WO2001063254A1

WO2001063254A1 - Sub-micron sensitive turbidimeter

Info

Publication number: WO2001063254A1
Application number: PCT/US2000/004936
Authority: WO
Inventors: Ashim K. Banerjee
Original assignee: Hach Co
Current assignee: Hach Co
Priority date: 2000-02-25
Filing date: 2000-02-25
Publication date: 2001-08-30
Anticipated expiration: 2002-08-25

Abstract

A turbidimeter is described which is sensitive to very small particles present in a liquid medium. The turbidimeter is capable of sensing particles well below one micron in size and, therefore, it is useful in applications where it is important to obtain information concerning the presence of small particles (e.g., in drinking water processes or where ultra-pure water is needed).

Description

Description Sub-Micron Sensitive Turbidimeter

Technical Field

This invention relates to turbidimeters . More partic- ularly, this invention relates to turbidimeters having sensitivity to sub-micron particles present in a liquid medium.

Background Art

Conventional turbidimeters are used to determine the turbidity of liquids (such as water) which contain small particles. A light beam is directed through the liquid and light which is scattered by particles in the liquid is detected by one or more detectors.

Turbidimeters which are commercially available are simple, reliable and rugged instruments that have received widespread acceptance in the drinking water market.

Such turbidimeters are substantially insensitive, however, to particles having a size below one micron. This limitation renders them largely incapable of exploiting the fact that diminishing particle size corresponds to rapidly increasing numbers of particles. In other words, there are typically many times more small particles present than there are large particles.

Over the past few years, the use of particle counters in the drinking water field has expanded. Particle counters offer two significant advantages over conventional turbidimeters: (1) they are sensitive to small changes in the number of particles present, and (2) they can provide information as to the size of the particles counted. Typical modern particle counters used in the drinking water industry count all particles having a size greater than about 2 microns. However, in the absence of information such as the index of refraction, the value of the particle sizing aspect is dubious.

In the processing of drinking water, and in industries where ultra-pure water is desired, the water must be passed through one or more small pore filters to remove contaminants. Because such filters periodically fail, it would be very advantageous (1) to be able to detect the early onset of filter breakthrough, and (2) to have an early warning if the filter is passing pathogen-sized debris .

There has not heretofore been provided an economical instrument which reliably senses the presence of sub-micron particles in liquids such as water. For every particle having a size of two microns that a particle counter can detect, there are eight particles in the one micron size that the particle counter does not see. There would also be thousands of particles having a size less than 0.1 micron which the particle counter would not see.

Disclosure of the Invention

In accordance with the present invention, there is provided a turbidimeter which is sensitive to sub-micron particles present in a liquid medium. In one embodiment, the turbidimeter comprises: (a) a light source comprising laser diode means for directing a light beam into a liquid medium; and

(b) photomultiplier tube detector means adapted to detect light scattered by particles present in the liquid medium. The turbidimeter of this invention is a very sensitive process control tool with performance comparable or superior to many conventional particle counters. The combined effect of the high intensity laser light source, relatively small beam size, and photomultiplier tube detector enables the instrument of this invention to detect light scattered from very small particles in the liquid sample that cannot be sensed by means of conventional turbidimeters. Also, because the number of sub-micron particles is very large when compared to super-micron sized particles, the ability to detect them efficiently also provides the instrument with the ability to rapidly follow trends in a sample stream (e.g. in a continuous flow situation) . A conventional turbidimeter suffers a very significant time lag in comparison, before it can begin to respond to the same stimulus. Particle counters, no matter how sophisticated, suffer from having a designed cut-off of sensitivity to small particles. Thus, by definition, particle counters will miss all particles below this design threshold, regardless of how many in number they might be. Other features and advantages of the instrument of this invention will be apparent from the following detailed description and the accompanying drawings.

