Development of a Portable Water Turbidimeter … of a Portable Water Turbidimeter ... circuit and controlled by a ... used to constitute a constant current power supply circuit. When

Sensors & Transducers, Vol. 26, Special Issue, March 2014, pp. 75-83

75

SSSeeennnsssooorrrsss &&& TTTrrraaannnsssddduuuccceeerrrsss

© 2014 by IFSA Publishing, S. L. http://www.sensorsportal.com

Development of a Portable Water Turbidimeter Based on NIR Spectroscopy

Qin Zhang, Lihua Zheng, Xiuchen Mi, Yao Zhang, Minzan Li

Key Laboratory of Modern Precision Agriculture System Integration Research, Ministry of Education, China Agriculture University, Beijing, 100083, China

Tel.: 18612628349, fax: 86-62737924 E-mail: [email protected]

Received: 19 November 2013 /Accepted: 27 December 2013 /Published: 14 March 2014 Abstract: Turbidity of water is a very important parameter in the aquaculture. An integrated portable optical detector was developed based Near Infrared Spectrum spectroscopy to measure turbidity of water body rapidly and accurately. And the integrated absorbance measuring approach was proposed in this paper. The detector used AT89S52 microcontroller as its control chip and consisted of the light emission unit, detection unit and control unit. Wherein, the light emission unit comprised two sets of 860 nm LED light source and the corresponding collimating lens; the detection unit comprised two photoelectric sensors; the control unit included the light source driving circuit, two-stage amplification circuit and A/D conversion circuit. The integrated detector was calibrated in laboratory and carried out field experiments. The experiment results showed that the water turbidity detector achieved a high accuracy, its calibration R2 was 0.994 and validation R2 was 0.999. With the resolution of 0.01 NTU and the error of ± 2 %, the turbidimeter reached the practical level. Copyright © 2014 IFSA Publishing, S. L. Keywords: Aquaculture, Turbidity, Optical detector, NIR, Integrated approach. 1. Introduction

The water body is the living environment

for aquatic animals. The turbidity is one of the important indicators of water quality [1]. The turbidity is an optical effect which exhibits the degree of light going through the aqueous layer when subjected to blocking by water body, it is mainly affected by the amount of particles in water and their size and shape [2-3]. The turbidity analysis methods are categorized into the following several ways according to the light reception mode: Transmission method [4], Scattering method [5-6], Surface scattering method [7], Scattered light-Transmission

light ratio method [8] and the column integral method [9]. Above methods have been put into production.

At present, many foreign instrument companies produced turbidity meters with advanced technology and excellent performance. Such as, 2100p portable turbidity meter of Hash (HACH) company in the United States, Turb355T/355IR portable turbidity meter of Germany WTW instrument company, Turbiquant 1500IR portable turbidity meter of Germany MERCK company, etc. All of those products have high accuracy, good sensitivity and reliability. However the resolutions of them are limited within the high turbidity range.

In China, almost 80 % of water turbidimeters are imported. Although varieties of independent

Article number P_SI_544


76

developed turbidity meters have been launched in recent years, the type of the products are one fold and most of them are desk-mounted so that the aquacultural applications were limited. Besides, their measurement ranges and application environment are strictly limited as well [10]. This study put forward a new turbidity measurement approach and developed an integrated turbidity meter to solve the above problems.

2. Measurement Approach

2.1. Measurement Method

When a bunch of infrared light with luminous

intensity of oI emits into water sample, due to the

absorption and scattering by the suspended solid and impurity in water samples and water body itself, the transmission light through the water sample

is attenuated to the luminous intensity of tI , and the

intensity of absorption and scattering of light are aI

and sI respectively. According to the law of Beer,

when the path length of the cuvette of d is a constant, the absorbance of A of the measured solution is proportional to the concentration of c. The precondition applying the Beer law is restricted as follows: (1) the tested solution is transparent; (2) when the light emits into the solution, only light absorption occurs at all the same time, with no light reflection, scattering and fluorescence occurring [11]. In practice, by measuring the transmission light intensity of the standard solution (distilled water) and test solution (water sample), the absorbance of A can be calculated as equation (1).

