Laserlenz Technotes
From time to time, President Leonard Formanek will offer his valuable insights in the field of particle counter calibration, the water industry and various other topics. He has had the pleasure of being a speaker on particle counter calibration topics at various Southeastern conferences.


Below are a list of topics Len has discussed:

Particle Counter Functionality.
Speech given at the KWWOA (Kentucky Water & Wastewater Operators Association) Conference, Owensboro, KY: "Technology not only helps us do our jobs more efficiently and effectively, but acquiring new technology provides us a path to examine how we do things"...read on...

Laser Particle Sensors & Laser Diode Life.
Speech given with James Hart, President, Laserpure at the KWWOA (Kentucky Water & Wastewater Operators Association) Conference, Owensboro, KY: "In general, low power laser diodes in properly designed circuits do not degrade significantly during thousands of hours of use or when powered on or off. For example, compact disc players' laser printers and bar code scanners are readily and confidently utilized throughout the world without concern of laser diode failure"....read on...





Particle Counter Functionality
– Leonard Formanek, President, Laserlenz
Speech given at the KWWOA (Kentucky Water & Wastewater Operators Association) Conference, Owensboro, KY

Technology not only helps us do our jobs more efficiently and effectively, but acquiring new technology provides us a path to examine how we do things. To make new technology effective, we have to look at how we are doing our job, make assessments on whether a different way of operating might be better and then use technology to support and further that change.

Particle counters provide the necessary single particle visibility required to optimize the filtration process to combat organisms.

The constant and rapid changes in particle counting equipment have left many users with out-dated equipment not supported by the manufacturer.

Several services are offered to assist the end user in upgrading or replacing these units. Evaluation and testing of equipment determines suitability for current standards.

What are particle counters?
     • Particle counters are used to count microscopic particles not visible to man in the air and in water. Particles are sorted and counted according to size.

     • Particle counters are much more sensitive than turbidimeters. Turbidimeters only give a relative measure of the cloudiness of the water (turbidity) and cannot distinguish the individual particles in the water. However, particle counters and turbidimeters together provide a more complete picture of the water treatment process.

     • Particle counters monitor the performance of the filters in the size range of disease causing parasites such as cryptosporidium and giardia.

     • Particle counters have developed into a useful and cost-effective means of evaluating water quality.

     • Particle removal is a critical factor in assuring the high quality of drinking water.

     • The primary particle sensor design for today's applications is the light blocking, laser diode sensor. These sensors are very rugged and reliable. As a result of increased stability, the verification procedure can assure proper sensor/counter operation to a high degree of confidence.

One of the most basic tests to determine particle counter functionality is the 'zero count' test. All particle sensors should count almost zero particles per milliliter when there are not particles present.

The size calibration fluid, polystyrene latex spheres, should be introduced to the sensor so that it appears as a normal influent. It should not produce out of tolerance sensor flow or introduce undue turbulence, bubbles or excessive background counts.

Simply calibrating particle counters with the same standard does not ensure all particle counters will count the same fluid the same way. Field verification of calibration does ensure the sensor is functioning to the manufacturer's standards and is stable.

Turbidimeters and particle counters measure different characteristics of the water. Turbidimeters measure the clarity or cloudiness of the water based on scattering of light. Particle counters on the other hand, measure the number of particles in a particular size range. These two instruments compliment each other in a way that they measure two different and distinct characteristics of water. Particle counters do not replace turbidimeters.

The purpose of a municipal drinking water facility is to remove particulates and pathogens from the water supply without injury to health, while supplying a tasteful and affordable product.

Calibration: Process of generating the correspondence table of voltage pulse height to known size challenge particles. It can also involve adjusting the light beam intensity, focus and shape.

