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Universal Liquid Level Sensor Employing Fresnel Coefficient Based
Discrete Fiber Optic Measurement Technique
Syed H. Murshid1,2
1
SPIE member
2
Optronics Laboratory
Department of Electrical and Computer Engineering, Florida Institute of Technology
150 W. University Boulevard, Melbourne, FL 32901, USA
[email protected],
ABSTRACT
A compact and light weight liquid-level-measuring system based on fiber-optics sensor technology is presented as
alternative to systems based on float gauges and other conventional level sensors for liquids that pose fire, corrosion and
explosion hazards. These Fresnel reflection based fiber-optic sensors are inherently safer because they do not include
electrical connections inside fuel/chemical tanks, and they exploit changes in internal reflection of guided electromagnetic
modes as a result of contact between the outer surface of optical fiber and a liquid. Discrete changes in light
transmission/reflection are used to indicate that liquid has come into contact with a suitably designed fiber optic probe at
the output end of the fiber. This endeavor presents a quasi-continuous fiber optic level detection system that measures
liquid level to within known increments of depth, by placing the probes of a number of such sensors at known depths in a
tank where each probe effectively serves as a level switch. Due to the fiber optic nature of the design, the system can
operate from cryogenic applications to boiling fluids. Experimental results for liquid nitrogen and water are presented.
Keywords: Liquid level sensor, fiber optic sensor, optical sensor, cryogenic level sensor, fiber optic level sensor
1. INTRODUCTION
Electronic means of liquid level measurements are reliable and inexpensive; however, certain liquid storage environments,
such as fuel tanks, liquid hydrogen and oxygen storage facilities and hypergolic fuel storage reservoirs are vulnerable to
spark generated explosions and can cause serious threat to safety of personnel and property. Alternative designs, such as
float gauges, typically do not allow for precise, reliable and accurate measurement of fluid levels1. Fiber optic sensors2-5
such as strain gauges6-8 etc., have been used in broad range of industrial applications. They are inherently safe, can be used
in hazardous environments and can be adapted to challenging environments such as zero G applications9-11. Furthermore,
they offer significantly higher sensitivities when compared to mechanical devices such as float gauges. Generally fiber
optic liquid level sensors utilize optical strategies such as Bragg gratings12-23, interferometers24-31, and evanescent based
sensors32-35; however, these devices demand precision during manufacturing process and tend to require either expensive
optical components or costly electronic readout or both. Alternative designs attempting to address these issues have been
reported36, 37 during the last few years. These include reliable and inexpensive liquid level sensors38, 39 that offer all the
inherent advantages of optical fiber sensors despite their simple and inexpensive design. This endeavor presents a low cost
and uncomplicated fluid level detection scheme based on Fresnel reflections.
The wave guiding properties of fibers tend to deviate from the normal at dielectric interface, and its reflection and
transmission properties depend on the surrounding medium that contacts it. Fresnel’s equations explain transmission and
reflection properties of light incident at the interface of two dielectric media. If a cleaved fiber is immersed in fluid, it
encounters a reflection coefficient that is smaller than that of air. The opposite is true for transmitted intensities. This is
caused by difference in the refractive indices of fluid and air. Therefore, fluid level in a tank can be determined by
measuring the amount of light transmitted or reflected by the fiber, in presence or absence of target fluid. The reflection
based approach is extremely desirable due to its intrinsic nature and the advantages that the fiber optic sensor technology
offers over the conventional sensing schemes.
Photonics Applications for Aviation, Aerospace, Commercial, and Harsh Environments V,
edited by Alex A. Kazemi, et. al., Proc. of SPIE Vol. 9202, 92020W · © 2014 SPIE
CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2083398
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The resultant approach eliminates almost every possibility of fire and explosion in hazardous areas that may be caused by
sensor malfunctions. This enhancement in safety owes to the fact that only light and passive optical fiber cables come in
contact with the target fluid. As a result, it can be safely used for challenging applications such as rocket propellant inside
space vehicle fuel tanks. The tank is not subjected to metallic parts or electrical signals. Lightweight and small size of this
system is inherent to fiber optics while better accuracy for this scheme is derived from the fact that the sensor directly
detects the presence or absence of the fluid. It does not utilize indirect approaches like level floats, pressure within the fuel
tank, and absorption or attenuation of radio frequencies inside the liquid vessel etc. Indirect approaches tend to falter due
to many variables including voids, which can be formed because of boiling of liquefied gasses. Another very important
feature of this liquid level sensor is the ability of the system to operate without any field calibration owing to a simple
signal processing approach that leads to a reliable threshold and quasi-continuous output. Therefore, this design can be
used to accurately measure levels of liquid to any desired resolution. Furthermore, the system is potentially very low cost
due to minimum number of components and practically no special parts. The small size, weight, geometrical versatility,
and physical as well as chemical compatibility of the system to any specific set of requirements are additional benefits that
will reduce the overall deployment/flyaway cost of the final system.
