UNDERSTANDING VACUUM
People always ask me "How much pressure is exerted on a project that has been placed inside a vacuum bag?" The answer to this is simple! If you take a 1" x 1" square tube that stretches from outer space to sea level the amount of air pressure inside that tube will be 14.7 pounds per square inch (PSI or psia).
It is almost impossible to achieve a 100% vacuum environment without using the most sophisticated (and very expensive) vacuum pumps. Most electric vacuum systems built for woodworking, produce between 11 to 13 lbs of vacuum pressure. This is more than enough pressure to vacuum most veneer projects. 13 PSI may not sound like a lot of pressure, but remember that to calculate the true pressure over the entire area of your project you need to multiply 13 PSI x the area. If your project is 12" x 36" the calculation would be 12 x 36 x 13 = 5616 Pounds The amount of overall pressure being exerted on your project will be over 5600 pounds, more than driving an SUV onto your veneer! The good thing about vacuum bag pressure is that the weight is evenly distributed over your entire project.
To understand vacuum you need to know a little about how it is measured. Here in North America, the easiest measurement for vacuum is pounds per square inch. Another common way to measure vacuum is in "HgA". This is a measurement of how far vacuum can pull mercury up a tube. 14.7 lbs of vacuum is 29.92 HgA. This vacuum conversion chart shows the different measurements based on the amount of vacuum produced:
Another question I am frequently asked is "How much vacuum does a household vacuum cleaner make?" This type of blower or fan pump will produce not much more than 5.8 PSI, not enough pressure to properly vacuum veneer with. A vacuum cleaner is a high volume/low PSI pump - lots of air flow but little real power. On the other hand, an electric vacuum pump that would be used for vacuum veneering can produce up to 13 PSI of pressure. It is a low volume/high PSI pump - not much airflow but lots of power. The Thin Air Press manual pump is also a high PSI pump, capable of producing 13 lbs of vacuum pressure. It is however, a low volume pump depending on how much elbow grease is applied.
Below is a chart showing the differences between the three pumps. Because you need at least 11 lbs of vacuum pressure to properly vacuum veneer, only the electric and manual pumps have sufficient pumping power to do the job.
The last question that I am asked a lot is "Do you need a vacuum gauge to monitor a vacuum bag project?" A vacuum gauge is a luxury, but not a neccessity for vacuuming woodworking projects. It is very easy to monitor your vacuum project by pulling the edges of the vinyl bag once in a while after you have pumped the air out. You will know right away if air is leaking back into the bag, as you will be able to pull the vinyl away from your project.
Here is a handy online conversion calculator that I created. It will convert PSI to HG and give you the total pounds of pressure inside your TAP vacuum bag.
Here is and interesting question: "Does water based glue boil in the vacuum bag while laminating a project?"
Link to larger Chart The answer is: Yes!
I hope this answers some basic questions about vacuum pressure.
Ted Hunter