Vacuum can pull sap from one side of the tree to the other...but ONLY after SEVERAL days of no-freeze conditions. The reality is that in many seasons a good share of the flows only last a few hours to one day long before the tree freezes again, so we are not ever getting to that condition (vacuum pulling sap across the tree). We did not drain the tree system to the largest extent possible given the time available. That means we are leaving sap inside the tree that we might have extracted had we been able to exploit the anatomy of the tree more optimally with a better spout design. It is not a question of how MUCH sap we can get out....it becomes a matter of how FAST we can get sap out before it is not possible to get it all out (due to a freeze).
If your goal was to put out a house fire, would you rather do it with a garden hose or a firehose? It is a matter of getting the most flow out of the system as quickly as possible. The only difference is we're not trying to put out a fire, we're trying to extract sap ($) from the tree as quickly as we can.
I do agree that there is a fine line. As we wrote, it's a "Goldilocks" problem. Too shallow driven is bad...too deeply driven is bad...just right is good. Finding that "sweet spot" is not always easy, and it is probably best to go just a hair deeper (to reduce heaving) than a hair too shallow.It's kind of like the studies I've read about driving your spouts in too far I have been very focused on this in the last couple years and it has caused me more harm than good.
Clearly overdriving is bad and clearly blocking off too much wood with regular spouts is also not optimal. Otherwise, why not drive a 3/4" deep taphole and drive it in 1/2" and leave only a 1/4" gap in the tree. I guarantee you that doing that will result in poor yield even with great vacuum. We need to sever a certain amount of wood vessels to maximize flow from the taphole. The next step is to maximize flow RATE from the taphole by reducing wood blockage in the shallowest wood rings.
We have found several ways to get MORE sap from a tree over a season, some are simply not doable for one reason or another...others are not economical given current syrup prices (dual lateral/dual droplines are a good example). But there is clearly more sap available in a tree than what we are getting via current standard practices.At the end of the day you can only get so much sap out of a tree in a season ...
You are right that seasons are all different. Some years we saw small (but detectable) gains in sap yields with the new spout (shorter barrel with barbs) if there was an abundance of long-slow thaws. Other years we saw sizeable gains with it if there were a lot of short-fast thaws followed by a freeze. All depended upon the characteristics of the flow periods.
Agreed.I dunno proofs in the pudding