Can real-time turbidity online monitoring be realized easily?

In fields such as meteorological environment monitoring and product quality inspections, turbidity is related to many physical quantities. This is why people often need to measure turbidity, particularly to perform real-time online turbidity monitoring. These tests require a high-accuracy and high-speed measuring system. So,

Turbidity measurement is often necessary in fields like meteorological environment monitoring and product quality inspections. Turbidity refers to the obstruction degree of the suspended matter and colloidal matter in colorless transparent liquids for light transmission. Under ideal conditions, it can be obtained through theoretical calculation; however, in practical projects, it is greatly influenced by the size, shape, surface structure, and properties of the particles in the colorless transparent liquids. Thus, it is necessary in practical projects to conduct actual measurement to obtain accurate turbidity parameters.

To solve this problem, a method of laser transmission is used for measurement in light of the relationship between transmittance and absorbance. Only high-accuracy and high-speed data acquisition digitizers can complete the data acquisition in this process and guarantee real-time online monitoring for the system. In addition to the stability of the laser driver circuit and the quality of the signal conditioning circuit, the reliability and operating speed of the data acquisition platform will be a major challenge in achieving the real-time online monitoring objective of the system.

Considerations in the system design

See Fig. 1 for the structural diagram of the system. After passing through a beam splitter, the modulated laser is divided into two beams. One of the two beams goes through the turbid liquid to be measured and meets the receiving end 1; the other meets directly with receiving end 0. After signal processing at the receiving ends, the two beams form two input signals and enter a data acquisition system to constitute a two-channel liquid turbidity measurement device.

Hardware design

(1) Light source driver circuit
The system uses a 635nm semiconductor laser as the light source. In order to reduce the interference of the noise signal and make it convenient to detect the desired signals, the laser source is modulated to be a 1KHz square-wave pulse. The frequency source is a 1MHz oscillator with high stability and accuracy. A frequency dividing circuit is used for a 1000-frequency division so the 1MHz signal can output a 1KHz square-wave signal with good frequency stability.

(2) Signal conditioning circuit
The system adopts a reciprocal receiving mode and uses a photodiode as a photoelectric detector. The photoelectric detector converts the optical signal into a current signal and then performs I/V conversion and pre-amplification to change the current signal into a voltage signal with larger intensity. In order to obtain the desired signal, a band-pass filter is used for filtering the voltage signal to obtain the desired 1kHz signal. In order to improve the intensity of the acquired signal, an in-phase proportional amplifier circuit is used for amplification.

(3) Data acquisition digitizer hardware
The data acquisition module of the system is designed with a high-performance PCI-9846H data acquisition digitizer. As a high-performance 16-bit four-channel product of ADLINK Technology Inc. (Fig. 2.), PCI-9846H data acquisition digitizers can realize sampling with a sampling rate of 40MS/s for input signals with a wide dynamic range and a high frequency range (up to 20MHz). The high sampling rate provides an effective guarantee for real-time online measurement. Using 512MB of onboard memory cache, PCI-9846H data acquisition digitizers are not limited by the transmission rate of the PCI bus and can record the waveform information over a longer time, allowing them to be deployed more extensively. Each PCI-9846H data acquisition digitizer is equipped with four high-linearity 16-bit A/D converters and can meet the ideal application need of equipment with large dynamic ranges. Each PCI-9846H data acquisition digitizer also has four synchronous analog input channels for synchronous sampling, with maximum sampling rates of 10MS/s, 20MS/s and 40MS/s to effectively meet the need for synchronous multi-channel data acquisition and guarantee synchronization for the data of the four channels, providing an effective guarantee for realizing a highly accurate measuring system.

The system is a two-channel measuring system which uses two (CH0 and CH1) of four available channels. The input signal of the CH0 channel is a reference signal, and the input signal of the CH1 signal is a measured signal which passes through the solution to be measured.