( ) lg[ ( ) / ( )]s tA I I , (1)

where ( )A is the absorbance at a certain

wavelength of light, ( )sI is the transmitted light

intensity of the standard solution and ( )tI is the

transmitted light intensity of the test solution. This detector used two incident light beams to

emit into the water sample at the same time. The scattering and transmission integrated light intensity of the standard solution (distilled water) was

measured and represented using '( )sI . Then that of

the test solution (water sample) was measured as

( )zI . By using '( )sI and ( )zI , the absorbance

was calculated with equation (2).

( ) lg[ '( ) / ( )]s zA I I , (1)

where '( )sI is the scattering and transmission

integrated light intensity of the distilled water

and ( )tI is the scattering and transmission

integrated light intensity of the water sample. With this method, the preconditions of Beer's law were better met than equation (1) and it guaranteed the linear relationship between the water turbidity and absorbance. 2.2. Design and Development of Turbidimeter 2.2.1. Overall Structure

According to the principle of measurement and

the functions that need to be implemented, the turbidimeter was designed as show in Fig. 1. It mainly consists of optical device, sample cell and control circuit.

Fig. 1. The overall structure diagram.


77

2.2.2. Design and Development of Optical Device

Using 860 nm LED lamp as the light sources [12],

two beams of light perpendicular to each other and vertical to the sample emitted into the water body at the same time. The photoelectric sensor installed at the opposite of each light source is to receive scattering/transmission integrated light intensity at the same time. To ensure the light incident emit into the water body parallel, the collimating lens is added behind each light source. To improve detecting accuracy, the average of the two turbidity that were measured by two photoelectric sensors were calculated. The optical structure of the turbidimeter is shown in Fig. 2.

2.2.3. Control Circuit Design and Development

1) The block diagram of signal processing (Fig.3). The equipment used a 9 V battery power supply

for each circuit after converted by the switching circuit and controlled by a master switch on the device panel. The optical signals were emitted by LEDs upright incidences to the sample and then were received by the photoelectric sensors. Then the obtained current signals of I were amplified by two-stage amplifier circuits and U1 were got. After simple

filter processing, they were respectively inputted into the A/D converters and digital signals were obtained accordingly. Finally the digital signals of U2 were transmitted into the microcontroller to be processed. The processed data could be storage by U disk and also could be displayed on LCD in real-time.

2) Power conversion circuit and constant-current circuit.

The portable turbidity meter used a 9 V battery power as its power source. Since the operating voltage of each chip in the device is 5 V, a power conversion circuit was developed to achieve power conversion. LM7805 Series has 3-terminal positive voltage regulator circuit and the TO-220 package, and it can provide a variety of fixed output voltage and a wide range of applications. It contains the protection circuits for overcurrent, overheating and overload. With the heat sink, the output current can be up to 1 A. Voltage conversion circuit is shown in Fig. 4.

Due to the instability of power supply can cause fluctuations of light intensity and spectrum, a constant current source circuit was developed to supply LED light. As shown in Fig. 5, LM317 was used to constitute a constant current power supply circuit. When LM317 input was grounded, 1.25 V stable voltage could be provided between the output end and the control-end, and hence the constant-current source for LED could be supplied.

(a)

(b)

Fig. 2. Structure of the optical device.

Fig. 3. Block diagram of signal processing.


78

Fig. 4. Voltage conversion circuit.

GND

Vout2

Vin3

LM317

0.1uFC13

100ΩR19

20Ω

R20D2LED

LED2

Fig. 5. Constant current source circuit.

3) Two-stage amplifier circuit. The turbidity meter used two-stage amplifier

circuit which was shown in Fig. 6. First level amplifier circuit was mainly constituted by CA3140 chip, its working process was as follows. The silicon photocell converted the collected signal into the current signal, then the current signal was converted to voltage signal through CA3140; its pins of 4, 5 were connected to R1, R2 and R4; by adjusting R4, the dark current could be eliminated, and by adjusting R5, the magnification factor could be changed to achieve the first stage amplification.

