SHREWSBURY, Vt.—In the December 2015 issue of Maple News, a person discussing 3’16” tubing said that one needed 10’ of fall for the system to work.  At best, 10’ would only give a vacuum of 9” of mercury (9” hg on your vacuum guage).  Not exactly a high vacuum system.  40’ or 50’ of fall is required for high vacuum.   If set up right, these systems will develop a high vacuum in the tubing itself without a pump or releaser.

To see why this is so we must understand some basic physics.  Look at the diagrams in figures 1 and 2.  Figure 1 shows the first mercury barometer developed by Torcelli in Italy in 1643.  Figure 2 shows how it compares with a water or sap barometer. Shortly thereafter, Pascal in France built one using red wine with a 46’ glass tube.  The empty space in the top of the tube is practically a perfect vacuum.  Note 30” of mercury is equal to 34’ of water.  This is because mercury is much more dense. As far as I know, neither Torcelli nor Pascal made maple syrup.  We had to wait almost 400 years for Tim Wilmot to apply their concept to our industry.

Our 3/16” system is simply a sap barometer without the cup at the bottom, tilted at the slope of the sugarbush.  Sap has about the same density as water, but since the sap in a 3/16” line is interspersed with air, you actually need 40’ to 50’ of fall to get a good vacuum.  You really w/ant to see this fall happen below your lowest tap, but this is not always possible.  3/16” systems necessarily have long tails after the lowest tap and before the mainline or tank.  See Figure 3.

The ideal situation would be to have all your taps on a plateau with a vertical cliff giving a 50’ fall.  You put your tank at the bottom of the cliff and all you need is a 50’ tail.  However, these sites are very rare and most of us have more gradual slopes.  For a 30% slope (a very steep hill) a 50’ fall would require 150’ of tail; 10% 500’; 3% 1500’.  3% seems almost flat.   A steep bush works best.  Mainlines are short and really it is better if you can hook the end of your 3/16” tube directly to the holding tank.  These ends must be above the sap level in the tank.

With these long lines economics dictate that you should have as many taps on each line as possible.  I found 35 to be okay, however some people have found the system is overloaded during high flows with more that 25.  With short mainlines and no vacuum pump or releaser this is a cheap system and pretty easy to install.  

There is a limit here.  The theoretical highest vacuum is 30” hg.  That is only obtainable at sea level.  The highest vacuum that you can get is equal to the atmospheric pressure at your site.  The higher in elevation, the lower the pressure.  My sugaærbush is at 1600’ and the highest vacuum I could possibly get is  28”.   You should put a vacuum guage at the top of every line.

We do get readings in the 25” hg range.  The temperature needs to be at least 40 degrees F and the sap flowing well  before high vacuum is developed.  A slight leak diminishes the vacuum considerably, so walk your lines.  Because of the practical layout of the bush, we do have some lines with falls that are quite low.  The highest vacuums on these are in the 10” to 15” hg range.  Falls of more than 50’ don’t really add more vacuum.  Vacuum diminishes the further down the  line you go and is zero at the bottom end.

March 2016