Reverse vacuum at the top of my line

[Obidiah]

All my lines are 3/16 gravity fed with 3/16 drops, at the top of one of my lines the sap just sits there. If I unhook the last tee all the sap shoots down the line like crazy, I can figure out why it isn't running. If I unhook farther down the sap actually sucks back towards the top of the line. Everything is running at recommendation for pitch and all my lines end with a tap but this is the only one doing this. Any and all recommendations are appreciated.

[wnybassman]

Does sap come out of that line at the bottom?

[Obidiah]

Very little compared to the other lines

[RileySugarbush]

That is totally normal. There is a vacuum at the top tap. But you have a nice tight seal on that spile so no air is getting in the line. Thats good. The tree is apparently not giving off any gas, so no bubbles. If that is the case, the sap sill only move as fast as the tree releases it, which for a single tap is very slow. If you pull that tap, the vacuum is broken and the whole volume races down the hill. If you open the line half way down, the lower half will fall down the same way, but the vacuum in the upper half will suck the sap up hill a bit.

[tgormley358]

That is totally normal. There is a vacuum at the top tap. But you have a nice tight seal on that spile so no air is getting in the line. Thats good. The tree is apparently not giving off any gas, so no bubbles. If that is the case, the sap sill only move as fast as the tree releases it, which for a single tap is very slow. If you pull that tap, the vacuum is broken and the whole volume races down the hill. If you open the line half way down, the lower half will fall down the same way, but the vacuum in the upper half will suck the sap up hill a bit.

Id like to understand this better, why it’s normal. I’m seeing similar results to Obidiah - multiple taps and drops with sap sitting in them not moving or flowing into the lateral. My lines are 3/16 gravity with 20 taps on slope between 10 and 20’, each 150-250’ long. Taps and drops are all new. I started with all 5/16 drops but when I saw this problem I changed out some drops and tees to 3/16 cause I ran out of 5/16 fittings. So now I have some of each in drops. I think I’ve ruled out clogs in tees because when I pull a tap the sap flow right down the drop through tee into the lateral. Holding the tap in the tap hole by hand it looks like the natural vac is pulling sap out of the tree vs before there was no flow. What I ended up trying is lightly tap the tap back into the tree until this vac hissing stops but sap and bubbles still flow out the tap and down the drop. If I tapped just one bit harder the sap and bubbles stopped completely, so I back off a bit. I did this in a 3rd of drops and saw an increase in flow at the bottom collection tank.

[BAP]

You may have a small leak down the line.

[Sugar Bear]

Id like to understand this better, why it’s normal. I’m seeing similar results to Obidiah - multiple taps and drops with sap sitting in them not moving or flowing into the lateral. My lines are 3/16 gravity with 20 taps on slope between 10 and 20’, each 150-250’ long. Taps and drops are all new. I started with all 5/16 drops but when I saw this problem I changed out some drops and tees to 3/16 cause I ran out of 5/16 fittings. So now I have some of each in drops. I think I’ve ruled out clogs in tees because when I pull a tap the sap flow right down the drop through tee into the lateral. Holding the tap in the tap hole by hand it looks like the natural vac is pulling sap out of the tree vs before there was no flow. What I ended up trying is lightly tap the tap back into the tree until this vac hissing stops but sap and bubbles still flow out the tap and down the drop. If I tapped just one bit harder the sap and bubbles stopped completely, so I back off a bit. I did this in a 3rd of drops and saw an increase in flow at the bottom collection tank.

I think drops to individual buckets are the best unless you have the money to spend on a pump. Obviously they are not for those who do not enjoy exercise.

CV's with drops to individual buckets outperform my tubing lines no matter what I do.

I have not tried 3/16 tubing however, but the more I read about it the less sleep I get. And I have lost enough as is.

I had a similar problem to you on 5/16 tubing back in February. I increased the pitch from my lowest tap to my collection barrel by shortening the distance between the two from about 90 feet to 30 feet (this steepened the pitch significantly ) and that helped a lot.

It seems to me the final pitch to the collection barrel is what makes or brakes a good gravity run! The closer to vertical as you can get it the better. And no fittings in it to muck the flow up into the barrel. Class 5 rapid would be the best.

But the hydrodynamics of sap in lines are very convoluted and while this is a very informative website on many frontiers nobody on it has yet to de-convolute hydrodynamics in sap lines to any great extent. Nothing personal to anybody. If somebody writes a good book on it I will buy six copies.

40 taps for me this year so far. 8 to a barrel, 7 to a barrel, 6 to a barrel, 5 to same barrel as the 6, 2 to a bucket and 12 on individual buckets. I think that make 40. Some with CV taps some without.

I have kept track of individual gallons of sap retrieved from each.

The 12 on individual buckets and the 2 on a bucket have put out half my sap this year. One of the problems with it is I end up with 5 gallon buckets overflowing even on 1 tap.

To date I have gotten 369 gallons of sap and about 9 gallons of syrup on the 40 taps.

Hope this weekend does not knock anybody out. Me especially. I am all rock so that gives me a good chance.