The second stage amplification was achieved by the LM358 chip. LM358 consists of two operational independent amplifiers with high internal frequency compensation and wide voltage ranges. The two signals from two first level independent amplifier circuits shared this chip together. The magnification factor could be changed by adjusting R9, and the second stage amplification was main portion of amplification. A low-pass filter consisted of R10 and C3 could achieve a simple signal filtering and reduce the noise effectively.

Fig. 6. Amplifier circuit.

4) A/D conversion circuit. Two amplified analog signals were converted into

digital signals via the A/D conversion circuit shown in Fig. 7. Then they could be transmitted into AT89S52 for further processing. A/D conversion circuit was mainly composed of an analog-digital conversion chip of AD7705 and REF195. AD7705 is a 16 bit resolution, 2 fully-differential analog input channels with three-wire serial input that can reduce the hardware space. Using SPI compatible three-wire serial interface, it can be easily connected to the MCU, and it is better than a parallel interface in saving IO of MCU. Through connecting with REF195, it can make reference voltage to 5 V to meet the requirements.

Fig. 7. A/D conversion circuit.


79

5) U-disk module. Taking into account the equipment's portability,

U-disk was used for instant storage. In order to save pins and memory of the microcontroller, Nanjing Heng Qin’s production was selected and its serial version U-disk file reading and writing modules were used. The communication between the microcontroller and storage module was achieved via MAX232 chip. MAX232 chip and U-disk module were shown in Fig. 8.

Fig. 8. MAX232 chip and U-disk module. 6) The other chips used in the circuit.

AT89S52 microcontroller was used as its control chip. AT89S52 is a low-power, high-performance CMOS 8-bit micro-controller, with 8 k in-system programmable Flash memorizer and suitable for general programming. LCD display circuit uses LCM128645B LCD module, with display of 128*64 dot matrix, dot size of 0.48*0.48 mm2, the dot pitch of 0.04 mm, Chinese database of more than 8000 characters, and serial/parallel dual-use interfaces. The serial interface was used. The keyboard module contained the start button, measurement button and calibration button.

2.2.4. Software Development

In accordance with the functions that the detector was designed to achieve, the software embedded

in the device was developed. It consisted of a main module and several sub modules.

The main module was mainly to complete the system initialization, LCD initialization and display the main window of the system, judgment button and call the subprogram, etc. The sub modules mainly consisted of turbidity calculating module, communication and storage module, LCD display module, A/D conversion control module and keyboard input module. The processing flow chart of the main module and turbidity calculating module were shown in Fig. 9.

Fig. 9. Flow diagrams of the main module and turbidity calculating module.


80

2.3. Experiments 2.3.1. Performance Experiment

For investigating the reliability of the principle of the detector, the comparing experiment between the detector and Shimadzu spectrometer is done. Firstly, The Formazine standard solution of 4000 NTU was prepared by mixed hydrazine sulfate solution and hexamethylene tetramine solution by the proportion. Then it was diluted with distilled water and finally 24 samples were obtained with the turbidity of 0, 2, 4, 6, 8, 10, 20......100, 200......1000 NTU.

The experiment process: The absorbance values of 24 Formazine standard samples were measured using Shimadzu UV-2450 with quartzose cuvette and the developed turbidity detector respectively on April 22th, 2013. We total did four experiments, and calculated the corresponding average values.

2.3.2. Calibration Experiment

To complete calibration of the instrument. The Formazine standard solution of 4000 NTU was prepared and diluted to 24 samples as the above. The water turbidity detection model was calibated using the standard samples. Firstly the transmitted and scattering light through the distilled water sample were detected and the corresponding average voltage value was calculated and stored as the reference datum. Then those through each standard sample were detected and their average value was calculated. Fanally, the absorbance of each standard sample was calculated by using the formula (2). With 19 samples as calibration group and the remaining data as validation group.

