Overview of Liquid Level Detection Technologies with Performance Characteristics Assessment and Energy Cost Saving for Household Water Pumps

Abstract

Usually, the arises to measure liquid levels in containers, in large industries; where large volumes of liquids are stored, in small scale industries and residential buildings in developing countries that sees many households implementing their own domestic water supply system. Measurements by humans may be influenced by sentiments, fatigues, lack of concentration, and so on, which has led to economic losses due lost liquid and wasted electric power. Therefore, researches have been directed toward development of automatic liquid level sensing technologies. This paper thus attempts to review available current technologies in automatic liquid level detection systems published in reputable literature. The reviewed technologies are based on optical devices such as light emitting diode (LED), LASER, photocell, photodiode, light dependent resistor (LDR), fibre optics cable and ultrasonic sensors. Other liquid level detection schemes studied are bases on capacitance measurement, edge detection technique in a computer vision system for inspecting the over and under fill liquid level of bottles. This paper also compared the full scale length to resolution ratio of some of the proposals reviewed in a discussion of performance characteristics of liquid level sensors. Finally, evaluated energy cost savings possible when liquid level sensors are used in water pumping systems. This paper will assist instrumentation researchers to know the state of the art in level sensing technology, and practitioners in selecting the right kind of level sensors for a particular application.

Full text

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WWJMRD 2017; 3(9): 287-294
www.wwjmrd.com
International Journal
Peer Reviewed Journal
Refereed Journal
Indexed Journal
UGC Approved Journal
Impact Factor MJIF: 4.25
e-ISSN: 2454-6615
Frederick Ehiagwina
Electrical and Electronics
Engineering, Federal
Polytechnic, Offa, Kwara
State, Nigeria
Lateef Afolabi
Electrical and Electronics
Engineering, Federal
Polytechnic, Offa, Kwara
State, Nigeria
Olufemi Kehinde
Electrical and Electronics
Engineering, Federal
Polytechnic, Offa, Kwara
State, Nigeria
Abdulmutolib Olaoye
Physics Electronics, Federal
Polytechnic, Offa, Kwara
State, Nigeria
Anifowose Jamiu Jibola
Touchpoint Analytics Nigeria
Ltd (MTN Connect Offa),
Kwara State, Nigeria
Correspondence:
Frederick Ehiagwina
Electrical and Electronics
Engineering, Federal
Polytechnic, Offa, Kwara
State, Nigeria
Overview of Liquid Level Detection Technologies with
Performance Characteristics Assessment and Energy
Cost Saving for Household Water Pumps
Frederick Ehiagwina, Lateef Afolabi, Olufemi Kehinde, Abdulmutolib
Olaoye, Anifowose Jamiu Jibola
Abstract
Usually, the arises to measure liquid levels in containers, in large industries; where large volumes of
liquids are stored, in small scale industries and residential buildings in developing countries that sees
many households implementing their own domestic water supply system. Measurements by humans
may be influenced by sentiments, fatigues, lack of concentration, and so on, which has led to
economic losses due lost liquid and wasted electric power. Therefore, researches have been directed
toward development of automatic liquid level sensing technologies. This paper thus attempts to
review available current technologies in automatic liquid level detection systems published in
reputable literature. The reviewed technologies are based on optical devices such as light emitting
diode (LED), LASER, photocell, photodiode, light dependent resistor (LDR), fibre optics cable and
ultrasonic sensors. Other liquid level detection schemes studied are bases on capacitance
measurement, edge detection technique in a computer vision system for inspecting the over and under
fill liquid level of bottles. This paper also compared the full scale length to resolution ratio of some of
the proposals reviewed in a discussion of performance characteristics of liquid level sensors. Finally,
evaluated energy cost savings possible when liquid level sensors are used in water pumping systems.
This paper will assist instrumentation researchers to know the state of the art in level sensing
technology, and practitioners in selecting the right kind of level sensors for a particular application.
Keywords: Capacitance measurement, edge detection, image processing, interferometer, liquid level,
optical fibre, ultrasonic sensors
Introduction
The need usually arise to measure liquid levels in containers, both in large industries where
large volumes of liquids are stored and in small scale industries, such as the vegetable oil
processing plants, gas service station, and so on. Other areas where there are needs to
measure liquid include public water supplies and treatment plants, fuel depots, even in
residential water supplies. Also breweries and bottling company have critical need for liquid
level sensing so as to maintain standard and minimize economic loss. These liquids may be
inert or highly flammable, conductive or nonconductive[1,2]. Manual or mechanical
measurements may be flawed, due to a multiplicity of factors such as human sentiments,
fatigues, lack of concentration, instrument error and so on. Therefore, researches have been
directed toward development of automatic liquid level sensing technologies.
Meanwhile, Zhou et al.[3] in doing a review of applications of plastic optical fibre (POF)
displacement sensor, highlighted examples and applications of liquid level sensors (LLSs).
However, the review was not comprehensive, since less than 20 literature were reviewed. In
this paper, we reviewed literature discussing liquid level sensing techniques, such as optical,
capacitive and ultrasonic sensors base liquid level technologies. Moreover, technologies
based on edge detection, which is especially useful in bottling companies are highlighted.
The rest of this article is arranged as follows: Section 2 presents a review of fibre optic,
ultrasonic, capacitive and other miscellaneous technologies based systems. Section 3 presents
a comparison of the obtained resolutions of various systems discussed. Finally, Section 4
Wo rld Wide Journa l of Mu ltid iscip linary Re searc h a nd De velo pmen t

