Sensors for control of water transparency in optical and microwave ranges

V F Kravchenko, E V Krivenko, V I Lutsenko, I V Popov

(1. Kotel’nikov Institute of Radio Engineering and Electronics of Russian Academy of Sciences， Moscow 125009， Russia；2. Bauman Moscow State Technical University， Moscow 105005， Russia；3. Scientific and Technological Center of Unique Instrumentation of Russian Academy of Sciences， Moscow 119992， Russia；4. Usikov Institute of Radiophysics and Electronics of National Academy of Sciences of Ukraine， Kharkiv 61085， Ukraine)

Abstract： In the face of deteriorating environmental conditions in the world, water quality control is an urgent task. It can be solved by creating sensors with high accuracy and low cost, which requires the development of fundamentally new radiophysical methods that take advantage of the optical, microwave and millimeter wavelengths that have a significantly greater sensitivity to low concentrations of pollutants and a lower inertia. The article presents prototypes of measuring cells of the microwave and optical ranges as well as the results of an experimental study of water of various degrees of pollution with their help. The results show that the use of the highly sensitive method of capillary-waveguide resonance makes it possible to detect the presence of micro impurities in water with concentrations up to 0.1% and to identify water even from sources of various natural origins. In addition, the use of measurements at several frequencies in the optical range will make it possible to solve the problem of creating water control sensors with high sensitivity to pollution and low cost. It can be concluded that the possibility of complex use of multiwave sensors (optical, infrared and microwave ranges) allows to increase the sensitivity and reliability of water quality assessment.

Key words： optical sensor; light attenuation in the medium; optical transparency; capillary-waveguide resonator; dielectric characteristics; Q-factor and depths of resonance absorption

0 Introduction

One of the key problems of the 21st century will be the problem of providing mankind with water of the required quality and in sufficient quantity. To solve it, it is necessary to develop the scientific basis for providing water in the future using smart-grid technologies. The first attempts to create them[1-5]showed that one of the key moments will be the development of intelligent water quality sensors combining high metrological characteristics with a sufficiently low cost and the possibility of mass production. In this paper, we will consider the possibility of creating such sensors in the optical, infrared (IR) and microwave wavelength ranges.

1 Optical method

Transparency is one of the important characteristics that determine the consumer quality of water. Monitoring of transparency or turbidity may be based on a registration in the optical range of transmission and scattering coefficients of light for a controlled pattern of water for several wavelengths.

As dimness of water is usually accompanied by the change of its color (from bluish to yellow), it is expedient to use sensors in several wavelengths.

Figs.1 and 2 show the layout of water transparency meter for 5 optical wavelengths (650 nm, red; 550 nm, green; 450 nm, blue) and infrared wavelengths (850 nm and 940 nm). It uses 5 emitters operating at different wavelengths and a broadband receiver. Measurement of attenuation at different wavelengths is carried out by one channel receiver, sequentially in time.

To improve noise immunity against external light sources, low-frequency modulation of the radiation source (1 kHz) is used and the narrowband synchronous receiving of radiation is transmitted through the sample. The enlarged block diagram of sensor is shown in Fig.1, and a general view of a sensor unit in which the cell with tested water is placed is shown in Fig.2.

Fig.1 Flow diagram: 1 is a microcontroller, 2 is arithmetic-logics unit (ALU) of microcontroller, 3 is ADC of microcontroller, 4 are emitters (light-emitting diodes LED), 5 is the investigated object, 6 is a photodetector, 7 is a display

Fig.2 General view of multi-wave measuring device of transparency of water: 1 is measurement cuvette; 2 is a block for a cuvette with emitters and receiver; 3 is a display

Spectral sensitivity used therein broadband photo-receiver (opt101) is shown in Fig.3. There are plotted wavelengths are used to transmit through the sample also. A more detailed description of the sensor is given in Ref.[6].