2.3.3. Field Experiment

The field experiment was carried out in Beijing Olympic forest park on May 4th, 2013. 64 water samples were collected and their turbidities were measured by using the developed detector and WGZ-B respectively. WGZ-B is a portable turbidimeter according to the principle of 90° scattering. 2.3.4. Reproducibility Experiment

In order to verify the stability and reproducibility of the instrument. We measured partial turbidity standard solution(0, 10, 20……100, 200, ……1000) at different way. Test 1 and test 2 were measured at different times: Test 1 was done when the apparatus switched on for five minutes and test 2 was done when the apparatus switched on for two hours. Test 3 and 4 were measured at different instruments:

Test 3 was done with instrument 1 and test 4 was done with instrument 2, and both were at the same time.

3. Results and Discussions 3.1. The Performance Experiment

and Discussions

According to the performance experiment. Correlation analysis was carried out and the result was shown in Fig. 10. It is easy to find that there have high correlation between these two devices. To prove that the the principle of the detector is reliable.

Fig. 10. The verification results of performance test.

3.2. Calibration Experiment and Discussions

The water turbidity model was established as shown in the equation (3). As shown in Fig. 11, its calibration R2 was 0.994 and the R2 was 0.999.

16.999-1121.8x=y , (1)

where y is the turbidity of water sample, x is the

absorbance of water sample.

Fig. 11. The calibration modeling.


81

According to the equation, we could caculate the minimum value of y was 13NTU. The resolution of the detector was 0.01 NTU when turbidity value was during 13~100 NTU, while the resolution was 0.1 NTU during 100~1000 NTU. Because the aquaculture water contains high nutrients, so the water turbidity can not too low. For this reason the turbidimeter can satisfy the measurement requirements.

The results showed that the model could be embedded into the turbidity meter to measure turbidity of real water samples. The instrument measuring range was 13~1000 NTU. Comparing with the turbidimeter under the same precision level, the detector improved the measurement range (WGZ-B, with measuring range of 0~200 NTU, produced by Shanghai XinRui company). Comparing with the turbidimeter under the same detecting range, the detector improved its resolution (Germany WTW production Turb355T/ir 355 portable turbidity meter, value accuracy is 1 NTU during 100~1000 NTU).

3.3. The Field Experiment and Discussions

After eliminating 4 outlier samples and 10 unpredictable values, correlation analysis results are shown in Fig. 12.

Fig. 12. The sampling test results.

The results showed that the data measured by the developed turbidity meter and commercial product had a high correlation. Due to many solid particles were contained in the samples, the turbidity was obviously changing with the solid particles moving. Hence it was difficult to ensure that both the measuring condition keeping definitely consistent. Therefore there were errors occoured when using different instruments.

The results showed that the developed turbidity meter could work continuously and had high stability. The results of stability test are shown in Fig.

13, and the reproducibility experiment results are shown in Fig. 14.

(a) Data of Test 1

(b) Data of Test 2

(c) Correlation between two data set

Fig. 13. The results of stability test.

Although the detectors in production should have certain difference, the result showed that there was still a very strong correlation between two data set


82

detected by two instruments. Hence it can be concluded that the developed turbidity meter have high reproducibility and could be used in aquaculture production.

(a) Data of Test 3

(b) Data of Test 4

(c) Correlation between two data set

Fig. 14. The results of reproducibility test.

3.4. The Field Experiment and Discussions

According to the data obtained from the field experiment, correlation analysis results are shown in Fig. 15 after eliminating 4 outlier samples.

The results showed that there was a high correlation between the data measured by the developed turbidity meter and commercial product respectively. Due to many solid particles contained in the water samples, the turbidity was obviously changing with the solid particles moving. Hence it was difficult to ensure that both the measuring condition keeping definitely consistent. Therefore there were errors occoured when using different instruments.

Fig. 15. The sampling test results. 4. Conclusions

This study proposed a turbidity measurement principle based on the near-infrared light absorbance integrated approach. An integrated portable optical detector was developed on the basis of that principle, and performance and reproducibility tests were carried out. The turbidity meter could measure the scattering/transmission integrated light intensity simultaneously. By using standard turbidity water samples, the turbidity calibration model was established based on the integrated absorbance intensity. Then the model was embedded in the detector to carry out the field experiment. The following conclusions were achieved.