[Super Sapper]

Id like to understand this better, why it’s normal. I’m seeing similar results to Obidiah - multiple taps and drops with sap sitting in them not moving or flowing into the lateral. My lines are 3/16 gravity with 20 taps on slope between 10 and 20’, each 150-250’ long. Taps and drops are all new. I started with all 5/16 drops but when I saw this problem I changed out some drops and tees to 3/16 cause I ran out of 5/16 fittings. So now I have some of each in drops. I think I’ve ruled out clogs in tees because when I pull a tap the sap flow right down the drop through tee into the lateral. Holding the tap in the tap hole by hand it looks like the natural vac is pulling sap out of the tree vs before there was no flow. What I ended up trying is lightly tap the tap back into the tree until this vac hissing stops but sap and bubbles still flow out the tap and down the drop. If I tapped just one bit harder the sap and bubbles stopped completely, so I back off a bit. I did this in a 3rd of drops and saw an increase in flow at the bottom collection tank.

The weight of the sap in the line is what creates your vacuum. If you open a tap mid way down you have decreased the weight causing the vacuum and having less weight pulling down, the vacuum pulls the sap up until the weight of the sap equals the vacuum. It is like reducing the weight on a spring, it will pull upwards.

[RileySugarbush]

Another way to think about it is something you can try in your kitchen. Take a tall glass of water, insert a straw to the bottom, cover top of straw with your finger and lift it out. The water stays in the straw just like the sap stays in your line. The weight of it is actually pulling a very small vacuum on your finger. ( If the straw was 33 feet long, you would get a near perfect vacuum on your finger) Lift your finger and and it all flows out. Satisfying to watch, but it is over when the sap has run out and the vacuum is gone!


It your try the same experiment with a bigger tube like a 1/2" inch hose, the trick doesn't work because bubbles of air can flow past the water and let the water run out. Diameter is even more critical with the straw held at a more horizontal angle since gravity pulls the water to one side and lets the bubble get past more easily. This is exactly why 3/16 tubing works creating natural vacuum and 5/16 doesn't. In such a small diameter, surface tension on the bubbles is stronger than the effect of gravity on the sap so the bubbles can't get past. So even with bubbles and sap in the line, the sap hangs there and it's weight pulls a vacuum on the upper end of the line.


Id like to understand this better, why it’s normal. I’m seeing similar results to Obidiah - multiple taps and drops with sap sitting in them not moving or flowing into the lateral. My lines are 3/16 gravity with 20 taps on slope between 10 and 20’, each 150-250’ long. Taps and drops are all new. I started with all 5/16 drops but when I saw this problem I changed out some drops and tees to 3/16 cause I ran out of 5/16 fittings. So now I have some of each in drops. I think I’ve ruled out clogs in tees because when I pull a tap the sap flow right down the drop through tee into the lateral. Holding the tap in the tap hole by hand it looks like the natural vac is pulling sap out of the tree vs before there was no flow. What I ended up trying is lightly tap the tap back into the tree until this vac hissing stops but sap and bubbles still flow out the tap and down the drop. If I tapped just one bit harder the sap and bubbles stopped completely, so I back off a bit. I did this in a 3rd of drops and saw an increase in flow at the bottom collection tank.

[tgormley358]

Another way to think about it is something you can try in your kitchen. Take a tall glass of water, insert a straw to the bottom, cover top of straw with your finger and lift it out. The water stays in the straw just like the sap stays in your line. The weight of it is actually pulling a very small vacuum on your finger. ( If the straw was 33 feet long, you would get a near perfect vacuum on your finger) Lift your finger and and it all flows out. Satisfying to watch, but it is over when the sap has run out and the vacuum is gone!


It your try the same experiment with a bigger tube like a 1/2" inch hose, the trick doesn't work because bubbles of air can flow past the water and let the water run out. Diameter is even more critical with the straw held at a more horizontal angle since gravity pulls the water to one side and lets the bubble get past more easily. This is exactly why 3/16 tubing works creating natural vacuum and 5/16 doesn't. In such a small diameter, surface tension on the bubbles is stronger than the effect of gravity on the sap so the bubbles can't get past. So even with bubbles and sap in the line, the sap hangs there and it's weight pulls a vacuum on the upper end of the line.

Thanks John. I remember the finger straw experiment. It helps with my understanding. So when I see sap hanging in a drop seeming to not move down to the lateral, that can be ok since it’s cintinuing to pull sap from the tree, I think you’re saying. But if the lateral has flow and the tee is not clogged, how does the stuck sap in the drop get moving? It sounds like my attempt at getting the sap moving, withdrawing the two slightly so I see sap and bubbles moving, but no vacuum leakage sound, are not really helping. Anyway, I’m getting more sap now that temps have warmed. I’m happy. But I’ll continue to noodle over it as everyone does.

[RileySugarbush]

Tgormley: That is right. The sap in a 3/16 line will not move unless the trees upstream are letting sap or air/gas into the system either from the tree or a leak. In a bigger diameter drop, you can see the flow since it is depart from the air in the tube and either drips down or runs down the side of th tube.

If the trees aren't running, the sap won't move. The exception is when you have a plugged up line. Your quick check by opening the line briefly will let you know if that's the case. If the sap runs down with an open top fitting, you aren't plugged.