I believe the vacuum generated will be about .85"/ft drop or there abouts when sap is flowing good and somewhat less on slow flows. The more sap in the 3/16 the greater the vacuum.
Somehow if I'm picturing your set up question right, I think a tail from the last back down to the main will be counter productive. It seems that would pull sap from the main if it was full enough, using the sap in the line generated from the first line going past all the taps until you achieved a balance and then the vacuum might be equal in both directions, if it picked up no sap from the main, it would be like venting the top tap, which kills any vacuum.
The property of vacuum generation using 3/16 is that the sap does not pass the gases (bubbles) in the line.
While you were able to get what looked like good flow with 500' of 5/16 and 110 taps, you were getting less than what a vacuum pump could have because you were over loading the line and line friction held it back some.
Others have posted similar results in the past on 5/16 too, but just do an experiment. Connect a pump to push water thru a 500' roll of 5/16 and measure the flow for a minute, then do the same using 50 or 75' of 5/16 and measure again. The difference is due to line friction.
Heck, even when using air hoses to power a higher consumption air tool, you lose from line friction if the air line is too long.
Back to your question, at 3-5% slope you can generate vacuum on 3/16 even without the ideal 30' drop in elevation and in that case, longer is better as long as you do not exceed about 30-35 taps on the line (one study by, yes the college degree) Tim Wilmott suggested that 37 might be the max, I have 41 on one line that is 1200' long, I'll be changing that to 2 lines for this next season. And I have about 30' drop in the lower 60-70'.
With 3% slope if only 200' long (total 6' drop) you might get somewhere near 5" at the top and progressively less as you go down the line, if that line is 500' you should get 2.5 times that at the top, then less as you go downhill. If the drop was an average of 5% the vacuum increases proportionally.
Last edited by maple flats; 11-19-2017 at 07:15 AM.
Dave Klish, I recently ordered 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.