Fig.3 Spectral sensitivity of receiver of multiwave sensor

The device generates the pulse sequence which is produced by 5 LED light sources with different wavelength emitted signals (sources from 1 to 5, left to right). The receiver of the scattered signal is the photodetector (opt101). After the signal is amplified by the photodetector, the signal is fed to the input of the 12-bit ADC in the microcontroller. The microcontroller performs the modulation of the emitted signal, and then digital synchronous detection of the signal is received by the photodetector, finally digital filtering and display of the information about the received signal level on the photodetector from each source (from 1 to 5, from left to right, from top to bottom) are completed on the display screen. The use of modulation makes it possible to exclude the influence of natural light, filtering allows to increase the signal-to-noise ratio, and 32-time sampling allows to increase the effective ADC resolution to 14 bits, which increases the dynamic range of the system. This device allows you to investigate the transmission and reflection coefficients of media at several wavelengths in the optical range. To reduce measurement errors caused by uneven spectral sensitivity of the receiver, the device is calibrated when the measuring cuvette is filled with air and distilled water with the same volume as the liquid being examined.

It is realized that the dynamic range of measurements of the luminous flux attenuates up to 30 dB to control the transparency (turbidity) of water in a wide range. The multiwave mode can increase the accuracy of measurements and to differentiate the sizes of polluting particles.

The dependence of the attenuation of the optical signal on the degree of contamination of ground water for the different wavelengths is shown in Figs.4 and 5, and the changing of its transparency at that. To control the transparency, we use the standard method of estimation of resolution as used in television. It should be noted that in a sufficiently wide range of contaminants (from 0 to 80%), the attenuation on the degree of contamination is satisfactorily described by a linear relationship. Accuracy evaluation of the degree of contamination by this method does not exceed 2%, which is quite acceptable in the practice of using it for water quality control.

The conducted researches have shown that the use of measurements at several frequencies of the optical range will allow to solve the problem of creating water monitoring sensors that have high sensitivity to pollution and low cost. A similar approach is used in Ref.[8] to assess the degree of water pollution in dishwashers and washing machines.

Fig.4 Attenuation on the degree of contamination of water (brown color) for different waves lengths in optical and IR ranges: 1 is 940 nm, 2 is 850 nm, 3 is red, 4 is green, 5 is blue, pollution

Fig.5 Optical transparency of water of brown color

2 Measurements in millimeter wavelength range

The presence of contaminants in water affects its dielectric properties. This circumstance can be used to control the degree of contamination of water. To detect contaminants, techniques of superhigh frequency (SHF) and extremely high frequency (EHF) dielectrometry may be used. Thus, the use of a highly sensitive capillary-waveguide resonance method[9]makes it possible to detect the presence of microimpurities in water with concentrations up to 0.1%, and, as studies shown, it makes it possible to identify water even from sources of different natural origins[10-15]. The cell of the dielectrometer is a capillary-waveguide resonator based on a rectangular waveguide with a cross-section 3.6 mm×1.8 mm and a dielectric capillary made of fluoroplastic with an outside diameter of 2re=1.5 mm and an internal capillary of 2ri=1.1 mm passing through the wide walls of the waveguide, as shown in Figs.6 and 7.

Fig.6 Laboratory unit for determination of characteristics of liquids by the method of capillary-waveguide resonance: 1 is capillary-waveguide resonator, 2 is a general view of the lab setting based on panoramic EHF-meter of Р2-69 type

Fig.7 Capillary-waveguide resonator

Measuring of the frequency and depth of attenuation is performed at resonance. Application of the reference polarization attenuator in the measuring path can reduce measurement error of decay to the level of 0.5 dB. The measurement error of resonance frequency and bandwidth does not exceed 0.1 GHz. Due to the resonances in the multilayer capillary inside of which liquid is under investigation, the system is sufficiently sensitive to microimpurities.

