Sensors and Actuators A 109 (2003) 21–24

A novel liquid level detection method and its implementationEldar Musayev, Sait Eser Karlik∗

Department of Electronics Engineering, Faculty of Engineering and Architecture, Uludag University, 16059 Gorukle, Bursa, Turkey

Received 21 October 2002; received in revised form 21 October 2002; accepted 13 July 2003

Abstract

There are various methods and implementations for level detection. Optical methods display a rapid growth among these with their highsensitivities and low costs. In this paper, we propose a new method that uses reciprocally placed LEDs and phototransistors whose opticalaxes are shifted according to each other. Shifting distance is determined by the direction of the change in the fluid level. The minimumvalue of the shifting distance depends on dimensions of the LED and the phototransistor. The advantage of this method is that opticalproperties of the liquid do not have significant effects on level detection phenomenon. Our method can be used in detecting levels of waterand inflammable fluids (gasoline, fuel oil, alcohol, etc.).© 2003 Elsevier B.V. All rights reserved.

Keywords: Optics; Optoelectronics; Liquid level sensors; LED; Phototransistor

1. Introduction

Level detection plays an important role in commercial andtechnological applications. Level can be detected with vari-ous methods such as mechanical, capacitive, inductive, ultra-sonic[1], acoustic[2] or optical[3–16]. The basic principlethat must be considered in selecting the detection method isthe property of the material that will be detected. For exam-ple, mechanical and ultrasonic methods are used to detect thelevel of solid materials that are in the form of dust. Capaci-tive and optical methods give better results in detecting thelevel of fluids. For inflammable fluids, optical methods aremore appropriate since light does not have any effect on thematerial that is to be detected. Also optoelectronic methodsare more simple and can be adapted to any medium easily.A new method is proposed for level detection in this paper.

2. The novel level detection method

The material whose level will be detected is inserted be-tween reciprocally placed light source and photodetector andcuts off the light in classical optoelectronic level detectionmethods. But for liquids, applications of these methods are

∗ Corresponding author. Tel.:+90-224-4428178x1138;fax: +90-224-4428021.E-mail addresses: eldar@uludag.edu.tr (E. Musayev),ekarlik@uludag.edu.tr (S.E. Karlik).

limited with optical properties of the liquid, e.g. the levelof any transparent liquid cannot be detected by reciprocallyplaced LED-phototransistor method. That is, although theliquid is placed between LED and the phototransistor, lightcan reach to the phototransistor. This limits the applicationfields of optical methods.

To expand application areas of optical methods and tominimize the effect of liquid properties, we propose a newmethod which does not depend on liquid properties. Theoptical diagram of this method is shown inFig. 1.

As shown inFig. 1, where PT is the phototransistor,dLand dPT are diameters of LED and the phototransistor, re-spectively,ϕL is the angle of the LED radiation diagram,ϕPis the detection angle of the phototransistor,a is the distancebetween LED and the phototransistor,�L is the liquid levelvariation between the lower end of the phototransistor andthe higher end of LED, LED and phototransistor are placedreciprocally but their optical axes are shifted by a distancea. Let us explain our new detection principle.

When there is no liquid between LED and the phototran-sistor, light that is being emitted from LED can fall on thephototransistor. As liquid level rises, it begins to block thedetection surface of the phototransistor and some percentof the light reflects from the liquid surface. When the liq-uid level reaches toL + dL, detection surface of the pho-totransistor is completely blocked and LED light reflectsfrom the liquid surface. This goes on till the levelL + a.When the liquid level rises to a higher extent, LED light fallson the phototransistor by passing through the liquid. The

0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0924-4247(03)00347-9

22 E. Musayev, S.E. Karlik / Sensors and Actuators A 109 (2003) 21–24

Fig. 1. Optical diagram of the new level detection method.

transmitted power of light can be computed with

PL = KLIL = KLVCC

RL(1)

whereRL is the current-limiting resistor,VCC is the sourcevoltage andKL, which is generally given in catalogues, isthe conversion constant of current to light. For an infraredLED,

KL = PF

IF(2)

where IF is the LED nominal current and has a value of100 mA andPF is the power of light under nominal current.

Considering that the distance between LED and the pho-totransistor is small, the light power that reaches to the pho-totransistor can be given by Bugger–Ber Law

PPT = PLexp(−cd) (3)

wherec is a coefficient which depends on liquid properties,d is the distance between LED and the phototransistor. Ifτ = e−c is considered as the light permeability coefficientof the liquid, the power that reaches to the phototransistorcan be expressed as

PPT = KLVCC − VLED

RLτd (4)

As it is seen in (4), whereVLED is the LED threshold voltage,the amount of light that is transmitted to the phototransistordepends on the output power of LED, light permeabilitycoefficient of the liquid and the distance between LED andthe phototransistor.

Considering that the distance between LED and the pho-totransistor is small, one can assumeτd = 1 to simplifycomputations.

Using electrical connections in the control region can bedangerous in some cases, e.g. inflammable fluids. Opticalfiber connections can be used in these cases. The opticaldiagram of the level detection system that uses optical fiberconnection is shown inFig. 2.

Optical fiber connections allow the design of a more sen-sitive level detection sensor. Level detection principle is sim-ilar to the previous method. Optical axes of fibers are shiftedin this system and the distancea can be smaller. The valueof a is limited by dimensions of LED and the phototransis-tor in the previous system.

Fig. 2. Optical diagram of the level detection system with optical fiberconnection.

3. Design and experimental results of the optoelectroniclevel detection sensor

To simplify the research, water is used as the liquid. Thediagram that explain locations of sensors which detect min-imum and maximum levels of the water is shown inFig. 3.

