Vacuum is a totally different animal than pressure! With pressure, given the right equipment, the amount of pressure is infinite. Vacuum, however, has a limit. The definition of a true vacuum is the absence of all molecules. This, of course, is unobtainable, even in outer space or the perfect laboratory setting. However, we can get fairly close with a good vacuum pump.
The 2 most common measurements for vacuum pump ratings of the amount of vacuum in the US is "inches of mercury" ("Hg) or microns. With "Hg, the higher the number, the higher the vacuum. With microns, the lower the number, the better the vacuum. A perfect vacuum (remember, this is not obtainable!) is 29.92" Hg at sea level on a standard atmospheric day (59° F) or 0 microns. For every 1,000 feet above sea level you live, you loose roughly 1" Hg of vacuum. However, since the air is thinner and the atmospheric pressure is less the higher you go, you still get the same effect at the lower vacuum reading since there is less air to begin with. It is really better to think of vacuum performance as it relates to stabilizing in terms of % of vacuum.
For example, at sea level, a 29" Hg reading on your vacuum gauge indicates a 96.9% vacuum or removal of air inside your blank (29/29.92). A 25" reading on your gauge at sea level indicates an 83.6% vacuum or removal of air inside the blank. However, in Denver, which is 5,000' +/- above sea level, the maximum theoretical vacuum is 24.976" Hg so a reading of 24.5" Hg on your gauge indicates a 98.09% vacuum or removal of the air inside your blank. In other words, make sure you know your elevation above sea level before you freak out that you are not getting enough vacuum! One person's reading is not necessarily the same amount of vacuum as another person's! Calculate your maximum theoretical vacuum with my new calculator!
Now, the big question...how much vacuum is enough? Get as much vacuum as you can get and you will get better stabilized blanks! It only makes sense. The reason we use vacuum when home stabilizing is to remove the atmospherically compressed air that is within the material, thus making room for the Cactus Juice. The more air you can remove, the more Cactus Juice you can get into the blank and the better the blank will be stabilized.
Here are some pictures I took of rubber shop glove that I tied off without any air in it to speak of. This is in my shop at 800 feet above sea level. My maximum theoretical vacuum is 29.055" Hg.
(Click to enlarge)
|25" Hg (86.04% vac)
|26" Hg (89.48% vac)
|27" Hg (92.29% vac)
|28" Hg (96.36% vac)
|28.5" Hg (98.08% vac)
As you can see by the photos above, there is a huge difference in the expansion of the air between 25" Hg and 28.5" Hg. How is the amount of inflation of a shop glove relevant to stabilizing? Well, a piece of wood, in very simplistic terms, is like a bundle of straws that are filled with air. The air inside the blank is compressed by atmospheric pressure. When you pull a vacuum, you are removing most of the atmospheric pressure that is compressing the air which allows it to expand. When it expands, it flows out of the blank and into your vacuum chamber and then is extracted by your vacuum pump. The more air you can remove, the more room there is inside those "straws" for your Cactus Juice!
Since the blanks are submerged in Cactus Juice and the fact that nature abhors a vacuum, when you release the vacuum, the space inside the blank tries to fill back up but instead of air, it fills up with Cactus Juice! Basically, the blank sucks up the resin with the help of atmospheric pressure. Then, the atmospheric pressure continues to hold the Cactus Juice inside the blank instead of allowing it to run out all over the shop floor! All you need to do now is cure the Cactus Juice and viola!, you have a Cactus Juice impregnated blank!