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World Wide Journal of Multidisciplinary Research and Development
presents the conclusion are reached and offers
recommendations.
Review of Existing Technologies
There are several approaches reported in the literature for
sensing the level of various kinds of liquid-volatile and
non-volatile, hot or cold, etc. Level sensing technologies
considered in this paper are classified in the taxonomy
shown in fig. 1.
Fig. 1: Taxonomy of liquid detection technology
Optical Sensor Based Technology
Optical devices such as light emitting diodes (LEDs),
LASERs, photocells, photodiodes, and light dependent
resistors (LDRs) have variety of applications in electronic
instrumentations. In addition, optical fibres are useful in
implementing fibre based liquid level sensors, which are
very useful in explosive environment due to their dielectric
and immunity to electromagnetic interference[2]. This
section of the paper examines trendy applications of optical
fibre to liquid level detection.
Birefringence Fibre Loop Mirror and Time Domain
Reflector Based Sensors
Bo et al., in[4], who developed a system with a high
birefringence-fibre loop mirror based on uniform-strength
cantilever beam for liquid sensing. The system applied
force at the end by using a hollow suspending pole. As the
liquid level changes, the applied force varies, which
consequently changes the transmission intensity of the laser
light. Subsequently, the wavelength of the light is used to
evaluate liquid level variations. Manik in[5] used an
optosensor to continuously measure liquid level. Its
approach was to establish an empirical relation for the
attenuation of the light from the light emitting diode (LED)
through a glass tube with varying distance. In its
application to water level sensing, a photo-sensor and a
light reflector in a glass tube floating on the water surface
are used. The intensity of the reflected light detected
depends on the distance between the sensor and the
reflection, which changes with liquid level up to 80 cm to
the nearest 0.5 mm. Additional aspect of this technology
was shown by Yang in[6], who demonstrated a multiplexing
of multiple liquid probes using an optical time domain
reflectometer (OTDR) device for application in liquid level
detection in cryogenic environment. The authors in[7]
reported a system for detecting liquid height in tanks. The
design was based on time-domain reflectometry and it uses
two-wire probe fixed t the wall of the tank- conductive or
nonconductive. Adaptability to container's shape is
enhanced by the use of flexible wire.
Fibre Bragg Grating Based Sensor
A fibre Bragg grating (FBG) liquid sensor based on
bending cantilever beam insensitive to temperature was
demonstrated by Guo, et al. in[8] and, in Sohn and Shim[9].
As the liquid level change, there is a broadening of the
FBG spectrum bandwidth and the consequent, reflection of
optical power. Another FBG based sensor, though
incorporating side- was reported in[10]. However, the FBG
used in this case was polished at the side. It senses level
variation of liquids of arbitrary refractive index, thus
making it more flexible than that reported in[8]. In
addition,[11] demonstrated and test liquid level and
temperature sensor based on tilted fibre Bragg grating. It
was reported that peak amplitude cladding modes and
liquid level are inversely related. Another point to note, is
the simplicity of the design.
Polished and Etched Silica Fibre Based Sensors
The bending losses of a partially polished polymer optical
fibre was harnessed by Montero et al. in[12] in the
development of a self-referencing optical intensity sensor
designed for measuring liquid level in flammable
environment, which is similar to that suggested in[13]. When
a bend on a multimode fiber is incorporated, the higher
order propagating modes in the fiber are refracted because
the angle of incidence increases in the interface core-
cladding, consequently leading to increase of the power
losses in the receiver. The schematic of the scheme
proposed by Montero et al. in[12] is shown in Fig. 2. If the
core refractive index of the fibre, is equal to the cladding
refractive index, it can be considered that the Fresnel
transmission coefficient T, for the beams refracted in an
optical fiber, is given[12] as shown in equation 2:
 
