WP402G Water Particle Counter

URUSHIBATA Shinichi¹

We have developed the WP402G Water Particle Counter and the WP412G Water Particle Counter software package. The WP402G can simultaneously measure the size and number of particles such as those of turbid matter in tap water. It is an on-line instrument for use in the field and is equipped with a sampling system to supply stable water flow into a detector. This paper gives an overview and principles of this counter, as well describes its superiority over turbidity meters, for example, in showing actual field test data.

1. Environmental & Analytical Products Business Div., Industrial Automation Business Headquarters

INTRODUCTION

Figure 1 WP402G Water Particle Counter (for Japanese market)

The Japanese Ministry of Health, Labor and Welfare has drawn up guidelines for the provisional measures against Cryptosporidiums in tap water and has instructed the water industry to maintain a finished water turbidity level of less than 0.1 degree. This demonstrates that turbidity control at filtration systems has become an important concern and the level of turbidity has been conventionally been used as an indicator for such systems. In addition, the spotlight has begun to be focused on a new filtration management method that directly measures the size and number of particles before and after filtration. This new method allows for more detailed monitoring of filtration conditions.

Under such circumstances, we developed the WP402G water particle counter that is capable of determining turbid matter containing particles that fall within the specific size ranges where protozoa such as Cryptosporidium (about 4 to 6 µm) and Giardia (about 5 to 15 µm) exist. The WP402G thus enables more advanced maintenance of filtration systems.

Cryptosporidium is a parasitic protozoan that belongs to the phylum Sporozoa. In water or food, it exists as an oocyst covered by a protective shell. If it infects human cells, it proliferates in the human body to cause diarrhea, nausea, or fever, and may even lead to death of a patient with immunity deficiency. The oocyst shell is so hard that it is said that chlorinating performed by regular water treatment plants cannot prevent the infection.

Supplying water to a detector at a steady rate is indispensable for stable on-line measurement over a long term. To meet this requirement, we created a particle counter system with excellent repeatability for continuous measurement by simultaneously developing and combining together a sampling system based on many years of experience and achievements which boasts improved flow rate control and a converter compliant to the JIS B9925 standard. Figure 1 shows an external view of the WP402G and table 1 lists its main specifications.

Table 1 WP402G Main Specifications

Measuring method	Semiconductor laser light extinction
Measuring range	Particle size: 2 to 400 µm (1 to 400 µm1)
Particle count setting	Size range: 2 to 100 µm (1 to 25 µm1) Number of size channels: Up to 8, programmable
Input signal	4 to 20 mA DC, 4 inputs
Output signal	RS-485 communication 4 to 20 mA DC, 4 outputs (optional)
Calibration	PSL (polystyrene latex) compliant to JIS Z8901
Repeatability	Within 5% of reading

Note *1: In the case of an optional measuring range

The WP412G is a software package for the WP402G that features simple operability and rich functionality, as well as providing an optimum environment for analyzing data measured with the WP402G. It can manage data in a centralized way from up to 32 WP402G units, for example, displaying trend graphs and alarm events, saving data, and calculating removal rates.

MEASUREMENT PRINCIPLE

Figure 2 Schematic Diagram of Detector

Figure 2 shows a schematic diagram of the detector, and figure 3 charts the voltage output from the detector. In figure 2, sample water runs through the flow cell less than 1 mm wide and made from fused silica. A collimated beam from a laser diode light source is continuously applied to the flow cell. At the opposite side of the light source is a photodiode with a photoreceptor for detecting the collimated beam. When a particle in sample water passes through the laser beam, it absorbs, reflects, or disperses the laser beam energy, thus reducing the light energy detected by the photodiode. The loss of light energy can be used to determine particle size, because it is proportional to the size of particle. Meanwhile, the sample water functioning as a medium to carry particles runs at a constant rate of flow, and the change in light energy comes out as pulses on the photoreceptor side, where one particle generates one pulse. The number of particles can be determined by counting the number of pulses.

Figure 3 Voltage Output of Detector

The minimum detectable size of a particle, or the minimum measurable particle size, depends on the relationship between the amount of loss of light energy and the level of light energy or electric noise. JIS B9925 defines the minimum measurable particle size as the lower limit of particle size that allows 50% or more of the theoretical number of particles to be measured. The minimum measurable particle size is normally around 1 to 2 µm for particle counters using the light extinction method. The minimum measurable particle size is 2 µm for the present WP402G which has a theoretical counting efficiency of 50%at a 2-µm measuring range, and 100% for particles of 3 µm or more. On the other hand, the maximum detectable size of particle mostly depends on the minimum dimensions of the detector structure including the flow cell through which the particle passes. Moreover, although the present measuring range of particle size is 2 to 400 µm, we are developing another option capable of a range of 1 to 400 µm to further reduce the minimum measurable particle size by a half.

The maximum detectable concentration of particles depends on the detector structure as well as the photoreceptor, the dynamic range and speed of signal processing per unit time of the electrical circuit. The higher concentration increases the probability of two or more particles irradiated by the laser beam. In this case, the WP402G detects a count of one less than the actual count. In other words, if the detected count of particles is 10% less than the actual count, for instance, it is referred to as a 10% loss caused by the simultaneous passage of particles. Figure 4 (a) shows an example where two particles are passing through the laser beam. Figure 4 (b) is a graph of the relationship between the actual count of particles and the particle concentration that the WP402G has detected. When the WP402G has a 10% loss, the maximum detectable concentration of particles is 15,000 particles/m < for a pulse width corresponding to a size of 2 µm.

