I know there are a lot of folks that know all about this so please just ignore it, however there are also lots who don't understand the basic process. I did do a search and could not find a similar thread, so here goes; ( Sorry, it's a bit long )[:I]
If you are considering stabilizing your own wood, but not sure which to choose, then I hope this will help.
For our purposes we will ignore terms such as 'absolute pressure' and for simplicities sake we will use Bar as a measure of pressure.
Air is a gas, and as such has a mass. The average pressure of air at sea level is 1 Bar. This is because of the mass of air above it. The higher we go, the thinner the air. These air particals are not as compressed as the ones at sea level.
If we seal a chamber at sea level with 1 Bar pressure in it, we can do two things with it. First, we can force more air into the chamber with a compressor. If we increase this pressure until there is 2 Bar pressure inside the chamber, it is obvious that the air particles in the chamber have to occupy half the amount of space as before.
Second, we can remove ( most of ) the air in the chamber with a vacuum pump. If we remove half of the air in the chamber, we would have 0.5Bar, if we could remove all of it, then we would have 0 Bar, or a vacuum.
We can use both of these to our advantage.
If we submerge a piece of wood ( which is full of air pockets ) in a liquid and apply an extra 1 Bar pressure above it, then the air pressure would force the liquid ( which cannot be compressed ) into the wood. Some of the air in the wood, that near the surfaces would be forced out of the wood to be replaced by the liquid. The rest of the air trapped in the wood will be compressed by the liquid being forced further into the wood.
However, when the applied pressure is released, the trapped compressed air in the wood will expand until it reaches 1 Bar again. This forces some of the liquid back out of the wood. It also leaves air pockets still in the wood.
If instead we remove the air in the same chamber while allowing the wood to float in the liquid, then while the pressure is being reduced, the air particles are being allowed to expand to their non-compressed state. The air in the wood also expands and forces its way out of the wood. The liquid may appear to boil as the air bubbles are released. As the pressure is reduced further by the vacuum pump, the air is then being 'sucked' out of the chamber and the wood. These now voids ( or vacuums ) in the wood now suck liquid in to replace the lost air. Some air will still be trapped in the wood, but at a reduced, near vacuum level.
Now when the vacuum is released, the normal 1 Bar pressure now forces the liquid further into the wood, while the voids, or vacuums help suck liquid in as well.
Much has been said about the merits of each. The fact is that it is easier, quicker and cheaper to apply pressure than to create a vacuum. Vacuuming, even though more difficult and slower is infinitely safer. A catastrophic failure of the chamber while under pressure may create exploding missiles while the compressed air almost instantly expands to the normal ambient pressure. The same failure while under vacuum is easy to contain and unlikely to cause any serious problems as the pressure is imploding, sucking any missiles inwards.
A faulty compressor could kill you, a faulty vacuum pump just won't suck!!
Several Bar above normal air pressure may be required to achieve the same penetration as can be achieved by a vacuum.
Some procedures of treating wood use both, first vacuum then pressure. Others also introduce heat, which speeds up the process.
For the average 'do-it-yourselfer', who wishes to stabilize their own wood, vacuum IS the better safer choice.
I hope that someone may find this helpful and perhaps understand the risks a little more.
If you are considering stabilizing your own wood, but not sure which to choose, then I hope this will help.
For our purposes we will ignore terms such as 'absolute pressure' and for simplicities sake we will use Bar as a measure of pressure.
Air is a gas, and as such has a mass. The average pressure of air at sea level is 1 Bar. This is because of the mass of air above it. The higher we go, the thinner the air. These air particals are not as compressed as the ones at sea level.
If we seal a chamber at sea level with 1 Bar pressure in it, we can do two things with it. First, we can force more air into the chamber with a compressor. If we increase this pressure until there is 2 Bar pressure inside the chamber, it is obvious that the air particles in the chamber have to occupy half the amount of space as before.
Second, we can remove ( most of ) the air in the chamber with a vacuum pump. If we remove half of the air in the chamber, we would have 0.5Bar, if we could remove all of it, then we would have 0 Bar, or a vacuum.
We can use both of these to our advantage.
If we submerge a piece of wood ( which is full of air pockets ) in a liquid and apply an extra 1 Bar pressure above it, then the air pressure would force the liquid ( which cannot be compressed ) into the wood. Some of the air in the wood, that near the surfaces would be forced out of the wood to be replaced by the liquid. The rest of the air trapped in the wood will be compressed by the liquid being forced further into the wood.
However, when the applied pressure is released, the trapped compressed air in the wood will expand until it reaches 1 Bar again. This forces some of the liquid back out of the wood. It also leaves air pockets still in the wood.
If instead we remove the air in the same chamber while allowing the wood to float in the liquid, then while the pressure is being reduced, the air particles are being allowed to expand to their non-compressed state. The air in the wood also expands and forces its way out of the wood. The liquid may appear to boil as the air bubbles are released. As the pressure is reduced further by the vacuum pump, the air is then being 'sucked' out of the chamber and the wood. These now voids ( or vacuums ) in the wood now suck liquid in to replace the lost air. Some air will still be trapped in the wood, but at a reduced, near vacuum level.
Now when the vacuum is released, the normal 1 Bar pressure now forces the liquid further into the wood, while the voids, or vacuums help suck liquid in as well.
Much has been said about the merits of each. The fact is that it is easier, quicker and cheaper to apply pressure than to create a vacuum. Vacuuming, even though more difficult and slower is infinitely safer. A catastrophic failure of the chamber while under pressure may create exploding missiles while the compressed air almost instantly expands to the normal ambient pressure. The same failure while under vacuum is easy to contain and unlikely to cause any serious problems as the pressure is imploding, sucking any missiles inwards.
A faulty compressor could kill you, a faulty vacuum pump just won't suck!!
Several Bar above normal air pressure may be required to achieve the same penetration as can be achieved by a vacuum.
Some procedures of treating wood use both, first vacuum then pressure. Others also introduce heat, which speeds up the process.
For the average 'do-it-yourselfer', who wishes to stabilize their own wood, vacuum IS the better safer choice.
I hope that someone may find this helpful and perhaps understand the risks a little more.