 
22
2
22
4cos cos cos
cos cos cos
c
c
T
  
  





(2)
Where:
θ = the angle of incidence for a certain beam with the
normal to the core surface
θc = the critical angle, which is the arcsine of the ratio of
cladding refractive index to core refractive index. When θ
is less than or equal to θc, the beam will be refracted from
the fiber core increasing the power losses

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World Wide Journal of Multidisciplinary Research and Development
Fig. 2: Plastic optical fibre sensor arrangement for obtaining liquid level[12]
In addition, Lomer et al.[14] presented a multipoint level
sensor based on lateral polishing bends in U-shapes in POF
– with the resultant power loss. The polished bared surface
is in direct contact with the liquid. Consequently, analysis
is done on the variation in optical and geometric parameters
of the multimode POF. Meanwhile, Linesh et al.[15]
proposed a very sensitive level detection technique with a
resolution of 100 mm, it was based on an optical source,
silicon fibres etched with hydrofluoric acid and a detector.
Total Internal Reflection Based Sensing Technique
Furthermore, Li and Yu in[16] developed a system based
frustrated total internal reflection (FTIR) based sensor
combined with single chip computer controlled stepping
motor for tracking and subsequently displaying liquid level.
The sensor head is made from standard multi-mode
communication silica fibre with a tapered tip. Another
FTIR based sensor was reported by Nath, et al., in[17]. The
methodology involved polishing the end face of a step-
index MMF to form a hemispherical tip, which allows part
of the forward propagating modes to be reflected in reverse
direction by the effect of TIR. When the tip of the fibre
touched the liquid, the medium changes, the critical angle
for the back reflected light, thus leading to a modulated
power output. The system is able to detect the level of
chemical liquids such as gasoline, diesel oil, and so on.
Romo-Medrano and Khotiaintsev in[18] used optical sensors
multiplexed using a matrix-type network of optical fibre
using a robust level tracking algorithm for measuring the
liquid level of nitrogen in large cryogenic systems. It
reported a resolution of 5 mm for a full scale measurement
of 1.6 m, which is higher than that reported by Description
of a fibre optic liquid level sensor based on intensity
modulation was presented by Golnabi in[19] and Pérez-
Ocón, et al., in[1]. The methodology used by Golnabi [19]
involves liquid touching the 450 faces of the 45-90-450
prism, leading the total internal reflection (TIR) to be
disturbed and subsequent modulation of the reflected light.
Whereas the approach used[1] involves the principle of TIR
of light with the fibre optic. Power attenuation, which
occurs in the fibre immersed in the liquid container is
measured when there is variation in the liquid level. In
order to minimize error in measurement, the LED light is
modulated before feeding it into the fibre. Pérez-Ocón, et
al. in[1] presented a system shown in fig. 2, and may
incorporate an alarm configurable to alert the observer on
any liquid level in the tank and a means of sending liquid
level data via internet in real time to a distant observer.
Fabry-Perot Fibre and Interferometer Based Sensors
Meanwhile, Lü and Yang in[13] had earlier used an extrinsic
fabry-perot (EFP) cavity comprising of the end of a single-
mode optical fibre and elastic Silicon layer to sense liquid
level. In this approach, liquid pressure change the cavity
length through action on the mechanical construct that
subsequently results in differential phase shifts observable
in the form of signal intensity. The developed system has a
resolution of 10 mm, with a linear range of 1.4 cm. Figure
3 shows the schematic diagram of the intensity based gauge
for liquid level measurement.
Fig. 3: Schematic diagram of the intensity based gauge for liquid
level measurement[1]
Meanwhile, an extrinsic fabry-perot optical fibre
interferometer with an all fused-silica structure and carbon
(IV) oxide laser heating fusion bonding technology, which
can be applied in optical fibre liquid level detection was
reported by Wang and Li in[20]. The developed sensor have
a resolution of 0.7 mm on a full length scale of 5 m, which
is an improved result when compared with the 2 mm
resolution over a range of 3.5 m reported in[13]. Another
fabry-perot cavity structure based system is reported in 21.
It is a white-light interferometric optical fibre sensor. It was
based on sensitivity to pressure, which is a function of
liquid depth. The reported sensitivity of the the system is
830,mmV per 106 N/m2 for up to a pressure of 6×106 N/m2

