In the past week or so, I have had several people walk into my office and ask a question that has been popular for as long as I can remember – – “What is the ratio of cross section to depth that defines the cleanability of a blind hole?” As is so often the case in cleaning world (and others too), the simple answer is, “It all depends!” I used to think that there must be a number that defines the problem and have probably told a couple of thousand people that it’s in the 1:4 to 1:6 range (that would be cross section vs. depth). And then, silly me, I started thinking more about it and came to conclusion that there is a lot more to it!
For the purpose of our discussion here, let’s define a “blind hole” as any cavity that has only one open end. Holes drilled into solid blocks and closed-end glass tubes will serve as good examples for our purposes. Secondly, let’s assume that in order to effectively clean inside the confined space of a blind hole there must be a means to introduce and remove air and liquid(s) from that confined space. For an “exchange” to take place, one media must be entering the blind hole through the single available access simultaneously with another escaping through the same access. Buoyancy, fluid dynamics and surface tension all come into play.
In most cases, blind holes are filled with air as they enter the cleaning process. The initial challenge is to remove the air from within the blind hole and replace it with liquid as the first step in cleaning. It’s not rocket science that if the open end of the blind hole is facing down, the buoyancy of the air will prevent its escape from the hole much like an inverted drinking glass or a diving bell. This can be easily demonstrated using a test tube suspended in water.
Inverting the tube so the opening of the blind hole is up allows the buoyancy of the air to cause it to exit the hole – – but will it always? Depending on the cross section of the hole, air may or may not escape to allow the hole to fill with liquid. This is not a problem with holes that are, for example, 1 square inch in cross section, but as the hole size becomes smaller, the forces of surface tension can prevent the liquid from giving way to the passage of the trapped air. The following video demonstrates just how effectively surface tension prevents this exchange in a closed-end glass tube 6 inches long and with an inside diameter of about 1/8″.
So, you say, “Reduce the ratio of depth to cross section.” OK, let’s do that. In the following video a closed-end glass tube approximately 1/8″ ID by 1″ long is immersed in water. (Note that this is about the same ratio of cross section to depth that we had with the test tube.)
Based on the above, it seems that a simple cross section to depth ratio won’t define which blind holes can be cleaned and which can not. In upcoming blogs, I intend to further investigate cleaning of blind holes and what makes a difference.
– FJF –