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04-01-2011, 08:24 PM   #46
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QuoteOriginally posted by Cannikin Quote
No, the principle is not the same. With water, the volume of a given mass cannot change, and therefore attempting to change the volume means that the water must be forced out at a given rate of flow, driven directly by the total force of the pump. The pump itself is directly pushing the water out. Flow (volume of water per unit time) is invariable (the size of the hole makes no difference in amount of water that is pumped through in a given amount of time). This is the principle behind hydraulics. The principles of pneumatics are quite different. A compressible gas can easily change its volume with compression, and flow is not directly dictated by the external force of a pump pushing it out, but by differential pressure (in essence, the air is pushing itself out). Unlike with liquids, flow is variable.

Anyway, the lens exerts no force in the zooming out (expansion) stage, which is when dust gets pushed inside. Zooming in ("pumping" stage) is irrelevant, because that forces air/dust out (and as I'll explain below, that makes no difference either). When expanding, all it is doing is allowing the air molecules inside to spread out more, lowering the pressure. Internal pressure is dictated solely by the volume of the internal cavity, and the quantity of air inside (and temperature, but that's not a concern here). Pressure differential is dictated solely by the difference between internal pressure (given) and external pressure (constant). No holes or whatever. Pressure is exactly the same on all internal surfaces (the same for all external surfaces).

There is no such thing as "suction" in physics. The only force, and the only thing that pushes anything here is atmospheric pressure, which is a constant 101,325 pascals (that's constant pressure, not constant force); this is not arguable. This is driven by the principles of static pressure, and zooming does nothing to change this. If you zoom at the same rate for both a small and big hole, the pressure differential change is exactly the same (both zooming in and out), and thus the net pressure at the interface (hole) is exactly the same. The size of the hole dictates flow rate (volume per unit time) by allowing more/less air to pass through in a given amount of time (opposite of a pumped liquid, where the size of the hole changes nothing about the flow rate), but the pressure and thus force exerted on an object of given surface area (dust) is constant.

Your idea of making a larger hole is indeed valid in terms of controlling where air flows, as the larger area means a greater proportion of the total air moves through that hole (compared to smaller holes that are simultaneously present), but the force experienced by any given particle of dust is constant.

Seriously, your theory and analogy are simply not correct. Any reference on fluid mechanics will tell you this. There are many examples I can give you (a balloon, a compressed gas cylinder, pneumatic machinery, etc.), but the end result is the same: the idea that a smaller hole creates more "suction force" given constant atmospheric pressure and a given internal pressure is nonsense, in more ways than one (pressure remains constant, total force decreases).
Oh dear, oh dear, you still don't get it, I think you are trying to write a doctor thesis here. And while we are at it lets ban the word "suction" because suction does not exist. And we also have gone down to "molecules" I guess atoms are next. You are "over explaing" things.

Anyway we are digressing and talking around circles, lets stick to the topic.

Greetings

04-01-2011, 08:48 PM   #47
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QuoteOriginally posted by Schraubstock Quote
Oh dear, oh dear, you still don't get it, I think you are trying to write a doctor thesis here. And while we are at it lets ban the word "suction" because suction does not exist. And we also have gone down to "molecules" I guess atoms are next. You are "over explaing" things.

Anyway we are digressing and talking around circles, lets stick to the topic.

Greetings
Sorry, I'm like that. I tend to go way too in depth in explanations, and end up confusing people. I probably wouldn't make a very good teacher.
Anyway, I'll just sum it up simply: any sized hole = constant pressure = constant force experienced by piece of dust (smaller than hole). Larger hole = greater flow volume.

Your idea to make a larger hole to control where the air enters is correct (more air, and thus dust, will flow into the big hole than the smaller ones), but the idea that the weather/dust sealing of the K-5 makes dust worse is not correct.
04-01-2011, 09:12 PM   #48
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QuoteOriginally posted by Cannikin Quote
but the idea that the weather/dust sealing of the K-5 makes dust worse is not correct.
Read my original post carefully
...is it possibly to making the problem worse ?

I did not say it does .

But the jury is still out.

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04-01-2011, 09:25 PM   #49
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QuoteOriginally posted by Cannikin Quote

Your idea to make a larger hole to control where the air enters is correct (more air, and thus dust, will flow into the big hole than the smaller ones) .
Glad you confirm this at least, that was the very idea for this little experiment and that is I wanted to achieve. Any effect positive/negative air pressure may or may not have on the parts of the body inside was just a secundary consideration.

Greetings

04-01-2011, 09:30 PM   #50
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Looks like my M42 lenses are goners!
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