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World Wide Journal of Multidisciplinary Research and Development
Hence, it is suitable for liquid level detection in
environment containing flammable or explosive substances.
Moreover, in[22], the authors reported a level sensor, which
consists of elliptical multilayer-core fibre having one end
silver coated. Due to the establishment of a Michelson
interferometer, interference shifts related to liquid level can
be observed.
Multimode Interference Effect Based Sensor Furthermore,
Antonio-Lopez, et al., in[24] demonstrated a fibre optic
liquid sensing scheme whose functionality depends on
multimode interference effect. The demonstration involved
fabrication of a single discrete level detection, using a
105/125 step-index multimode fibre, which possesses the
ability of discriminating the refractive index of the liquid
during the level detection. The light source for the system
is tunable laser with wavelength of 1460 to 1580 nm. In
addition, Antonio-Lopez, et al.[23] used the liquid around
the MMF to modify the self-imaging properties of the MMI
device, thus, detecting liquid level. When the effective
refractive index and the diameter of the fundamental mode
of MMF is
MMF
n
and
MMF
d
, with total length
L
, then the
free space wavelength is given as equation (1)[23] as:
220
044
MMFn MMFn n MMF MMF
n d L L
nd
L L L L

   

   
   
(1)
MMFn
n
And
MMFn
d
are the effective refractive index of the
section with liquid and the diameter of the fundamental
mode of MMF section with liquid respectively. Figure 4
shows a fibre sensor, with section covered by liquid
n
L
and
other sections not touching the liquid
0
L
.
Fig. 4: Schematic of the fiber-optic liquid level sensor[23]
Ultrasonic Based Technology
The use of ultrasonic to detect liquid level, with LM567 as
the ultrasonic detecting integrated circuit was presented by
Jian-long[25]. The methodology involved measuring the
ultrasonic velocity and the time delay in the detection by
LM567. Later, Ling[26] evaluated several ultrasonic liquid
level measurement methods, pointed out that with
appropriate modification of the ultrasonic speed table, the
liquid level of various liquids at various temperature can be
determined. He went on to develop a system of ultrasonic
liquid level measurement on ektesine of seal vessel.
Gao et al.[27] presented an ultrasonic based liquid level
detection device. The authors further proposed the
observation of weak signals based on chaotic oscillator,
with the aim of enhancing the system's stability against
noise disturbance. Due to the improved accuracy, the
proposed device may be deployed in the measurement of
oil rig. However, Sakharov et al.[28] presented a system
based on ultrasonic lamb waves for the detection of liquid
level in an enclosed metallic tank under high pressure. It is
a non-invasive measurement technique. The developed
system uses two sets of wedge transducer to generate and
receive ultrasonic lamb waves along the circumference of
the tank. Properties of the wave is dependent on the
availability of liquid in the tank, thickness of the wall and
the constituent of the tank wall.
Capacitive Based Technology
Planar Capacitive Sensor
A sensor consisting of planar electrode structure, a
microcontroller and a capacitance controlled oscillator was
presented by Toth, et al.[29]. Resolution of 0.1 mm over 4 m
range within 0.2s was obtained. Reverter and Meijer[30]
developed a remote grounded capacitive sensor. It used a
stainless steel rod and a PTFE-insulated wire. It
incorporated a cable with active shielding to connect the
sensor to an interface circuit which relies on a relaxation
oscillation and a microcontroller. It was used to measure
the level of water in a grounded metallic container with a
resolution of 0.10 mm over a range of 70 cm for a
measuring time of 0.02 s. Ref.[31] uses a mutual calibrated
output function along with taking new fluid as an on-line
reference to develop a multifunctional sensor for
monitoring liquid level and water content of brake fluid
using capacitive method.
Capacitive Glocal Technique
Khan et al.[32] demonstrated the concept of continuous level
monitoring through the development of a capacitive electric
field methods for different fluids. Consequently, finite
element method analysis in the process examining the
sensitivity of the developed sensor in various liquid.
Capacitive Type Differential Sensor
Moreover, a hydrostatic pressure based level detection
device using a very sensitive pulsating capacitive type
differential sensor incorporating a customized oil
manometer aimed at overcoming bubbling effect by air
movements, spurious information of measured level
occasioned by ambient temperature variation, etc. was
presented by Pozo, et al.[33]. Owing to high cost of
commercially available water level sensor and limited
scope of existing system, Loizou and Koutroulis[34]
presented the simulated performance characteristics of a
new capacitive-type water level sensor, comparable to
commercially available ultrasonic based water level
detection schemes, with a much lower cost of production.
Edge Detection Based Technology
Pithadiya, et al.[35] discusses optimal and template based
edge detection technique in a computer vision system for
inspecting the over and underfill liquid level of bottle. It