In general, either analog or relay type systems are used inlevel detection. The method that we propose can be used inboth systems. Minimum and maximum levels of the liquidare determined and sensors are set to these levels in relaytype level detection systems, as shown inFig. 3. Sensorsin Fig. 3 are composed of LED1–PT1 and LED2–PT2 pairswhich are located atLmax andLmin levels. The liquid whoselevel will be detected is moving up and down in a basinbetween LED-phototransistor pairs. The minimum level ofthe liquid is detected byT1–R1 pair and the maximum levelis detected byT2–R2 pair, as shown inFig. 3.

Each transmitter is composed of a LED and a bipolartransistor while phototransistors, which are connected in theform of emitter followers, are used as receivers. Simplifiedcircuit diagram of the level detection sensor is shown inFig. 4. The system is controlled by a microprocessor. Pulsesthat have a duration of 100�s and a duty cycle of 25%are generated at A and B outputs of the microprocessor.These pulses are shifted in time domain with a duration of150�s according to each other and are given to sensors forminimum and maximum level detection. Pulse shifting intime domain prevents low and high level detection sensorsfrom effecting each other, i.e. if these sensors are located tooclose to each other, light coming fromL1 can be detected by

Fig. 3. Location of sensors detecting minimum and maximum levels.

E. Musayev, S.E. Karlik / Sensors and Actuators A 109 (2003) 21–24 23

Fig. 4. Circuit diagram of the level detection sensor.

PT2 while light coming fromL2 can be detected by PT1 andthis will cause a serious problem unless there is a precautionlike pulse shifting in time domain. Operating transmittersin pulse mode decreases the amount of the current suppliedfrom the source.

Passing through the basin of the liquid, pulses generatedby transmitters are detected with phototransistors PT1 andPT2 and are converted to photosignals. Having been pro-cessed in comparators, these photosignals arrive to inputsIn1 and In2 of the microprocessor. The microprocessor pro-duces a signal proportional to the liquid level. Some othercircuits can be connected to the output of the microproces-sor according to system requirements.

An SFH409 type LED and an SFH309F type phototran-sistor are used in the structure of the level sensor. Some im-portant characteristics of these devices are given inTable 1.The distance between the transmitter and the receiver is de-signed to bed = 6 mm in the implementation.

Designed sensors are located to minimum and maximumlevels of the basin of the liquid and are connected to themaster electronic board. First, the distance between opticalaxes of the transmitter and the receiver (LED and the pho-totransistor) is selected to be

a = amin = dL

2+ dPT

2= 3 mm

Changing the level of the liquid, the normalized photosignalvariation for one of the sensors is found as shown inFig. 5.

Then, the distance between axes of LED and the photo-transistor is changed toa = 8 mm and the graphic of photo-signal variation versus liquid level is obtained as shown inFig. 6.

Table 1Characteristics of LED and the phototransistor used in level sensors

LED SFH 409 Phototransistor SFH 309F

Forward current (IF) (mA) 100 Collector-emitter voltage (VCE) (V) 35Forward voltage (IF = 100 mA) (V) 1.5 Collector current (IC) (mA) 15Total radiant flux (IF = 100 mA) (mW) 15 Dark current (VCE0 = 25 V, E = 0) (nA) 1Wavelength at peak emission (λpeak) (nm) 950 Wavelength of maximum Sensitivity (λmax) (nm) 900Half angle (ϕL ) (◦) 20

Fig. 5. Normalized photosignal variation vs. change in the fluid levelwhen the distance between the axes of LED and the phototransistor isa = 3 mm.

Fig. 6. Normalized photosignal variation vs. change in the liquid levelwhen the distance between the axes of LED and the phototransistor isa = 8 mm.

As it can be easily seen fromFigs. 5 and 6, increasingthe value ofa causes a gap to occur and does not effectthe shape of the graphic. If the comparator is adjusted tothe half-level of the photosignal, maximum error of such asensor can be computed as

δmax = Lr − (L + dL/2)

Lr× 100% (5)

whereLr is the real level of the liquid detected with a moresensitive method,dL is the diameter of LED anddL = dPT.

The error depends on the radius of the LED as it is shownin (5). To decrease this error, transceivers with smaller radiican be used, e.g. optical fiber connections.

24 E. Musayev, S.E. Karlik / Sensors and Actuators A 109 (2003) 21–24

4. Conclusion

A new optical level detection method has been proposedin this study. Axes of LEDs and phototransistors are shiftedaccording to each other in our method. Shifting distanceis determined by the direction of the change in the liquidlevel. Minimum value of the shifting distance depends ondimensions of LED and the phototransistor. It is found thatamin = dL = dPT. An optoelectronic level detection systembased on this method has been designed and graphics ofphotosignal variation versus fluid level have been obtained.

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Biographies

Eldar Musayev received his MSc degree in electrical and electronics en-gineering from Fergana Polytechnique University, Fergana, Uzbekistan,in 1974. Between 1979 and 1986, he received his PhD and associate pro-fessor degrees from Moscow Giredmet Scientific and Technical ResearchCenter, Taskent Technical University and Fergana Technical University.Currently, he is an associate professor in Electronics Engineering De-partment of Uludag University, Bursa, Turkey. Dr. Musayev is the authoror co-author of 6 books, 4 course notes, 98 scientific papers and holds19 international patents. His research interests include electronic and es-pecially optoelectronic circuit and system design, optoelectronic sensorsand optical measurement techniques.

Sait Eser Karlik received his BSc degree in electronics and telecommu-nication engineering from Istanbul Technical University, Istanbul, Turkeyin 1997 and his MSc degree in electronics engineering from UludagUniversity, Bursa, Turkey in 1999. Currently, he is a research assis-tant and working toward his PhD degree at Electronics Engineering De-partment of Uludag University. His research interests involve fiber op-tic communication systems, distributed fiber sensors and optoelectronictransducers.