Brief Description of Drawings The turbidimeter of this invention is described in more detail hereinafter with reference to the accompanying drawings wherein like reference characters refer to the same parts throughout the several views and in which:

FIGURE 1 is a sectional view illustrating one embodiment of a turbidimeter of the invention;

FIGURE 2 is a sectional view illustrating another embodiment of a turbidimeter of the invention;

FIGURE 3 is a schematic illustrating one manner in which the performance of the turbidimeter of the invention is compared to that of conventional particle counters and a conventional turbidimeter;

FIGURE 4A is a graph representing total counts data produced by particle counter 1 in Figure 3; FIGURE 4B is a graph representing data produced by the turbidimeter of this invention (as used in Figure 3);

FIGURE 4C is a graph representing data produced by a conventional turbidimeter (as used in Figure 3); FIGURE 5A is a graph representing data produced by the turbidimeter of this invention in Figure 3;

FIGURE 5B is a graph representing data produced by the 0.2 micron channel of particle counter 2 in Figure 3.

Best Mode for Carrying Out the Invention As illustrated in Figure 1, one embodiment of the turbidimeter 10 of the invention includes a housing or enclosure 12 in which there is a chamber 12A for receiving the liquid to be tested. A bubble trap 17 may be included to remove bubbles from the liquid to be tested. A light source comprising a laser diode module 14 (including laser diode, power supply and lens) generates a light beam 14A which is directed into the liquid being tested where particles present in the water cause the light to be scattered. Light trap 19 at the lower end of the sample chamber reduces stray light in the chamber 12A.

The scattered light 15 is received at a fixed location by one end 16A of an optical fiber 16. The opposite end 16B of the optical fiber is operably connected to a photomultiplier tube detector 18. In the embodiment shown in Figure 1, the scattered light is being gathered at a point which is at 90° relative to the axis of the light beam 14A. The optical fiber 16 preferably has a plastic core which is 1 mm in diameter. This is much larger than other fibers used in light scattering instrumentation (typically from 5 to 200 microns) and is designed specifically to maximize the amount of scattered light it can collect and carry to the detector (e.g. the photomultiplier tube detector) . The laser diode light source 14 generates an intense light beam which preferably has a diameter in the range of about 0.5 to 3.0 mm. One useful laser diode is a 660 nm, 30 mW device which is commercially available. It is possible to use other types of laser diodes. For example, a laser diode capable of generating a light beam with shorter wavelengths than 660 nm (e.g., 400 nm) could be used which would scatter light more efficiently when striking very small particles. The photomultiplier tube detector module is commercially available (e.g., HC120 manufactured by Hamamatsu) . When coupled with the optical fiber 16, light which is scattered by the small particles in the water and received by the optical fiber is conveyed to the photosensitive cathode of the photomultiplier tube detector. A series of dynodes are used to electronically multiply this signal, thereby producing a highly amplified current signal at the photomultiplier tube anode. This current is converted to a voltage, further amplified as necessary using operational amplifiers and then used as a measure of the sample's turbidity, calibrated as nephelometric turbidity units (NTU) .

Another type of detector which may be used in this invention is an avalanche photodiode which is less sensitive than a photomultiplier tube detector but is also less expensive. The avalanche photodiode is more sensitive to higher wavelengths of light and therefore if it is used, the laser light source should preferably be the type which generates red or infrared light beams. When the size of the particles being measured is of the order of 15 times smaller than the wavelength of light in the light beam which is being used, the pattern of the scattering intensity is largely uniform as a function of the scattering angle. This means that a detector positioned at 90° relative to the axis of the light beam will see the same amount of scattered light as one at any other angle. Since the 90 degree angle is least susceptible to undesirable influences such as flare light, it becomes the angle of choice. When the particles have a size which is approximately equal to the wavelength of the light in the light beam being used, the scattering intensity pattern is described by a more complicated function. In essence, more light is scattered in the forward direction than sideways or backwards. Thus, a detector placed so as to capture scattering at a forward angle (about 10 to 50 degrees) is desirable. In the event that the size of the particles in the sample is not known, having multiple optical fibers at a forward, side and back scatter angles enable the instrument to have higher sensitivity (due to signal addition by the multiple fibers) while at the same time being able to exploit any non- symmetrical scattering profiles .

