The physics
How natural vacuum works
A sealed tubing line running downhill fills with sap. That column of liquid has weight, and as it drains it pulls on everything above it. At the taphole that pull is a vacuum, and vacuum is what raises yield: the 2022 Producers Manual puts the gain at about 5 to 7% more sap for every inch of mercury at the tree.
The arithmetic starts with water: one foot of water column equals 0.88 inches of mercury. Tubing never holds pure liquid, so the Manual gives 0.6 to 0.8 inHg per foot of drop in practice and UVM Proctor's 2017 note says about 0.75 to 0.80. UVM's Tim Wilmot used the example of a full 20-foot drop making 17.6 inHg, “however, the line will never be completely full.” Trees low on the slope see less vacuum than trees higher up, and the vacuum only develops while the line is mostly full, which means during heavy flows.
| Drop from top tap to tank | At 0.6 inHg per ft | At 0.8 inHg per ft |
|---|---|---|
| 10 ft | ≈ 6 inHg | ≈ 8 inHg |
| 20 ft | ≈ 12 inHg | ≈ 16 inHg |
| 30 ft | ≈ 18 inHg | ≈ 24 inHg |
Those are ceilings, not promises. In the field, UVM's 3/16-inch lines of 8 to 20 taps “reached and maintained vacuum of 24 inHg or more” for the whole season, and Cornell measured 24 to 27 inHg at the top taps of steep drops. The small bore is the whole trick: 5/16-inch tubing needs 25 to 50 taps per lateral to fill enough to pull vacuum, and even then does it only in strong runs. One more rule follows from the physics: vented spouts kill it. The Manual says venting “serves no useful purpose,” eliminates natural vacuum and lets microbes in.
Sources disagree
How much slope you need (3% or 5%)
Here the two best sources part ways, so here are both. UVM Proctor (2017): the drop “should be on the order of at least 3%, but steeper is better.” The 2022 Producers Manual: 3/16-inch tubing should be used only on sloped land, with “5+ percent” optimal, because on low slopes or flat ground the narrow bore adds friction, negates the benefit and can even build pressure in the line.
For tubing in general the Manual calls 2% or more optimal and 0.5% the extreme-care minimum. Within a system, laterals should pitch downhill as steeply as practical with no sags, and UVM's 2024 tip sheet gives mainlines an optimal 2 to 4% slope with laterals running uphill from them. A 3% grade is about 3 feet of drop in 100 feet of run; if you cannot find that, read the last section.
Two ranges in one book
Taps per lateral
The 2022 Manual gives two answers. Its tubing-design list says 15 to 35 taps per 3/16-inch lateral, with or without pumped vacuum. Its natural-vacuum section says 10 to 25 taps, and adds that the lines “can be very long.” We record both rather than pick. The research lines were on the low side: Cornell's 2015 cooperator ran 220-foot laterals with 8 to 11 taps, and UVM's trial lines carried 8 to 20.
Compare 5/16-inch tubing under a pump, where the rule is “strive for five, no more than ten” taps per lateral (Cornell's beginner notebook says 5 to 6 is ideal). The opposite design, 25 to 50 taps on a 5/16-inch gravity lateral, exists only to fill the line and coax a little natural vacuum out of a big bore.
Cornell vs UVM Proctor
The results, year by year
Cornell's single-season numbers made 3/16-inch tubing famous. UVM Proctor's multi-year trial explains why some producers were disappointed by year three. Both are right about what they measured.
| Trial | 3/16-inch | 5/16-inch | Note |
|---|---|---|---|
| Cornell demonstration, 2014 (8 taps each) | 10.3 gal | 5.8 gal | Tubing Notebook. On a 32-ft drop: 16.6 vs 3.3, where the 3/16 line appears to have stolen sap from the adjacent 5/16 spouts. |
| Cornell cooperator, 2015 (23-ft drop, 8–11 taps per line) | 18.2 gal | 11.25 gal | +62%. Tanks on the 3/16 side overflowed between collections. |
| UVM Proctor, 2015 (new systems) | +12% | baseline | Three-season sanitation trial, new spouts every year. |
| UVM Proctor, 2016 | +3.8% | baseline | Edge shrinking. |
| UVM Proctor, 2017 | about 10% less | baseline | Third season: 3/16 fell below 5/16 despite new spouts. |
| Cornell Uihlein Forest, 2017 | 19.5 gal | 18.5 gal | Not statistically significant. |
Two things carry across every row. New 3/16-inch tubing on a real slope outperforms new 5/16-inch gravity tubing. And the gap is not permanent on its own.
Plugged fittings
Why the advantage fades, and the fix
The Manual's explanation is plain: 3/16-inch systems are “susceptible to microbial plugging at tees and unions over time.” The bore that pulls the vacuum also clogs first. In UVM's trial, sanitizing the lines with bleach raised 3/16-inch yield by 53%, against 21% for 5/16-inch, which points to fitting plugging as the main loss. The Manual's remedy is good sanitation or replacing the drops and all fittings every 2 to 3 years.
The spout research was done on 5/16-inch
Flat ground and pumps
When 5/16-inch is the better choice
- Flat or gently sloped woods. Below the 3 to 5% thresholds above, 3/16-inch tubing adds friction without adding vacuum. Use 5/16-inch and, if you want vacuum, a pump.
- Pumped systems. With a pump the standard is 5/16-inch laterals of five to ten taps. A small diaphragm pump needs no releaser or moisture trap and works well holding a 15 inHg base; liquid-ring pumps reach 26 to 28 inHg if their cooling liquid stays cold.
- Hybrids add up. Natural vacuum plus pumped vacuum equals total vacuum: the Manual's example is a pump holding 15 inHg in the mainline plus 10 inHg from 3/16-inch laterals for 25 inHg at the tree. UVM Proctor says the two are additive.
- 3/16-inch drops into buckets. UVM Proctor (2017) says any benefit “would be marginal,” since the spout would have to sit higher to gain head. A 2014 farm-press column on Wilmot's early work claimed single droplines into buckets did yield more. Both are on record; the research note is the more cautious one.
Whichever bore you run, plan 1.5 gallons of sap storage per tap per day of flow for gravity tubing, and 2 gallons under vacuum. Cornell's 2015 tanks overflowed on the 3/16-inch side. Use the sap forecast to see the big days coming.