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World Wide Journal of Multidisciplinary Research and Development
reported that Shen-Castan’s ISEF optimal edge detection
algorithm results were better than results from methods like
Roberts, Prewitt, Sobel, Marr-Hilderth LoG algorithm and
Canny algorithm. Upon review of existing techniques of
bottle inspection in terms of cap closure, overfilling or
under-filling, The authors in [36] proposed a feature
extraction algorithm for achieving the aforementioned three
tasks optimally.
Huang, et al.[37] proposed a system with a field rod for
automatically detecting liquid level in a transparent bottle.
The system, which is based on computer vision,
incorporates positioning techniques used in determining the
rod’s position, which seems broken at the liquid surface
due to the phenomenon of refraction.
Miscellaneous Technologies
Some other techniques are based on acoustic signal
generated by mechanical impact but received by a
piezoelectric polyvinylidene fluoride sensor attached to the
liquid container[38]; PTC thermistor[39]. Meanwhile, a
mmWave radar sensor incorporating stepped frequency
radar methodology operating at 29,720 to 37,700 MHz.
used among other things to observe continuously varying
liquid level in a container[40,41].
Gu et al.[42] presented a system that is sensitive to
microscopic variations in liquid level. The proposed system
was based on near-field microwave microscopy platform
based on an evanescent microwave coaxial probe
electromagnetically coupled with the liquid and Network
analyzer. The electromagnetic coupling was achieved by
means of a tunable broadband matching network placed
between the probe and the test port of the analyzer
operating at about 0.86GHz. An earlier but similar system
based on fibre optic micro displacement sensor was
reported in[43]. Moreover, the authors in[44] demonstrated a
level to frequency converter used for monitoring
intravenous drip liquid level. A merit of this system is its
small dimension, which is 1.96 mm by 2.00mm. This
methodology was different from technique used in[45] for
observing the liquid level in infusion bottle. In addition, the
authors in[46] highlighted some techniques and steps such as
edge detection, binarization, filtering, image projection and
motion detection. In a yet another image processing-based
measure, Wang et al.[47] used a digital camera and a circular
float to observe more precisely and accurately liquid level
in tank using the pixel counts of the uniquely coloured float
in the image captured by the camera and the photographing
distance.
Meanwhile, the authors in[48] proposed a contactless liquid
level monitoring device, by measuring the phase shift and
attenuation of the wave reflected from the surface of the
liquid. Due to being contactless, it can be used to detect
level of fluid inside automotive engine. While, the authors
in[49] presented a prototype of a density independent and
non-contact technique for observing liquid level. It was
based on Hough transform and image normalisation. A
merit of this system is that there is no need to recalibrate it,
in detection of the level of different liquid. However
Atojoko et al.[50] uses energy efficient passive UHF RFID
tags as liquid level detectors. The RFID reader is made as
part of the tank in close proximity to the tags. Liquid level
is deduced by observing variation of the tag reading fed to
a computer database. An earlier related work is a low cost
application to restaurant beverage glass as seen in[51]
and[52], where a wireless glassware was developed for
sensing liquid level. Other contactless liquid level sensors
are based on hall effect[53], capacitance measurement[52],
optical effect.[54]
Furthermore, the authors in[55] designed a piezoelectric-
exited millimetre-sized cantilever for sensing liquid level
and subsequently evaluated the dynamic characteristics of
the device for various depth of tip immersion scenarios
using Euler-Bernoulli beam theory. Meanwhile, the device
can observe a level difference of 8um. Furthermore, the
authors in[56] reported a liquid level detection mechanism
that can help in estimating the height of liquid in a blast
furnace hearth. The basis for the system is theoretical hot
metal and slag generation rate observed from both specific
oxygen and calculated drainage rates. Many of the
available methods will fail because of the high temperature
and pressure inside hearth. Liang in[57] worked on fuzzy
PID control for a coupled-tank liquid level control system
having lumping lag and characteristics that are not linear.
Meanwhile, a trend in liquid quantification is the
integration of several sensors in a silicon bases CMOS for
monitoring various parameters of fluid including liquid
level[44,58].
Discussion
Liquid level sensor are instrumentation system so they are
assessed by the following performance characteristics:
reliability, speed of measurement, accuracy, resolution, full
scale range, cost, simplicity of design, and so on. We next
present in Table 1 a discussion of the resolution of some of
the technologies reviewed in this paper.
Table 1: Comparing the resolution of optical and capacitance sensors liquid level detection technology
Author
Technology
Full Scale Length (mm)
Resolution (mm)
% resolution (mm) over full scale length
[5]
Optical sensor
800
0.5
0.0625
[18]
Optical sensor
1600
5
0.3125
[13]
Optical sensor
3500
2
0.0571
[20]
Optical sensor
5000
0.7
0.0140
[29]
Capacitive sensor
4000
0.1
0.0025
[30]
Capacitive sensor
700
0.1
0.0143
[59]
Optical sensor
750
9
0.012
[4]
Optical sensor
140
10
The capacitive sensor based device reported in[29] have a
lower full scale length to resolution ratio of 0.000025. But
Wang and Li[20] reported a scheme with the highest full
scale length of 5 m.
A Case for Applications of Liquid Level Sensor to
Water Pump Control
How to provide and sustain available water resource in
several countries of the World has been a subject of several
discourses. This problem is related to poor water allocation,

