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In recent blogs we have been talking, for the most part, about particles that are relatively large – ones you can see, touch and feel. There is a world out there, however, that worries about particles that are very tiny. I’m talking about particles that are smaller than tens of microns in dimension. An earlier blog talked about the size of a micron – How Big is a Micron. Particles less than 10 microns in size can not be seen with the naked eye, weigh nearly nothing and, for the most part, defy detection with the methods discussed so far. Among other problems, filters to perform “millipore testing,” although available in sub-micron ratings are expensive and filtration is deadly slow. Futhermore, the entire testing process, for all practical purposes, has to be carried out in a “bunny suit” style clean room. Even the cleanest ambient air will contaminate samples with thousands of sub-micron sized particles in seconds.
In cases where such small particles count, liquid particle counting may be the answer. The liquid counting device consists of a small transparent tube through which liquid containing the particles to be counted is forced by one of many means – a peristaltic pump or syringe are commonly used. A laser beam is directed at the liquid in the tube and special sensors placed critically with respect the the laser beam record the light absorbed and reflected as the particles suspended in the liquid pass by. By computer analysis of the data signals received from the sensors, it is possible to determine both the number and size of the particles that pass by in a given period of time. The flow rate or sample size, of course, must remain fixed for the data to be of any real value.
But Wait! In fact, the liquid particle counter or LPC is not really counting the actual particles that pass through the device. Instead, it is recording and interpreting data based on algorithms developed using latex spheres of known quantities and sizes. The actual particles being counted, if they are not latex spheres, may give results that differ from reality. In fact, this really isn’t a problem because most liquid particle counting is based on some basis of data collected using the same measuring means. Changes in particle numbers and sizes will be relative and since the base evaluation is maintained the same, the results are, therefore, extremely useful. For readers interested in more information on LPC technology I recommend the following website Particle Measuring Systems. This company also periodically offers their “Particle College” which I highly recommend. I am a “graduate.”
One interesting implementation of liquid particle counting utilizes ultrasonics to disloge particles from surfaces for counting. A setup very similar to that is described in a previous blog Secondary Vessel Used to Collect Particles. In the case of liquid particle counting, the liquid (usually water) used in the secondary collection vessel is pumped to or sampled for delivery to a liquid particle counter for evaluation. In an extension of this same concept, liquid particle counting is being incorporated in the final rinse tank(s) of cleaning systems cleaning parts that are used in applications that are sensitive to small particles. Computer printer heads are one example. The system provides a real-time analysis of particle counts to either assure that the parts are suitably clean before exiting the machine or, in some cases, actually control the duration of the rinse as needed to assure the required cleanliness.
Liquid particle counting is a very sound technology that is still growing. You may not need LPC now but it is a good thing to be aware of because it may well be in your future.
– FJF –