Why the last few inches take so long
A vane pump is a volume machine. Every revolution it swallows the same volume of whatever is in the bag and throws it out, so as the bag empties, each revolution removes less and less air. That gives a logarithmic pull down rather than a linear one, and the practical consequence is that the minutes per inch of mercury climb steeply as you approach the top of the gauge.
On the default figures here the first ten inches of mercury take 0.21 of a minute and the last five take 0.37, which is 0.021 minutes an inch against 0.073 — three and a half times the price per inch by the time you are near the top. Raise the cut-out from 25 to 27 and the whole pull goes from 0.94 minutes to 1.21, which is 29 percent more time for 8 percent more pressure. There is nothing wrong with wanting that pressure; it is worth knowing what it costs in minutes, and that a bigger bag scales the whole curve rather than part of it.
What the gauge is actually telling you
A vacuum gauge reads the difference between the bag and the room, which is exactly the pressure squeezing the panel. Twenty five inches of mercury is 12.3 pounds per square inch, and over a 24 by 48 panel that is a bit over fourteen thousand pounds. That is why a bag does what a hundred clamps cannot, and it is also why a bag is worth respecting when it is down.
Elevation matters here in a way that surprises people. The pump is not weaker in Denver. There is simply less atmosphere, so the whole scale is shorter: local atmospheric is around 24.9 inches of mercury at five thousand feet, which means a perfect vacuum reads 24.9 on the gauge and a setting of 27 does not exist. The page caps the target at what is physically available and says so.
The leak test is ten minutes and it is the only honest input
Pull the bag down, switch the pump off, and watch. How far the gauge falls in ten minutes is a measurement of your bag, your closure and your fittings on that day, and nothing published anywhere can substitute for it. Two inches in ten minutes on the default bag works out to a cycle every twenty five minutes, a duty cycle under two percent, and about nine and a half pump starts over a four hour press.
The number that should make you look at the bag is the duty cycle rather than the leak rate. A big bag with a slow leak and a small bag with the same leak behave completely differently, because the small bag loses its vacuum proportionally faster while taking less time to recover. Comparing two bags on inches per minute alone is comparing the wrong quantity.
Where this sits
The clamp side of the same question, including pressure targets by species and how many bar clamps a glue-up needs, is the glue and clamp calculator, which is where to go if the answer is a row of clamps rather than a bag. What goes into the bag is usually a veneer layup or a bent lamination. If the pump shares a compressor circuit with anything else, the demand side is the air compressor CFM calculator.
Questions people ask
What air gap should I use for the bag volume?
The average standoff between the bag and everything under it, once the work, the platen and the breather mesh are in and the bag is closed but not yet pumped. On a flat panel job that is small, an inch or two, because the bag is already lying on the work. On a curved form with a lot of shape under it, it can be several inches, and the pull down takes proportionally longer. If you would rather measure it than guess, fill the closed bag with a known volume of packing and work backwards, or just use the second mode and enter a volume you trust.
Why does my pump never reach the number on its plate?
Several reasons that this page cannot distinguish between: the plate figure is usually measured on the pump alone with a short connection and a fresh charge of oil, your bag has a real leak rate that fights the last inch, the hose and fittings add restriction, and local atmospheric sets a hard ceiling that is below the plate figure everywhere except at sea level on a high pressure day. The pull down time here assumes the pump moves its rated displacement, so it is a floor rather than a promise.
Is the duty cycle figure telling me my pump is undersized?
No, and the page deliberately does not make that call. A duty cycle is a description of one bag with one leak and one pump, and whether a given pump is happy running that share of the time is a manufacturer question with a real answer that varies by pump type. What the number is genuinely useful for is comparison: fix the bag, change nothing else, run the test again, and see whether the duty cycle moved.
Does more vacuum give a better joint?
That is a glue and layup question and this page will not answer it. What is true is arithmetic: pressure across the bag is the gauge reading times 0.4912 psi, and it acts on every square inch under the bag at once. What pressure a given adhesive and a given veneer thickness want, whether more can crush a soft substrate or starve a joint, and how long it should be held are all on the data sheet and in your own test panels. The page prints the psi and, if you enter your own requirement, prints that beside it without comment.
Can I use these numbers for a bag I have not built yet?
For the pull down side, yes, because that only needs a volume and a displacement, and you can estimate both before anything exists. For the cycling side, no. The leak rate is a property of a real bag, a real closure and real fittings, and there is no way to predict it from dimensions. Build the bag, run the ten minute test, and come back.