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World Wide Journal of Multidisciplinary Research and Development
inefficient use, and lack of adequate and integrated water
management. Therefore, efficient use and water
monitoring, have necessitated research into various water
level sensing technologies, and collection methods. To
prevent or minimize water wastage during pumping and to
care for future need for large volumes of water, the concept
of automatic pumping machine with microcontroller based
water level controller was developed in our previous
work[60] based on the conductivity of a copper wire, as
shown in Fig. 4.
Fig. 4: Hardware prototype of the automatic water level
controller[60]
We show in this section of the paper that energy can be
saved when motorized boreholes pumps in most cities in
Nigeria incorporates liquid level sensor. The quantity of
energy consumed by a water pump is a product of the pump
power rating and the time of operation in pumping up water
to a collection tank. is the number of horse power of
the pump. The quantity of energy (kWh) per minute, of
operation is as shown in equation (2).
(2)
At cost of ₦23.5 ($0.0642) per unit (1 kWh), the cost of
energy usage in a minute will be as shown in equation (3).
(3)
With the assumption of a household pump up water three
times per week, and the average rating of the pump is 1.5
horse power, Figure 5 shows the cost of energy that could
be save for 1000 motorized pumps, if a water pump is not
allowed to stay ON for an extra minute after the collection
tank is filled to capacity.
Fig. 5: Estimated energy cost savings for incorporating liquid
level detector in boreholes pumps
Conclusion
This paper have shown that liquid level detection
technology have drawn the attention of researchers. In this
regard, trendy technologies are based on the following:
fibre optics sensors, ultrasonic sensors and edge detection.
There are ongoing efforts to improve the accuracy,
resolution and dynamic range of operation of the liquid
level sensor instrumentation. Other efforts are directed
towards development of level sensors that be used in harsh
environment such as the compartment of an internal
combustion engine and the blast furnace of an iron
processor.
Cost of energy per household with private borehole based
water scheme would be reduced, if a liquid level detector
based water pump controller is implemented with every
water pump system. In addition, water resource would be
conserved. Meanwhile, much still needed to be done in
terms of integrating liquid level sensors in small scale
liquid storage facilities in developing nations as typified by
Nigeria, where a lot of facilities still rely on deep-sticks and
other crude methods of observing liquid levels.
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