In general vacuum as it relates to sap collection is when we remove some of the atmospheric pressure on the taphole. A true vacuum is the removal of all pressure, i maple we attempt to remove as much as we can afford to remove financially. That is tempered with an eye towards how much gain we can expect with the cost to attain that level of vacuum.
That's the basic, in practice we remove the air pressure or rather a portion of that pressure in the taphole. As the tree starts to move sap, it is trying to push sap, the lifeblood to the highest buds on the tree. We remove some of the air pressure and in response more sap is collected. On average, each inch of additional vacuum (vacuum is usually for our purpose measured in inches of mercury) will typically yield 5-7% more sap. This gain is compounded, like a good interest bearing investment.
Even without vacuum the maple tree yields more sap when the air pressure outside the taphole is lower, like in a low pressure event, using mechanical vacuum we reduce the pressure far more than a low pressure front.
Dave Klish, I recently bought a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.