Is It Clean? – Oil and Hydrophobic Films – More Simple Tests

I will get to the “high power” stuff in upcoming blogs but thought it would be a good idea to introduce a few more tests that are simple and require little or no specialized paraphernalia.  Simple is good in my opinion especially when you are troubleshooting in a remote location or when you are working on a shoestring.

In a previous blog, we’ve talked about the water break test.  One of the shortcomings of the water break test is that it does not work well on surfaces with complex contours created by machining and other processes.  The tests we are going to talk about here are similar to the water break test in that they detect the difference between a hydrophilic (clean) and a hydrophobic (contaminated) surface but, although exceedingly simple, are applicable on a wider variety surface contours.

Breath Test

Here’s one requiring only a breathing human being!  One simply breathes on a dried part which has been allowed to cool (so that condensation will occur).  By examining the characteristics of the resulting condensation droplets one can judge cleanliness.  A clean part will show an even fog of imperceptibly small droplets.  On a part contaminated with a hydrophobic film, the imperceptibly small droplets will join together to form larger, usually visible droplets.  Before relying on this test to make any critical decisions, I would recommend that you try it on parts that are known to be clean and others that are known to be dirty to assure that it will reveal the difference.  This test, like the water break test discussed earlier and any other test that relies on surface activity to detect the presence of a hydrophobic film can be easily “fooled” if there are surfactant or soap residues present.  One good thing about this test is that it is performed using, essentially, distilled water eliminating the possibility of contaminated water as a variable in the testing process.

Spray Pattern or Mist Test

This test is similar to the breath test but is performed on a larger scale using a spray gun to spray distilled water on the part being tested while it is still wet from the final rinse.  The part is  usually hung vertically.  By spraying with distilled water, water quality in the rinse is eliminated as a variable with the distilled water serving as a “final” rinse.  Spraying rather than immersion also makes the test more sensitive.  Similar to the water break test, the surface will show a pattern of wetted and non wetted areas after a period of time.  Because there is less water involved than in the water break test where the part is initially totally immersed, this test can detect contaminated areas in undercuts and inside acute angles where water might otherwise accumulate due to its natural surface tension.  This test has been used to produce quantitative results by adding a colorant to the water and manually plotting or photographing the patterns and analyzing them using computer-aided techniques.  It should be noted that to produce meaningful results from test to test, the spray distance, pressure and duration must be carefully controlled.

Atomizer Test

Finally, an atomizer or nebulizer can be used to apply distilled water to a surface being tested.  This test is usually performed after a part has dried (similar to the breath test) and using far less water than the Spray Test described above.  Again, the pattern and size of droplets that collect on the surface will indicate the degree of cleanliness.

The tests described in this and a previous blog about the water break test are limited in that they are only intended to detect the presence of a hydrophobic film which is usually oil or grease.  These tests do not detect particles unless they are so large that you probably don’t need anything more than the naked eye to detect them.  We will talk more about detecting particles in upcoming blogs.

–  FJF  –

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