Understanding Vacuum and Pressure.

Understanding 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.

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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