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Laser Particle Sensors & Laser Diode Life
– James Hart, President, Laserpure
Speech given with James Hart, President, Laserpure at the KWWOA (Kentucky Water & Wastewater Operators Association) Conference, Owensboro, KY

In general, low power laser diodes in properly designed circuits do not degrade significantly during thousands of hours of use or when powered on or off. For example, compact disc players' laser printers and bar code scanners are readily and confidently utilized throughout the world without concern of laser diode failure. Usually the user feels complete product satisfaction. However, it doesn't take much to blow diodes when utilized in demanding or harsh environments where static discharge or voltage spikes are present. High voltage pumps, vibration, lightning strikes, relays, and power brown-outs can all dramatically affect the operation and life of a laser diode integrated into a laser particle sensor. Particle sensors can operate continuously for more than 1 year with no noticeable change in performance. This doesn't necessarily mean that the laser diode itself isn't gradually degrading in some way - just that the automatic power control is still able to compensate fully. This means that the current of the laser diode is increasing to insure that the optical power in a flow cell is maintained to the same level it was when the sensor was new.

Some datasheets list expected lifetimes for laser diodes exceeding 100,000 hours - over 12 years of continuous operation. Of course, I trust these about as much as the latest hard drive mean-time-to-failure rate of 1 million hours. What the manufacturers of laser particle sensors don't tell you is that the laser diode life is rated at 100,000 hours only when the diode is operated at less then 10% of its maximum optical output. For example, most water particle sensors use 5 mW, 780 nm laser diodes from Sharp, Rohm, Panasonic, Hitachi, Mitsubishi or others. The particle counters are usually adjusted to an optical power level for 3 mW output. According to the manufacturer's laser data, 25% of all of the laser diodes in the world will fail when they are operated at 3 mW at 60 C for 10,000 hours.

Laser diodes that fail prematurely were either defective to begin with or their driver circuitry was inadequate, or they experience some 'event' resulting in momentary (greater than a few microseconds) over-current. What this means is that with cheap driver electronics such as those found in many laser pointers, leaving the device on continuously may result in much longer life than repeatedly pulsing it.

Temperature Effects
High temperature operation can reduce life expectancy, possibly by as much as a factor of 2 for each 10C rise above the temperature quoted in the device's specifications. Thus, a laser diode with a quoted life of 10,000 hours at 25C, might only last 125 hours at 55C. Not that it will actually fail at 125 hours and 1 second, but its maximum output power will be reduced by 50 percent. I expect that there is a wide variation on the extent to which this applies depending on device type, how close it is operated to its specified maximum power, and all sorts of other factors. Of course, in the grand scheme of things, even LEDs gradually lose brightness with use.

There is one thing to keep in mind about laser diode lifetimes. The time to failure probability distribution is quite wide, meaning that some laser's lifetime will be significantly less than the 5,000 hour mean, and some will be much, much longer than the mean. Lasers are not like light bulbs where they "wear out" and have a predictable lifetime. The main life limiting factors in a laser diode are related to how many crystal defects are present in the device when it is made. If you are lucky to have a diode with very few defects, then your laser may last nearly forever. If you are not so lucky, it may only last a few hours.

Sensor Connections
Compression fittings must be used under pressure. Particle sensors that use long drain lines generate negative pressure. Air can be pushed by atmospheric pressure into the tubing due to lack of positive pressure in the compression fitting. This causes bubbles to be introduced into the sensors and flow meters. Keep the tubing short. Bubbles in the tube can prevent water from flowing. Do not use ball flow meters. Use digital turbine flow meters with a display. This will prevent false confidence in water flow from stuck metering balls in flow meters.

Filters
Use a 600 um mesh in the front of each sensor. Large mesh is not acceptable. Particles from aged pipe, specifically rust, plugs the particle sensors. Typical flow cells are 800 um x 800 um in size. Most strainers are 2500 um x 2500 um in size. The strainer must be at least 200 um smaller then the flow cell size.

Bubbles
Do not collect data from sensors that have poor fitting connections or bubbles. Bubbles cause high counts and will set off alarms. A 800 um x 800 um particle sensor with a single photo detector can not see more then 35,000 particle per ml under any conditions. As such, the high counts in influent or effluent is suspect and will leave you open to EPA scrutiny and liability.

Clean Monthly
Use a pump and circulate a mild cleaner through the system every month to clean the tubing. Clean the tubing separate from the sensor. If you clean the tubing and sensor together you will force particles into the sensor orifice and cause bigger problems.


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