At any interface between two optical media of differing refractive indices, any incident light will be divided into reflected
and transmitted components. Fresnel’s laws of reflection40, 41, precisely describe the amplitude and phase relationship
between the reflected and incident light at the boundary between two dielectric media. For optical energies, in a medium
of refractive index n0, incident normally on an interface to a dielectric medium of refractive index n1, the relative amplitude
of reflected light, r, can be given by equation 1. Figure 1 shows the percentage of reflected light as a function of the
refractive index of the target fluid, where n1, the refractive index of glass fiber is 1.5 and n0 is either the refractive index
of air, 1.0, or that of the target fluid and may vary between 1.0 and 1.5.
n n0
r 1
n1 no
2
(1)
Reflected Light (%)
Reflected Light -vs- Refractive Index
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
1
1.1
1.2
1.3
1.4
1.5
Refractive Index of Target Liquid
Figure 1: Percentage of reflected light as a function of the refractive index of the target fluid.
An experimental breadboard laboratory fiber-optic liquid level detection system was built and tested for initial proof of
principle demonstration. This experiment involved a standard 2×2 multimode optical fiber coupler. The output ends of the
coupler were cleaved to serve as sensing fibers. A beaker containing water at room temperature was used to measure the
response of the sensing system for two levels of water column. The system block diagram is composed of a suitable optical
transmitter (TX) and a matching optical receiver (RX) connected to a 3-dB coupler with the output ends of the coupler
serving as optical switches, S1 and S2. This block diagram is shown in figure 2.
TX
RX
Po
3-dB
S2
Coupler
S1
Figure 2: Block diagram of discrete liquid level sensor system.
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2. EXPERIMENTAL SETUP & RESULTS
The experimental set up for the Fresnel reflection based quasi-continuous fiber optic liquid level detector system is
illustrated in figure 3, where an LED source is used to launch light into one of the two input arms of a 2×2 fiber coupler.
A PIN diode is connected at the end of second input arm for detection of the sensing signal. This liquid level sensor
employs reflection mode, instead of transmission mode; hence, reflections from output ends of the coupler is measured
and processed to detect fluid levels. This system configuration forces light launched from the optical source to split into
two while traveling towards the two output ends of the 2×2 coupler. It should be noted that a 2xN coupler could also be
used. When light reaches the other end of the fiber, most of it is transmitted into the interface medium; however, a small
fraction of the total optical power is reflected back towards the detector and in accordance with Fresnel’s laws, the intensity
of the reflection received at the detector is a function of the difference between the refractive indices of the optical fiber
and the interface medium. The amount of reflection can be calculated by equations 1. Experimental results show that two
distinct reflection levels are seen by the detector, depending on the presence and absence of fluid at the ends of the two
fibers. This distinction in the reflection coefficients owes to the differences in the refractive indices at the ‘fiberenvironment’ interface due to presence or absence of target fluid at the interface. Figure 4 shows the response of this quasicontinuous or stair-case digital liquid level sensor system to two different levels of water.
LED Source
2×2 Coupler
PIN Diode Detector
Liquid Tank with
sensing fibers
Figure 3: Schematic diagram of Discrete Liquid Propellant Level Sensor System
Orf -10ÄmV-CV1 mV M .095
RefA 10.OmV 5.005
RefO 70.0mV 5.095
Figure 4: Two typical output response traces of the discrete liquid level sensor system to water.
Figure 4 shows two oscilloscope traces corresponding to two identical test runs. These experimental results were saved as
Reference A and Reference B on the Oscilloscope. The ‘A’ and ‘B’ followed by the small arrow to the extreme left of the
oscilloscope screen indicate the ground level for each individual trace. The top trace, Reference B, is at its maximum value
during the first two divisions of the time scale, i.e. for approximately ten seconds. The output is at its maximum as both
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the ends of the 2×2 coupler are in air and the reflection is also at its maximum. After approximately ten seconds, one of
the output arms of the coupler is submerged in water with a resultant dip in output level due to reduction in reflection.
