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Understanding Vacuum and Pressure.
Posted on September 12th, 2010 No commentsUnderstanding mechanics is nothing to do with mathematics, it is the reality of what is actually happening and why it is happening.
A vacuum is the complete absence of any gas or any other substance that could exert a pressure. A diamond in a vacuum chamber would not effect the vacuum in any way. Most other substances and gases would. Note: Total absence of pressure has never been achieved.
Luckily we very rarely have to consider such a tight meaning, because we are usually only considering a partial vacuum and its effects. Note that intergalactic space is not a vacuum, it is just at a much lower pressure than we are used to. If it was a vacuum our atmosphere would disappear like dust up a vacuum cleaner hose.
Note:- The reason we retain our atmosphere is that although you would expect it to be sucked into the partial vacuum of space, this is balanced by the gravitational force of the Earth acting on the gases forming our atmosphere.
I have worked on the design of pressure systems operating up to 30.000 pounds per square inch, (including B.O.P s and other oil and gas equipment, to be topical.) but the problems of designing vacuum systems is a lot more difficult, especially considering we are only dealing with a maximum pressure difference of about 14 pounds per square inch. This indicates that something strange happens at low pressures.
Very low pressure systems are difficult to make and costly to run. The difficulty is due to the problems of sealing against leaks. Joints that seal perfectly at pressures of 10,000 pounds per square inch may leak like a sieve under very low pressures. Materials of the vessels themselves will leak under vacuum, meaning that special materials must be used. Even with the best materials there is some leakage, which means that the vacuum pumps must be run continually to sustain any vacuum attained.
At low pressures, atoms and molecules that retain their stability at normal atmospheric pressure, begin to act oddly. As the pressure reduces they expand. (They are a in compressed state whilst a normal pressure.) This expansion means that the they take up more space in the containing vessel and also weigh less,. (The mass decreases, but please note that the individual atoms/molecules still have the same mass, but there will now be less of them taking up the same space, therefore the weight for each cubic centimetre is less)
Note:- Removing the atoms/molecules is rather like removing a flock of sheep from a field. Initially you have no difficulty removing large groups of them, but as the numbers reduce you have difficulty in rounding up the few remaining ones.
Why does a partial vacuum work better than pressure in certain cases?
Normally you are taught that a suction pump operates due to the pressure difference, and that by removing the pressure on the suction side, the the atmospheric pressure forces the gas or liquid to move. In general, this is a working principle that includes most normal situations.
However, consider a pipe with a suction pump at one end, the other end lying in a tank of sludge. When you switch on the pump the partial vacuum at the pump end allows the liquid at that end to expand and become lighter, making it easier to move and it will start to move through the pump. This expansion of the fluid will in itself cause some of the fluid to pass through the pump.
The expansion effect will pass back through the pipe until the effect of the reduced pressure reaches the sludge tank, when atmospheric pressure takes over. Even if you close off the pipe at the sludge tank end, the liquid expansion will still continue until you stop the pump.
This is why, when attempting to unblock the sink, it is the upstroke that finally clears the blockage, the expansion of the water molecules effectively breaking up the blockage. The downwards pressure of the plunger in most cases applying far more force on the blockage than the differential pressure set up by suction, but in most cases without shifting it.
Another apparently strange thing is that a car tyre with 30 psi pressure in it will stay up for years (Mine don,t). However, if you place it in a vacuum chamber at (say) 8psi absolute, it will leak. This not due to the increased pressure difference, (cars tyres can take a considerably higher pressure than that without leaking) but due to the atom/molecule changes under low pressures.
Note:- To be effective suction pumps should be of the positive displacement type. Vacuum cleaners, water pumps etcetera. are only capable of creating a relatively small vacuum effect. A sink plunger creates a far more effective vacuum.
A problem with many pumping systems which use non-positive displacement pumps is cavitation, which has the same cause, creating vapour or gas within the pump, which ceases to run correctly or even not work at all.—————————-
The old washing machines that used a central paddle arrangement that moved one way and then the other way, depended on vacuum for their cleaning efficiency. When the paddles reversed direction a partial vacuum was created which sucked water through the washing. Later washing machines are unable to do this and although the wet washing gets moved about, only a very small amount of water actually passes through the washing. Unfortunately the old system generally required top loading which is difficult to fit in modern kitchens.
Also read Dinosurs and the Expanding Earth – 2 -The Earth Mechanics
Author – Brian Williams
116 responses to “Understanding Vacuum and Pressure.”
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Enjoyed looking at this, quite great stuff, thanks .
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Certainly like your web site but you need to test the spelling on quite a few of your posts. Several of them are rife with spelling problems and I find it very bothersome to tell the truth however I’ll certainly come again again.
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I do find this type of comment mildly annoying. There are unlikely to be many spelling errors on my web site. A problem is that my website is an English site, and I am English therefore I write in the English language. There are thousands of spelling differences between the English and American languages. Also, many differences in the meaning and/or uses of particular words. In your above comments, in English you would check the spelling, NOT test it, and you would be looking for spelling mistakes not spelling problems.
It is refreshing to find a lawyer who admits to ” I find it very bothersome to tell the truth………”. Yes I know you didn’t mean that, but if you can’t correctly write in the English language I suggest you find more suitable employment.Brian
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The above comment from a SEO company. At this time (6/11/2012) the above posting is at number 6 on page 1 of the google listing out of 30,300,000 entries. It has been on page 1 for a number of months, (this is without any help from SEO companies or others.) I would suggest that she looks up entries for opticians in her area.
I checked under SEO companies and surprise, surprise, her company wasn’t on any of the first 10 pages out of 873,000,000 entries. (This figure would indicate that approximately 12.5% of the World’s population are SEO experts, I must admit that I find this a little surprising)
Brian -
Music began playing anytime I opened this web site, so irritating!
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As there are neither music or video on my web site it must be your computer. Perhaps it gets excited at going to a good website for a change.
Brian -
Very interesting and factual article, but in my opinion does not fully exhaust the subject.
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There are thousands of books and articles covering the subject of vacuum and pressure. However, knowing the technicalities of the subjects does not mean that you understand them. If you are involved in the subjects, keeping my article in mind should prevent you from making serious errors.
Brian -
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Kelly, it is impossible to prevent it happening unless the search engine owners decide to do it. This is unlikely to happen I rely on my publication dates if I need to sue. Plus I have the advantage in my physics related posts (My main concern), in that I can supply full technical backup leading up to the publication of the post. Physicists would not be able to do this and would be subject to ridicule if they tried.
Brian -
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Brian -
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