Dear 鄧Sir,
According to Wiki, the max [suction] of normal household vac cleaner is around 20Kpa, about 0.2 atm, significantly weaker than the vacuum made by condensed vapor(pressure drop may close to 1 atm).
請問如何可以防止器材發霉?
It never requires all the steam to condense to collapse a steel drum. I guess it would be about 0.5 atm. BTW, steel drums are much stronger than telescope optical tube.c++ 寫:Dear 鄧Sir,
According to Wiki, the max [suction] of normal household vac cleaner is around 20Kpa, about 0.2 atm, significantly weaker than the vacuum made by condensed vapor(pressure drop may close to 1 atm).
Yes, household vacumm cleaner is not design to achieve high vacuum and 0.2 atm seems like a reasonable figure.
What force is 0.2 atm representing? 1 atm = 101325 Pa, i.e. 101325 Newton per metre square. Assuming that you have a 90mm lens. Area = (0.09/2)^2 X 3.14 = 0.00636 m^2. The force on the lens will be 644N. It represents approximate putting a 64kg weight ON your lens. If you would like to put more than twelve big counterweights (my iEQ45 counterweight is 5kg each) on top of your lens, be my guest. [萬年奸臣]
Dear 鄧Sir,鄧登凳 寫:It never requires all the steam to condense to collapse a steel drum. I guess it would be about 0.5 atm. BTW, steel drums are much stronger than telescope optical tube.c++ 寫:Dear 鄧Sir,
According to Wiki, the max [suction] of normal household vac cleaner is around 20Kpa, about 0.2 atm, significantly weaker than the vacuum made by condensed vapor(pressure drop may close to 1 atm).
Yes, household vacumm cleaner is not design to achieve high vacuum and 0.2 atm seems like a reasonable figure.
What force is 0.2 atm representing? 1 atm = 101325 Pa, i.e. 101325 Newton per metre square. Assuming that you have a 90mm lens. Area = (0.09/2)^2 X 3.14 = 0.00636 m^2. The force on the lens will be 644N. It represents approximate putting a 64kg weight ON your lens. If you would like to put more than twelve big counterweights (my iEQ45 counterweight is 5kg each) on top of your lens, be my guest. [萬年奸臣]
Firstly, vacuum created by steam condensation can be easily exeed 0.9 atm, as satuated vapor presussure at room temprature is only 2~3 Kpa(~0.02 atm)
Secondary, as the pressure are actually apply to the dust cover rather than the lens itself, and the tube is far from air tight, so the pressure on both sides of the lens are well balenced, thus the lens is free from any stress on the front surface,as long as the dust cover survives.
Third, it seem that the number 64kgf needs to multiple by 0.2, thus the pressure on the dust cover is about 13kgf, it will quite probable survive.
1. As steam is itself a poor conductor of heat, the steam inside the container never reached room temperature when it collapses. If you have done similar experiment in school, you will find the can/container is still quite hot (50 to 60C) after it collapse.c++ 寫:
Dear 鄧Sir,
Firstly, vacuum created by steam condensation can be easily exeed 0.9 atm, as satuated vapor presussure at room temprature is only 2~3 Kpa(~0.02 atm)
Secondary, as the pressure are actually apply to the dust cover rather than the lens itself, and the tube is far from air tight, so the pressure on both sides of the lens are well balenced, thus the lens is free from any stress on the front surface,as long as the dust cover survives.
Third, it seem that the number 64kgf needs to multiple by 0.2, thus the pressure on the dust cover is about 13kgf, it will quite probable survive.
2. Never assume the telescope tube to be all round. With various devices such as focuser, attachment points for finder scope... there are many high stress points.
3. Even for 13kgf, see if you can put 13kg on top of your lens cap. Don't blame me if you bend it or break it. (I made a mistake, the initial calculation was based on a C8 SCT with 203mm front lens. That ends up with 65kgf, I forgot to change the calculation when changing to a smaller telescope)
A household vacuum food sealer has even less power than a home vacuum cleaner. According to you, it cannot crush a can, can it? See for yourself. http://www.youtube.com/watch?v=cIl7U9xbM1M
鄧登凳 寫:1. As steam is itself a poor conductor of heat, the steam inside the container never reached room temperature when it collapses. If you have done similar experiment in school, you will find the can/container is still quite hot (50 to 60C) after it collapse.c++ 寫:
Dear 鄧Sir,
Firstly, vacuum created by steam condensation can be easily exeed 0.9 atm, as satuated vapor presussure at room temprature is only 2~3 Kpa(~0.02 atm)
Secondary, as the pressure are actually apply to the dust cover rather than the lens itself, and the tube is far from air tight, so the pressure on both sides of the lens are well balenced, thus the lens is free from any stress on the front surface,as long as the dust cover survives.
Third, it seem that the number 64kgf needs to multiple by 0.2, thus the pressure on the dust cover is about 13kgf, it will quite probable survive.
2. Never assume the telescope tube to be all round. With various devices such as focuser, attachment points for finder scope... there are many high stress points.
3. Even for 13kgf, see if you can put 13kg on top of your lens cap. Don't blame me if you bend it or break it. (I made a mistake, the initial calculation was based on a C8 SCT with 203mm front lens. That ends up with 65kgf, I forgot to change the calculation when changing to a smaller telescope)
A household vacuum food sealer has even less power than a home vacuum cleaner. According to you, it cannot crush a can, can it? See for yourself. http://www.youtube.com/watch?v=cIl7U9xbM1M
Dear 鄧Sir,
1.Even at 60C, the saturated vapor pressure for water is only 0.2 atm(0.12 atm at 50C), the pressure difference from ambient air is 0.8~0.88 atm, 4 times stronger than the maximum ability to vacuum cleaner. This only proves how powerful vacuum created by condensation can be.
2.For uneven shape like finder base, the pressure cancels out except the face it attaches the OTA, thus to the OTA it feels no difference.
3.Yes for large tubes even weak vacuum is still a thread. But for smaller one as the previous example you provided, 13 kgf is evenly applied to the surface of the cover, and as the dust cover is rim-supported, it might not as bad as we think.
If we still have concern, just put one or two kitchen stainless steel disc on top of the dust cover will strengthen the cover quite a lot.
4. Vacuum food sealer is by no means weaker than vacuum cleaner in creating vacuum, unless suction data is provided. The logic is simple, though vacuum cleaner is very noisy and consumes more electricity power, it is designed to work with large air flow to suck the dirt. While food sealer is designed to create higher pressure drop for sealed bags--very low air flow required, so it can employ more specialized pumps.
4.1 For aluminum beverage cans, the wall can as thin as 0.09mm, normal people can crash it from the middle of the cylinder with TWO FINGERS. Can anyone(except for Kongfu masters) crash, or even dent any telescope tube with all of his fingers?
So my points are: A, vacuum is powerful, but not the weak one created by home cleaner, anyway its corrugated plastic pipe will collapse far before the OTA tube, I even push the open of cleaner hose to my hand to feel the suction before; B, for large OTA, the weak negative pressure might still be dangerous.
My suggestion to use vacuum bag is based on its air tight property, not that vacuum is needed or useful in storing instrument.
I made my warning about using vacuum cleaner to suck air out of a vacuum bag storing astronomical instrustment. Even if a OTA is not crushed completely by vacuum, excessive force may affect collimation of lenses etc. If you think it is safe, then you can do it your way and bear the consequences.
I made my warning about using vacuum cleaner to suck air out of a vacuum bag storing astronomical instrustment. Even if a OTA is not crushed completely by vacuum, excessive force may affect collimation of lenses etc. If you think it is safe, then you can do it your way and bear the consequences.
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