After a delay of additional 5 seconds, the other end of the coupler is also submerged in water. Immediately, the sensor
output dips to a value very close to the ground level and stays that way for the next two divisions. At that point, both the
arms of the sensor are withdrawn from water and nearly 11 seconds after withdrawal from water the sensor output
approaches its initial value.
The bottom trace depicting Reference A shows a rerun of the experiment described by Reference B. It can be seen that
both these traces are almost replicas of each other. The experiment was repeated multiple times over a period spanning
over three months and similar results were obtained.
A breadboard version of the fiber-optic liquid propellant level detection system as shown in figure 3 was also employed
to detect levels of liquid nitrogen. Liquid nitrogen was filled in a Dewar and the quasi-continuous/stair-case digital liquid
level sensor was inserted and withdrawn manually, to measure the response of the sensing system to different levels of
liquid nitrogen. The two output ends of the 2×2 coupler effectively behaved as two optical switches, where each switch
was able to detect the presence or absence of the target fluid. Figure 5 shows the simple design of the fiber optic sensor,
which consists of only standard optical fibers. Almost any number of fibers can fit into small openings and almost every
geometry. Hence, any desired resolution for the fluid level is possible. The sensor in figure 5 is shown against a dark
background to enhance visibility. This sensor is rugged and has survived multiple insertion and withdrawal cycles in
multiple fluids including liquid nitrogen.
Figure 5: Simple sensor assembly for the stair-case digital liquid propellant level detection system.
As discussed earlier, two discrete output intensity levels are seen at the detector, depending on the presence and absence
of liquid nitrogen at the end of the fiber. Figure 6 shows responses of the sensor to different levels of cryogenic liquid
nitrogen.
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nurse
Rei
Save
R ?t k
1-17
10OrnV CH2 10.0mV
FrJA 10 nrnV 5,00s
515.005
Reid 10.0mV 5.005
Figure 6: Distinct sensor response is seen for different levels of liquid nitrogen.
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The two oscilloscope traces presented in figure 6 correspond to two different experimental runs. These experimental results
were again saved as Reference A and Reference B on the Oscilloscope. The system response is also identical to that for
water. However, there are two significant differences. First, the change in output signal is smaller and it does not approach
the ground level, due to a smaller difference in refractive index. It should also be noted that though the change in the output
signal is smaller compared to that for water, there is still a significant change in the sensor output and a suitable threshold
level can easily be chosen. The second difference involves shorter delay in sensor recovery. The total recovery time for
liquefied nitrogen is about 5 seconds as opposed to approximately 11 seconds that the sensor took in order to approach its
initial value for water. It should be noted that this 5 seconds is the total time taken by the system to return to its initial
value. Properly set thresholds will reduce this delay to approximately 1-2 seconds and indigenous sensor housing designs
have the potential to reduce it further. This hysteresis cold be reduced to less than a second by orienting the fibers in a
fashion that assists the residue of the fluid to quickly channel away from the fiber end-face.
The sequence of events can easily be predicted by looking at the oscilloscope traces. The top trace, Reference B, is at its
maximum value during the first 11seconds. At that point one of the two ends of the sensor was submerged in liquid
nitrogen. This results in distinct reduction in the output. After waiting for approximately 4 seconds, the other end of the
sensor was also submerged in the cryogenic fluid, which results in another significant drop in the sensor output. The output
stays unchanged for nearly 5 seconds, as there was no activity. At that point both the arms of the sensor were withdrawn
from fluid, and approximately 4-5 second after withdrawal, the sensor output approached its initial value. The bottom trace
shown by Reference A is identical to Reference B. It can be seen that both these traces are almost replicas of each other.
This experiment with liquid nitrogen was repeated multiple times over a period spanning over three hours with similar
results. The system is discrete in nature with stair case digital or quasi-continuous output.
In a follow-up experiment, the total height of fluid tank was divided into eight equally spaced intervals, and the presence
or absence of fluid at pre-designated levels was successfully detected and measured using this quasi-continuous fiber optic
liquid level sensing scheme. The block diagram of the eight level fluid measurement system and its experimental
implementation is shown in figure 7, where SP1 through SP4 are simple threshold based signal processing units. The
individual fibers can be attached at any desired location inside the fluid tank to achieve the desired sensing resolution.