Software realization

Software systems are an important part of virtual instruments. The measuring system uses the software application of the Visual Basic development system. The software application has a very convenient interface design, a small programming workload and a short development period. Furthermore, ADLINK Technology Inc. also provides DAQPilot, a control interface suitable for the Visual Basic environment. The operational instructions for DAQPilot can be found in the related technical manual. DAQPilot is included with some program modules, making software development more convenient and software system development very easy. The software application interface of the system in a VB development environment is shown in Fig. 3.

The PCI-9846H data acquisition digitizer can be set in the DAQPilot wizard in the window program design interface. Choose channel 0 (CH0) and channel 1 (CH1) as the two ports for analog signal input. The sampling rate of each channel is set to 200000. The desired signal frequency input is 1KHz, so 200 data points can be effectively sampled in each signal period, fully meeting the accuracy requirements. Thus, the signal waveform data can be described very accurately. In addition to the abovementioned settings, the Properties sheet can be set for the X axis of the DGraph control to change the display status in the display window.

Test and performance analysis

(1) System test
In the actual system test, standard measuring tools were used for measuring the liquid volume, and some milk with an even concentration was titrated to change the turbidity of the solution to be measured. The test steps were as follows:

a) Purified water of a specific volume was added into a beaker.

b) The optical device was calibrated. After that, milk of a specific volume was added into the purified water. In the process, a rubber dropper was used for quantitative titration. After a drop of milk was added and evenly stirred and the turbid liquid became steady, the "Calculate" button on the system software was pressed. As a result, the intensity ratio of the two input signals was obtained. The intensity ratio is the transmittance. A group of data was measured following each titration.

The measured data was processed with MATLAB. The volume of the added milk was regarded as the horizontal axis, and the mean of the transmittance values measured was regarded as the vertical axis. Fig. 6 is the result of the point drawing.

According to the figure above, the tendency of the points complies with exponent characteristics and may be fitted by a proper exponential curve. The curve SSE fully meets the design requirements of the system.

(2) System performance analysis
If the volume of the added milk was regarded as the independent variable and the transmittance as the dependent variable, the independent and dependent variables met a certain functional relationship. From the comparison between the actual fitted curve and the theoretical derivation function, the following conclusions of the above system performance test were obtained:

a) The previous exponential factor of the fitted curve is 0.9928, which is smaller than but very close to the exponential factor 1 obtained based on the theoretical derivation formula. The difference between 0.9928 and 1 was caused by factors such as absorption and reflection. The actual measured transmittance is inevitably smaller than the theoretical value. Viewed from the actual fitted curve, the system, however, has high reliability.

b) The second factor in front of the second exponential term of the functional relationship is much smaller than 1. The exponential term is a deviation between the actual value and the theoretical value. According to the related technical data, if the particle concentration is high, there will be overlapping scattering and more light will be transmitted; in other words, the transmitted light will have larger intensity. This complies with the change tendency of the exponential term.

c) It may be concluded from the comparison between the mathematical model and the actual measured fitted curve function form that there is mainly one kind of particle in the milk which affects the intensity of the transmitted light. In order to verify the test result, we referred to the related data and found that milk with an even concentration mainly includes fat particles; the fat particles are bound to proteins, so they can exist stably in water; the milky white color of milk is mainly caused by the scattering of light on the fat particles bound to proteins. This reflexively proves that the fitted curve is consistent with actual conditions.
 

The system uses the PCI-9846H high-performance multi-channel data acquisition digitizers of ADLINK Technology Inc. As a result, the overall performance of the system is significantly improved, the measured data are highly reliable and accurate, and the expected development purpose is realized. Therefore, a photoelectric two-channel turbidity measuring system based on a high-speed data acquisition digitizer can realize real-time online monitoring. The design proposal of the system can be used in other application systems easily and has bright application prospects in fields such as environmental quality monitoring, water quality monitoring, meteorological parameter monitoring and food quality monitoring.

*LabVIEW™ is a registered trademark of National Instruments.

Related ADLINK Links:

• More about ADLINK Digitizers
• More about PCI-9816/9826/9846