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)
|0" Hg||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!
How to choose a vacuum pump:
I highly recommend that you use a rotary vane vacuum pump. Those are the type used by HVAC and refrigeration technicians and will generate the best vacuum for the money. When looking at vacuum pumps, you will see two ratings. One will be the amount of vacuum generated and one will be the CFM rating. Choose a pump with a vacuum rating of at least 29" Hg or or a maximum of 75 microns. I suggest you forget about the CFM rating for the most part! The CFM rating is how many cubic feet per minute of free air the pump is capable of moving. The CFM rating HAS NOTHING TO DO WITH HOW WELL YOU CAN STABILIZE!! It is basically just a measurement of how fast you can get your chamber to full vacuum. Some folks tend to think that if 3 CFM is fine, 10 has to be better and they end up throwing away good money to get that higher CFM pump. The fact of the matter is that the 1/4" fitting on your pump will only allow .75 CFM of vacuum through it! Therefore a 3 CFM pump will pull down my largest chamber just as fast as a 6 CFM pump since they are both being throttled down to .75 CFM at the pump connection! See the video embedded at the bottom for proof!
Next to consider is single stage vs. two stage pumps. With a single stage pump, you basically have one pump cartridge doing the work. With a two stage pump, you have 2 vacuum cartridges. The first pulls a medium vacuum and then the exhaust is run through a second vacuum cartridge that pulls it down even further. A single stage pump will do pretty good but a two stage pump will pull an even deepr vacuum, thus removing more air. However, two stage pumps usually cost considerably more. I would not hesitate to buy a single stage pump as long as it is rated at at least a 75 micron vacuum or less.
I am a big proponent of American made vacuum pumps. The cheap Asian imports are ok but they are not usually serviceable. If something breaks, you toss the pump and get another one while a quality American made pump can easily be re-built. A lot of folks buy Harbor Freight vacuum pumps and while some are happy, quite a few end up being disappointed. From the feedback I have received from customers, the HF pumps tend to put out a prominent oil "cloud" in their shop when it is running. There are better alternative out there, even if you budget does not allow for a quality American made pump! Take a look at the Robinair Vacumaster pumps. I have the 3 CFM pump linked below as a backup/"haul around to shows" pump and it does fine. It is an Asian import and a throw away pump but the price is good for a small pump that does a good job from a reputable company. I would, however, rather see you look in your local pawn shops for a used American made pump! I would much rather have a used high quality American pump than a new Asian import pump for the same money!
Sources for vacuum pumps:
Here are some links to vacuum pump sources to consider. I personally have a 6 CFM JB Eliminator which is a great quality, US made vacuum pump. I bought it at a local pawn shop for $125. The links below are by no means an endorsement of a particular vendor. They were just sources I found while doing a search to make it easier for you to find the model of pump listed.
Robinair 15310 VacuMaster 3 CFM Single Stage Pump $117 (I have one of these as a show pump and it works just fine. See it in the comparison video below)
6 CFM JB Eliminator Vac Pump $284 (I bought one of these at a pawn shop for $125, great pump!) **I recommend and prefer JB pumps.
2.5 CFM Harbor Freight Vac Pump $99 Note: I have had numerous customers complain that the HF pumps put out an oil mist in the air.
2 Stage 3 CFM Harbor Freight Vac Pump $149 Note: I have had numerous customers complain that the HF pumps put out an oil mist in the air
Battle of the Vacuum Pumps 3 CFM vs 6 CFM