Furthermore, this sensor is inherently safe in explosive, corrosive, and hazardous locations due to its all optical nature
which ensures that no electrical current is introduced inside the fluid tank. Choice of a broad range of silica and plastic
fibers allows the use of this sensor for a vast range of practical applications. It should also be noted that in addition to level
detection, this sensor can also be used to determine and characterize the fluid itself by employing appropriate signal
processing algorithms and a lookup table that correlates the intensity of the reflected signal to the refractive index of the
fluid and then matches the refractive index to a specific fluid. Tunable sources could also be used to provide better
discrimination between fluids with similar properties.
RX 3
SP3
TX 2
RX 2
SP2
TX 1
RX 1
SP1
1
2
3
4
5
6
Coupler 4
TX 3
Coupler 3
SP 4
Coupler 2
RX4
Coupler 1
TX 4
7
LED
DISPLAY
3dB Couplers
8
Figure 7: Simple system construction indicates feasibility of any desired resolution for fluid levels.
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This design lends itself to remote real-time operation as every component of this design including the transmitter (TX),
receiver (RX), signal processor (SP), display LEDs and the couplers can be housed at any desired remote location. Only
the standard communications grade optical fiber cable goes to the fluid tank and comes in contact with the measurand.
This leads to a versatile sensing scheme that is negligible in size, inexpensive, lightweight, highly reliable, and is almost
universal in nature and can be used in almost any application with a geometry that is adaptable. It has a straightforward
design with limited electronics utilizing simple optical transmitter and receiver circuits coupled with negligible signal
processing and requires minimal power to operate. Total current consumption for the eight level sensor is approximately
60mA. This current can be easily reduced to about 15mA if the transmitter circuits are operated in series. Pulse width
modulation of the circuit can further reduce the power requirements by orders of magnitude. The base line design uses
four 2×2 multimode couplers. The system efficiency could improve further with 2×4 and 2×8 couplers and other
multiplexing techniques. The transmitter circuit shown in figure 8, consists of a simple transistor (2N2222) circuit driving
an infrared LED (OPF-692).
vss
Vss =4.99y
I
Vrx
R1
I
15k
OPF-692
R2
47k
Vax
Figure 8: Simple transmitter circuitry provides threshold adjustment for the detection system.
The optical receiver circuit utilizes an inexpensive OPF-792 type PIN photo diode. In addition, the detector circuitry uses
two Op-Amps. One behaves as a non-inverting buffer while the second one acts as an inverting amplifier. The output of
the second amplifier is fed to a signal processing circuit that basically works as a comparator. The output signal from the
receiver circuit is compared to a predetermined threshold. The threshold level is adjustable; hence, the decision-making
circuitry can be tuned to any desired fluid. Figure 9 gives the schematic diagram of the detection circuit, which is a
combination of the receiver and signal processing circuitries.
Signal Processing
+5v
4
V
vLM324
11
12v
-12v
Receive; Circuit
OPF792
+12v
12v
4
.`- R3
-;:2200k
+12v
V+
R11
V
10k
4
LM324
11
V+
LM324
v-
o -12v
-12v
11
R1 100k
-12v
Figure 9: Schematic diagram of the signal detection and processing circuit.
Generally, successful operation of any intensity modulated optical system requires intensity compensation for undesired
environmental effects such as micro-bends or the drift in the transceiver output. Due to its digital nature, this design does
not require any intensity compensation schemes. In an extreme scenario, a portion of the output can be fed back to a
detector to cater for any deviations in the system. In this case, additional intelligence could be built into the system, and
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an algorithm can be developed that will use the feedback to determine the type and quality of the measurand. Wavelength
dependent methods could also be used to discriminate between different fluids.
3. CONCLUSION
A novel, quasi-continuous fiber optic liquid level sensor exploiting Fresnel reflections is presented. This design has been
employed to determine the levels of a broad range of fluids and successfully tested for a host of liquids including liquid
nitrogen, vegetable oils, and water. The proposed fiber-optic level sensor exploits changes in reflection of light when it
comes in contact with a target liquid to measure the fluid levels. The measuring system is inherently safe, compact,
lightweight, simple, and low cost in nature.
In short, this endeavor has produced a universal optical fiber liquid level sensor composed of point sensing optical switch
architecture, which can be used to detect presence or absence of a broad range of liquids under almost any environment.
This design significantly reduces the sensor diameter and eliminates stresses induced on the fiber due to physical conditions
such as tight bending radii. As a result, this leads to a mechanically rugged, low cost, and simple system that can endure
thermal stresses caused by cyclic cryogenic applications. The digital nature of this system greatly enhances the reliability
of the overall design. It is composed of readily available off-the-shelf components and can be deployed quickly in an
inexpensive fashion.
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