Now it’s time to see the results! Particles have been harvested, collected and are ready to either be weighed or counted. The first step in evaluating cleanliness using either method is to examine the specification. There are two basic types, part-specific and generic.
Part-Specific Specifications –
Part-specific specifications will state the sample size in number of parts to be re-washed to collect the required sample for evaluation. If the specification is not part-specific (just a sample size and no identification of the part) the test will be rather meaningless because, for example, a larger part will have more surface area and, more than likely, will yield a higher weight even if the same cleanliness is achieved as on a smaller part. This is one of the things that often happens when specifications are “borrowed.” Making specifications part-specific really makes things simple but may not be possible.
Generic Specifications –
In the interest of universality, many of the specifications widely used across several industries and in many applications will use a square area of part surface as the basis for evaluation. Since it isn’t practical to cut sample parts into pieces each with the appropriate surface area, there is a little math involved in determining how much residue will be allowed on a specific part. Calculating the surface area of a part can be a daunting task, especially in the case of castings which have multi-planar contours. It’s a good idea to have at least a couple of different people calculate the surface area of a part as mistakes and generalizations can yield widely varying results. If the specification allows a certain weight of residue on a specific square surface area then it becomes a simple matter of dividing the surface area of the part being tested by the reference surface area and multiplying the allowable weight by the resulting factor.
Gravimetric (weight) Analysis –
Until a couple decades ago, most specifications concentrated on the weight of the collected non-volatile residue from re-washing procedure. This frequently included both the particles and whatever else didn’t evaporate when the washings were dried. This kind of measurement, of course, did not require the filtration process described in a preceding blog – the washing solvent was evaporated by heating or other means and the weight of whatever remained was compared against the goal. More recently, only the weight of the particles is considered. Anything that does not pass through the filter is weighed.
Occasionally a specification will require an weight analysis of both the particles collected and, separately, the non-volatile residue. In this case, the particles collected on the filter are weighed as described above. Then the wash solvent that passed through the filter in the particle collection process is evaporated and the remaining non-volatile residue is weighed.
Particle Counting and Analysis –
At one time, particle counting was particle counting – each particle (visible under a specific magnification) was counted and compared against the goal. Today, the process of particle counting is often much more complex. Specifications frequently state the number of particles allowable in a number of size ranges eg. X particles from 1 to 5 microns in size, Y particles from 5 to 10 microns in size and so on. But then it gets even more complex! Specifications today often state state that the largest dimension of a particle not exceed a specific limit and/or that the particle must fit into a square with specific dimensions. There are also other particle attributes to be considered – some specifications differentiate between fibers and metallic particles while others do not allow particles known to be of a specific makup such as glass. Particle counting specifications are often hard to both write and interpret to to the wide range of variables involved. It is suggested that readers with an extended interest in this area read an excellent article on particle characteristics which is available under the Technical Library tab on the Cleaning Technologies Group website www.ctgclean.com. The article can also be accessed directly using the following link – Particle Analysis. Particle counting has become so complex that nearly all specifications requiring particle counting are administered using computer analyzed imaging.
– FJF –