Vacuum pump interpretation
Hi All,
I'm interested in building a vac bag system. I have been to Joe Woodworker's site and have learned a lot. My problem is understanding the relationship between different methods of describing the specs on pumps. JoeWW says that a pump should hold at least 25" hg and VacUPress systems also describe their pumps in terms of inches of mercury. However, when I look at offerings on eBay or on the internet, they often talk about microns of vacuum. How do I compare microns of vacuum to inches of mercury? Also, I assume that the CFM rating just indicates how fast the pump will get down to its maximum vacuum capacity. Is the size of the electric motor that drives it (I'm not interested in a compressor driven system) or the mechanism of the pump (piston, rotary vane, etc.) of major significance to a woodworker?
Thanks for your help,
Mark Blumer (East Lansing, MI)
Orifice & hose length is IMPORTANT(long)
No matter what pump you have or how low a vacuum you want, what you suck thru is just as important. The speed of the pull is inversely proportional to the square of the orifice or length of hose. If you double the length of the hose between the pump and the vacuum chamber, it takes FOUR times as long to do the same amount of work. A 3/8" hose will pump five times faster than a 1/4" hose. The smallest orifice in the system will determine the rate even if most of the system is larger. Some people use quick disconnect air fittings for vacuum service. The actual orifice on a 3/8" hose fitting is usually less than a 1/4".
A refrigeration mechanic uses vacuum to remove moisture from the system. He has to reduce the pressure to a point that the water in the freon tubing will BOIL at room temperature. When the vapor pressure of the water is greater than the atmospheric pressure inside the tubing, the water will boil, allowing the pump to then suck out the water vapor( low temp steam from the water boiling at 70 °F). This happens below 50 microns of pressure. If the water is not removed, it will freeze, stopping up the expansion valve which shuts down the cooling system.
On Mount Everest, with much lower atmospheric pressure, water boils considerably below 212 °F. Pressure cookers have the oposite effect. The boiling temp of water inside the cooker is raised because the vapor pressure has to overcome the higher local atmosheric pressure. Therefore the food cooks faster.
Vacuum can be expressed in quantities from either end of the scale. Thirty inches(29.92) is the amount below atmospheric you have gone.Zero pressure guage, the pressure at sea level is really 14.7 psi above true zero pressure(absolute). When we refer to torr or microns we are quantifing how much above absolute zero pressure we are at. 29.92" Hg of vacuum = 0 torr= 0 microns=0 psi absolute. To further add to this confusion, The US Weather Bureau corrects local barometric pressures to sea level. Here in the NC mountains when the televised BP is ± 29", it is really 26.5" due to our altitude of 2 to 4000 feet.
Bill in WNC mountains