Figure 4 Loss Caused by Simultaneous Passage of Particles

CONFIGURATION

Figure 5 WP402G Configuration

Figure 5 shows the configuration of the WP402G. The RS-485 communication feature allows users to manage measured data in a centralized way from up to 32 WP402G units when the WP412G software is installed on a personal computer, as well as greatly reduces wiring costs. Optional 4 to 20 mA DC outputs (four outputs) can be added as necessary. In addition, the WP402G comes standard with four analog inputs (selectable from 0 to 20 mA DC, 4 to 20 mA DC, and 0 to 10 V DC). Thus, it can input external signals such as turbidity and head pressure and use WP412G to manage those data in the same way as organizing particle count measurement data by size.

CONTROL OF LIGHT QUANTITY

Figure 6 Laser Output Control

In figure 3, the peak value of pulse signal that corresponds to the particle mainly depends on the intensity of the laser light source. Figure 6 shows control of the laser output. In figure 6, there is a laser-emitting diode (LD) and a photodiode (PD) at the back of the irradiating side inside the laser diode element. The PD monitors the amount of light leakage from the LD, which is proportional to the amount of the irradiating beam. Accordingly, the WP402G maintains the amount of the irradiating beam to a constant level by receiving feedback from the PD for controlling the current output to the LD.

 

CONTROL OF FLOW RATE

Figure 7 Sampling System for Low Pressure (20 to 50 kPa)

To maintain the measurement accuracy of the WP402G, it is necessary to supply sample water to the detector at a constant rate and thus control of the flow rate is also a key point for stable measurements. As the supply hydraulic pressure is very low at some installation sites, we have developed two types of sampling systems. Figure 7 shows a flow diagram of the sampling system for low pressure and figure 8 for high pressure. The low-pressure system maintains the flow at a constant level with a head difference between the deaerator and the moving drainage.

Meanwhile, the high-pressure system controls the flow with a pressure deaerator and a deaerated constant flow valve. The pressure deaerator not only removes air bubbles, but also suppresses its generation by maintaining the supply hydraulic pressure.

Furthermore, the sampling system for the Japanese market can integrate the TB500G high-sensitivity turbidimeter, suitable for low-turbidity measurements, or the TB600G laser turbidimeter, which is highly sensitive to micro particles.

 

MEASUREMENT EXAMPLE

We measured kaolin, formazine, silica, alumina, and PSL (polystyrene latex) which is used as the calibration particle, with the WP402G and received very satisfactory results with fine repeatability for each of them. Figure 9 shows the measurement results of kaolin (W company's, 0.1degree) as an example.

Figure 8 Sampling System for High Pressure (30 to 500 kPa)	Figure 9 Measurement Example of Kaolin(Repeatability Test)

MEASUREMENT AT FILTRATION SYSTEM

Figure 10 Measurement of Particle Count by Size at Filtration Outlet

Figure 10 shows an example of on-site measurement of particle counts by size in water sampled at an outlet of a filtration system. The particle sizes are mostly between 2 and 15 µm, and their distribution by size does not change although there are changes in the total count of measured particles. This is typical for on-site measurements.

Figure 11 shows a graph that plots the total particle count as shown in figure 10 (2 to ∞ µm) and the turbidity values measured with a light transmission and scattering type turbidimeter and a laser-type turbidimeter. In figure 11, the variation of the measured values by the two turbidimeters corresponds to that of the measured particle count by the WP402G, which suggests some correlation between them. However, the WP402G indicates larger variation in turbidity compared to the turbidimeters even if the particle size distribution of sample water does not greatly vary. The WP402G further allows users to clearly understand the variation by showing counts by size. This is because the range of micro particles in sample water measured at the filtration outlet is usually 2 to 15 µm and the turbidimeters cannot reflect the count change in the measurement values sufficiently. However, the WP402G can count the particles one by one, allowing a noticeable change in the measured results.

 

 

INFLUENCE OF CHANGE IN PARTICLE SIZE DISTRIBUTION

Table 2 Influence of Change in Particle Size Distribution

Influence of Addition of 5-µm PSL	WP402G Particle Counter (particles/ml)			Light Transmission and Scattering Turbidimeter (degree)
Size Range	2 to 4 µm	4 to 6 µm	Total: 2 to ∞ µm
Kaolin	264.67	59.50	343.53	0.049
Kaolin with 5-µm PSL	266.37	182.03	488.57	0.058
Indication change	No change	Large change	Small change	Large change

Table 2 shows a comparison of experiment results between the WP402G and a light transmission and scattering turbidimeter when the distribution of particle size changes. We added 5-µm PSL to kaolin so as to change the size distribution in this experiment. The WP402G can measure the particle count by size and thus give precise meaning and clarity to the change. However, the turbidimeter measures the change macroscopically and cannot show detailed factors. As turbidimeters have different sensitivity for each size of particle, the measurement results are characterized by an inclusive value of turbidity.

CONCLUSION

Figure 11 Measurement of Particle Count by Size and Turbidity at Filtration Outlet

Turbidity control at the filtration outlet will be increasingly strict in the future; nevertheless, turbidity measurement alone cannot allow users to grasp which size of particles will increase and raise turbidity. Either 1-or 2-µm particles can increase in size and raise turbidity. Generally, a sand filtration system can capture particles of 1 to 2 µm or more; smaller particles regularly pass through filtration but measuring them is not very useful. Contrary to expectation, it is important to continuously monitor particles larger than 1-to 2-µm which pass through filtration and take the necessary measures. This will require a monitoring system performing not only low-turbidity measurement but also using a particle counter.

As aforementioned, the WP402G water particle counter is a brand-new product that allows users to receive information that the conventional turbidimeter cannot detect. Installing the WP402G before and after filtration and calculating removal rates for each particle size can be a new indicator for filtration management. We believe that the WP402G can be very effective in improving filtration management by size distribution measurement, and taking countermeasures against protozoa such as Cryptosporidium, and thus contribute to optimal management.