1 

## Leck Mitchell, Ph.D., PE

11 Rhine Court Grand Junction, Colorado, 81507

970

- 244

- 8661; Leckmitchell@Bresnan.net

Steve Wendelken, Ph.D. Technical Support Center Ground Water and Drinking Water Mail Stop 140 26 West Martin Luther King Drive Cincinnati, Ohio 45268

Reference: Justification for a dding “Laser” to EPA Method M5331

DETERMINATION OF TURBIDITY BY LED NEPHELOMETRY

September 14, 2015

Dr. Wendelken:

Technology is the evolution and application of science based on research and development for

newe r and better ways of doing business. This is especially necessary when protecting the health and quality of life for the 310 million people of the United States. There has been a long history of advances in monitoring water. The earliest public use for t urbidity measurement began in

1900 with the Jackson C andle Unit

, eventually leading in the 1950s to incandescent light sources, and now in 2015 to LEDs and laser light sources providing additional wrinkles to the amazing changes in photonics

... green laser s a t 5 10 to 540 nm.

It is essential for suppliers to offer plant operators information and choices which are available technically

, which may be of benefit, and which have proven over the years to be reliable in a large number of applications, including wa ter and waste

- water treatment.

The use of a 525 +/

- 15 nm in a LED was selected for method M5331 Revision 1.1 because calculations show twice as much scattering of light by small particles as opposed to a 650 nm light source and six times the scattering of light by an 850 nm (ISO 7027). The monochromatic LEDs proved superior to incandescent and white

- light LEDs. As laser technology has evolved, it

too, has shown superior advantages over both poly

- and monochromatic LEDs.

L aser s provide a stronger and m ore energetic signal to the scattered

- light detector due to its stability in wavelength and its tightly focused beam, reducing the amount of stray light the detector must contend with. By comparison the LED light beam displays a broader cross section, all owing secondary scattering. The more focused beam of the laser allows higher intensity with less stray light. Hach suggests that a narrower beam results in higher scattered intensity from smaller particles. ( Introduction to Laser Nephelometry , p.14)

There are two factors in suggesting an alternate light source for the M5331 Method. The first factor is technical while the second is political. Technically : When lasers in the 510 to 540 nm2 became available, my major consideration was how to incorporate this technology and still meet the specifications for the M5331. Section 6.0 of M5331 specifies a light source of 525 +/

- 15 nm. Currently, laser s at 520 nm and 532 nm are easily available and within the allowable range.

Other choices , including the 525 n m laser, may become less expensive in the future and more

desirable

.

In comparing LED and laser in Method M5331:

• The divergence of the light source in the measurement area is the same for both laser and

LED.

• The distance traveled is the same in both t he laser and the LED.

• The detector is the same in both the laser and the LED.

• All mechanical features are identical ; only the light source is changed.

• Both the LED in M5331 and the proposed laser light source meet all quality control and performance ch aracteristics.

• Solid

- state, single

- chip monochromatic lasers reduce overall costs and could provide longer life of the light source than with the 525 nm LED

.

The cross

- section of the laser beam has a smaller cross

- section than the LED, which will result in

lower detection of stray background light. This will result in more reliable detection of turbidity at very low

- level s of concentration

, but will not display a detectable difference in conventional plant operation

. Detec tion at lower levels may facilitate membrane indirect integrity testing

.

The USEPA Membrane Filtration Guidance Manual found all approved turbidity meters acceptable for indirect integrity testing of membranes but suggested that a laser turbidity meter might be superior (Sec 5.2.)

The second factor in adding laser to M5331 is political. Virtually all optical lenses and LED s are now of Chinese origin, which would make availability of these components difficult in the event of a disruption in production stability. By contrast, several U.S. companies produce and support laser manufacturing, which allow s a smooth continuity between suppliers and end users, irrespective of global issues.

Notes:

(1) Lasers have an inherent collimated beam output; whereas , a LED requires multiple lenses.

The M5331 design for LED , including a laser

, provides multiple lenses for the detector so that the

detector beam is also col limated in the detection volume limiting stray light going into the3 detector. Use of lenses with a small detector limits the photodiode noise, which is a function of

size of the photodiode chip. L enses produce no noise.

(2) A recent review by Jan Tuner and Lars Hode covering scientific and medical journals indicated LEDs may be more destructive than lasers when used on organic structures, which might have some effect on NTU and should be further investigated. ( Laser Therapy, Clinical Practice and Scientific Background

, Prima Books, 2000, pp. 335

- 338 )

(3) Mitchell USPTO Patent # 7663751: NEPHELOMETER INSTRUMENT FOR MEASURING TURBIDITY OF WATER: February 16, 2010, states that wavelengths at 525 +/

- 15 nm provides for use of solid

- state LED, OLED, and laser light sources. The ad dition of

“laser” in the M5331 Method would already be covered under the Mitchell patent.

Respectfully submitted,

Leck Mitchell