Fig.8 Descriptions of resonant absorption of water solutions with alcohol: 1 is a distillate; 2 is distillate + 1% of alcohol; 3 is a distillate +2% of alcohol; 4 is a distillate +4% of alcohol

The possibilities of the method are illustrated by the dependence of the frequency of the absorption resonance on the alcohol concentration in the aqueous solution (Fig.8) and the effect on the quality factor of the absorption resonance (Fig.9)[9].

It can be seen that the addition to water of alcohol at concentrations in units of percent is confidently registered by this method. The limited sensitivity of the method by concentration is approximately 0.2%. For water and aqueous solutions, the resonant absorption was observed at frequencies 63-66 GHz with a quality factor of 20-65.

Fig.9 Dependences of changes of inductivity (a) and reverse Q-factor of absorption resonance (b) on the concentration of solution C

For example, Fig.10 shows the frequency dependence of signal attenuation for several water samples (artesian and distillate). The greatest depth of the dip of the resonant absorption curve of electromagnetic energy was observed for the distillate of double distillation and it was ～43 dB (curve 1), for a distillate of single distillation it decreased to 41 dB (curve 2), and for artesian water it was 37 dB (curve 3).

Curves for absorption resonance for different types of water are shown in Fig.10. Distillate, melted and tap water are shown on Fig.11.

Fig.10 Descriptions of resonant absorption of different types of water: 1 is water 721; 2 is a distillate; 3 is a bi-distillate

Fig.11 Descriptions of resonant absorption of different types of water: ■ is distillate; ? is melted snow; ● is tap water

It can be seen that the distillate is characterized by the deepest resonant absorption. Thus resonance frequency, depth and quality factor can serve as indicators for assessing the purity of drinking water. The development of specialized measuring devices, and not universal ones, which were used in laboratory experiments, can significantly reduce the size of the sensor and its power consumption. The transition from millimeter to decimeter wavelength can significantly increase the size of the measuring capillary and simplify the design of the sensor, but this requires further development.

In addition to the capillary-waveguide resonance absorption method, the classical resonance methods, in particular, “whispering gallery” resonances, can also be used to estimate water quality. For this purpose quasi-optical dielectric (QOD) and partially shielded quasi-optical dielectric resonators[9]can be used, in which inhomogeneity is introduced in a capillary filled with a medium under investigation. It is shown that such a resonator can be used as an element of stabilization of an oscillator based on a Gunn diode. In this case, the change in the characteristics of the inhomogeneity due to water contamination leads to changes in the oscillation frequency and the steepness of the electronic tuning of the generator[13-15].

3 Design features of industrial water quality sensors

Unlike laboratory water sensors, industrial designs must provide the ability to monitor the required volumes and have the minimum requirements for the user’s qualification to provide the possibility of working for a long time with the required quality characteristics in automatic mode[8].

Structurally, the sensors of the optical detector which consists of a receiver and several radiators (optical and infrared bands) are located on opposite sides of a pipe with a diameter of approximately 20-25 mm and a length of up to 50 cm through which the investigated water flows. And a radio wave detector (of the decimeter band) uses a tube with a diameter of approximately 5-6 mm in the length of the working area of approximately 50-60 mm, through which the water under test flows. The sensors are freely integrated into the investigated water flow either directly or through a bypass circuit (a tapping is carried out in parallel to the main flow channel).

4 Conclusion

Complex use of multiband sensors (optical, IR and microwave ranges) allows to increase the sensitivity and reliability of water quality assessment.

1) In the optical and infrared ranges, as an informative parameter the signal attenuation is used as it propagates through the medium under study.

2) In the microwave band, while using the phenomenon of capillary-waveguide resonance, the parameters of the absorption resonance (frequency, quality factor and absorption depth) are informative. When using the resonances of higher types, such as “whispering gallery”, informative features of their self resonance frequency and quality factor are changing, which manifest themselves in changes in frequency and steepness of electron generator tuning are stabilized by this resonator.