As illustrated in Figure 2, it is possible to use a plurality of optical fibers 16 for collecting scattered light at a plurality of different positions. For example, when testing ultra-pure water (such as is used in the semiconductor industry) , several optical fibers may be positioned at different angles relative to the axis of the light beam in order to collect scattered light at the different angles. The collecting fibers can be positioned to collect light scattered by the same part of the laser beam but at different angles. All of the collected light is then transmitted to the photomultiplier tube detector in order to obtain higher signal strengths due to signal addition from each of the optical fibers.

The instrument of this invention is at least 100 times more sensitive in detecting sub-micron particles than is a conventional turbidimeter. The cost of the instrument is comparable to a conventional turbidimeter and significantly less than the cost of a particle counter. Thus, the instrument of the invention is a low-cost and low- maintenance instrument which is capable of replacing particle counters as a process control tool in the drinking water and ultra-pure water markets. Figure 3 is a schematic representation of an experiment undertaken to collect data while comparing the performance of the instrument of this invention with commercially available turbidimeters and particle counters. Particle counter 1 is an instrument that is widely used in the drinking water industry. It is capable of counting particles having a size from 2 microns and greater. Particle counter 2 is a research grade particle counter used in the ultra-pure water industry. It is capable of counting particles in the size range from 0.2 to 2.0 microns.

Figure 4A shows data collected by particle counter 1, and Figure 4B shows data collected by the instrument of this invention. Figure 4C shows data collected by the conventional turbidimeter. Figure 5A shows data collected from the instrument of this invention, and Figure 5B shows data collected from the 0.2 micron channel of particle counter 2.

The above-described figures illustrate that the instrument of this invention very accurately detects sub- micron particles present in a liquid sample.

Other variants are possible without departing from the scope of this invention.

Claims

1. A turbidimeter having sensitivity to sub-micron particles present in a liquid medium, said turbidimeter comprising :

(a) a light source comprising laser diode means for directing a light beam into a liquid medium;

(b) photomultiplier tube detector means or avalanche photodiode detector means adapted to detect light scattered by particles present in said liquid medium.

2. A turbidimeter in accordance with claim 1, wherein said light scattered by said particles is detected at an angle of 90 degrees relative to the axis of said light beam.

3. A turbidimeter in accordance with claim 1, wherein said laser diode means produces a light beam having a diameter in the range of about 0.5 to 3.0 mm.

4. A turbidimeter in accordance with claim 3, wherein said laser diode means comprises a 660 nm, 30 mW laser diode .

5. A turbidimeter in accordance with claim 1, further comprising optical fiber means adapted to receive light scattered by said particles and transmit it to said detector means .

6. A turbidimeter in accordance with claim 5, wherein said optical fiber means comprises a plurality of optical fibers .

7. A turbidimeter in accordance with claim 6, wherein light scattered by said particles is detected at a plurality of different angles relative to the axis of said light beam.

8. A turbidimeter in accordance with claim 1, wherein said laser diode means produces a predominately blue light beam.

9. A turbidimeter in accordance with claim 1, wherein said detector means comprises an avalanche photodiode.

10. A method for determining the presence of sub- micron particles in a liquid medium, the method comprising the steps of:

(a) directing a light beam into said liquid medium; wherein said light beam is produced by a laser diode;

(b) detecting light scattered by particles present in said liquid medium by means of a photomultiplier tube detector.

11. A method in accordance with claim 10, wherein said light scattered by said particles is detected at an angle of 90 degrees relative to the axis of said light beam.

12. A method in accordance with claim 10, wherein said light beam has a diameter in the range of about 0.5 to 3.0 mm.

13. A method in accordance with claim 10, further comprising the step of transmitting light scattered by said particles to said detector means by means of optical fibers .

14. A method in accordance with claim 13, wherein light scattered by said particles is detected at a plurality of different angles relative to the axis of said light beam.