1) The scattering/transmission integrated light signals were detected simultaneously by emitting two perpendicular beams of light into the water body vertically at the same time. With this method, the preconditions of Beer's law were better met than equation (1) and it guaranteed the linear relationship between the water turbidity and absorbance.

2) The turbidity meter adopted two detectors to measure the absorbance simultaneously and the average absorbance could be calculated. It could reduce the error of measurement as well the over reliance on the external environment so as to improve


83

the detecting accuracy, achieve the high precision and provide a wide measurement range.

3) Performance test and field experiment results showed that the developed turbidity meter based on the principle of the integrated absorbance approach reached a very high accuracy and could be used to measure the turbidity in aquaculture. Stability and reproducibility test showed that the developed turbidity meter have relative high reproducibility and stability.

Acknowledgements This paper was supported by the National Science and Technology Support Plan Project (2011BAD21B01). References [1]. Tai Haijiang, Li Daoliang, Wei Yaoguang, et al.,

A Simple Temperature Compensation Method for Turbidity Sensor, Computer and Computing Technologies in Agriculture IV, Springer Berlin Heidelberg, 2011, pp. 650-658.

[2]. Orwin J. F., Smart C. C., An Inexpensive Turbidimeter for Monitoring Suspended Sediment, Geomorphology, Vol. 68, Issue 1, 2005, pp. 3-15.

[3]. Dong Dasheng, Cheng Qun, Chen Shizhe, et al., Design of Chlorophyll a and Turbidity Sensor, Instrument Technique and Sensor, Issue 10, 2012, pp. 15-16.

[4]. Zhang Daode, Design of the Digital Turbidity Sensor Based on Infrared Ray, Optoelectronic Technology, Vol. 24, Issue 4, 2004, pp. 246-256.

[5]. Wang Dong, Sheng Qiang, He Xiaogang, Experimental Design of Scattering-Light Turbidity Base on ATmega16, Journal of Taiyuan University of Technology, Issue 1, 2010, pp. 80-82.

[6]. Ye Yanlei, Yang Likun, Ye Bing, Qin Huawei, Low-power Underwater Turbidimeter System, Journal of Mechanical & Electrical Engineering, Vol. 30, Issue 5, 2013, pp. 574-576.

[7]. Hong Zhi, Ge Jianhong, Application of Optical Automatic Gain Control to Measurement of Turbidity, Opto-Electronic Engineering, Vol. 27, Issue 2, 2000, pp. 60-62.

[8]. Zhang Kai, Zhang Yujun, Yin Gaofang, et al., Measurement of Water Turbidity Combined with Scattering and Transmission Method, Journal of Atmospheric and Environmental Optics, Vol. 6, Issue 2, 2011, pp. 100-105.

[9]. Zhang Li, Han Guocai, Research and Function Test of a New-type Column Integrating On-line Intelligent Turbidimeter, Chinese Journal of Scientific Instrument, Vol. 27, Issue 8, 2006, pp. 916-919.

[10]. Yue Shunlin, Cheng Guoguan, Tong Jun et al., Measurement of the Turbidity of Water with Low Turbidity, Water Purification Technology, Vol. 29, Issue 3, 2010, pp. 48-53.

[11]. Li Minzan, Spectral Analysis Technique and its Principle, Science Press, 2006.

[12]. Bai Jinwei, Zhang Deyuan, Liu Chang, Research of Turbidity Measuring Influence Caused by Two Different Light Source, Optical Instruments, Vol. 30, Issue 2, 2008, pp. 1-3.

___________________

2014 Copyright ©, International Frequency Sensor Association (IFSA) Publishing, S. L. All rights reserved. (http://www.sensorsportal.com)

Documents

Development of a Portable Water Turbidimeter … of a Portable Water Turbidimeter ... circuit and controlled by a ... used to constitute a constant current power supply circuit. When