable. Due to the low power
consumption characteristics of the integrated circuits, which
were employed, the total power consumption of the proposed
water-level measuring system, including the sensor and
signal-conditioning circuits, is 12 mW. A script executed by
the ALIX 3d2 data-acquisition unit, was developed using the
Python programming language for communicating with the
AD7745 chip. A flowchart of this program is depicted in
Fig. 4. Initially, the internal registers of the AD7745 chip are
initialized for measuring the low-pass filter output voltage.
The capacitance of the proposed sensor is affected by the
changes of ambient air temperature. Thus, temperature
compensation of the water-level measurements provided by
the sensor is also performed in the software, using
measurements of the AD7745 internal temperature sensor. The
water level, L (m) is calculated using the measurements of
the low-pass filter output voltage, V f (V) and AD7745
internal temperature, Ta ( oC ), according to the following
equation:
L = α1 ⋅V f + (Ta − Tref ) ⋅ α 2 + α 3 (2)
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2000
1800
capacitance (pF)
1400
1200
1000
800
600
400
200
0
20 30 40 50 60 70 80 90 100 110 120
Water level (cm)
71
distribution networks are placed. Additionally, this flexibility 70
enables easy elimination of the tubes bending, during 69
installation of the proposed sensor, thus improving the
68
linearity of its response. The water-level sensor that was
67
developed has been experimentally tested under various
66
operating conditions. The first sets of experiments were 67 68 69 70 71 72 73 74
conducted in the laboratory, by immersing the sensor inside a Measurement of ultrasound sensor (%)
plastic container and using an LC-meter to measure the Fig. 8. The measurements acquired by the proposed capacitive-type
capacitance of the sensor. The experimentally measured total water-level sensor versus the corresponding measurements of the ultrasound
capacitance of the proposed sensor at various water levels is sensor, when both operating in a water storage tank of the Municipal
shown in Fig. 6. The non-linearity Root-Mean-Square (RMS) Enterprise for Water and Sewage of Chania (Greece).
error is 0.63 % and the Mean Absolute Error (MAE) is 0.61%.
installation in this tank, the proposed capacitive water-level
The variation of the DC output voltage produced by the sensor had been calibrated, using the ultrasound water-level
low-pass filter of the signal-conditioning circuit, which is then sensor as a reference. The two different types of sensors were
converted to digital using the AD7745 chip, at various water then set to operate in parallel for a time period of 6 hours. A
levels, is presented in Fig. 7. In this case, the non-linearity plot of the measurements acquired by the proposed capacitive
RMS and MAE errors are 0.69 % and 0.64%, respectively.
sensor versus the corresponding measurements of the
Also, the laboratory tests, which were conducted, indicated ultrasound sensor is presented in Fig. 8. The RMS and MAE
that the impact of water salinity on the performance of the of the deviation between the measurements obtained using the
proposed water-level measurement system was negligible. In proposed water-level sensor from the corresponding
order to evaluate its performance, the proposed sensor has also measurements of the reference ultrasound sensor are 0.88 %
been tested in a drinking water storage tank (constructed of and 0.81 %, respectively. This accuracy is acceptable for
concrete) of the Municipal Enterprise for Water and Sewage applying water management techniques in city-scale water
of the city of Chania (Greece). An experimental prototype of distribution networks.
the proposed sensor having 4 m length has been constructed
for that purpose, in order to adapt the sensor to the depth of The total time required for the software executed by the
the corresponding water storage tank, which has been ALIX 3d2 system board to produce a water-level
allocated for performing the experimentation process. An measurement (including the communication with the
ultrasound water-level sensor was already installed in that tank signal-conditioning circuit) is approximately 1.02 sec.
by the Municipal Enterprise for Water and Sewage of Chania,
for monitoring the water level of the tank, in order to apply the The proposed water-level sensor and signal-conditioning
appropriate water management procedures. Prior its circuits have been designed to operate using low cost
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materials and devices, thus reducing the total cost of the Greece), as well as the administration and technical staff of
overall water-level measurement system. In order to calculate “Georgios Liontas & Co. E.E.” for their assistance in
the total construction cost of the proposed water level sensor, conducting this research.
the prices of the required materials when provided in small
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Sewage of the city of Chania (Greece) for their contribution
during the sensor development and experimental performance
evaluation processes and the administration and
research/technical personnel of the Telecommunication
Systems Research Institute (Technical University of Crete,
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