Vacuum Through Laterals

[rhwells2003]

So vacuum is a tricky thing for me to get my head around. My dad met with a rep from one of the big maple producers probably 6mo ago when we were talking about setting up their woods. He mentioned something quickly that I've had a hard time getting my head around. He said vacuum doesn't transfer through sap. Your laterals are always full of sap I understand that, and the bubbles you see in your laterals are gas released by the trees, I also understand that. So if vacuum doesn't transfer through sap does that mean during a closed system and all your taps in the tree, and the laterals full of sap you have no vacuum at your tap? If so does the only way vacuum actually benefits your system is by pulling the sap from the lateral quicker at the saddle therefore allowing more sap to enter the lateral? If this is true and vacuum doesn't transfer through sap how do ladders work when your using star fittings? Would that mean the main prior to the ladder has vacuum, but the main after the ladder doesn't?

[GeneralStark]

It's really not about "transferring vacuum" but instead think of it as removal of air from the system. Air can move faster through the pipeline than sap so they compete unless you have a thin stream of sap in the bottom of the pipe and plenty of space for the air to move over it, or a dedicated line for each (wet/dry system). In a lateral the diameter is small so you are seeing the sap and air competing as they move through the tubing. Assuming the movement through the tubing is adequate for the removal of the air and sap, vac. (once again absence of air) is maintained at the taphole. If there is too much friction in the tubing (ie too long, lots of bends, many fittings, etc.) then there will be less vac. at the taphole due to reduced ability to remove air.

This is why with high vac. setups you strive for five (five or less taps) and less than 100'. Many go to 1-3 taps and keep them very straight and short as possible. The ideal would be mainline to each tree with a short drop of tubing to the saddle. Or even wet/dry laterals. There is a good article in the recent Maple Digest about research on vac. systems that gets into this in more detail.

Now it is starting to look like using 3/16" tubing for laterals (assuming they slope downhill) may be the better way to go (mainly speaking cost wise) as you are no longer as concerned about air removal to create the vac. but are instead taking advantage of the column of liquid moving downhill to create the vac.

The best way to see this is to put vac. gauges on the end of laterals. People get excited about 24" at the releaser, but that in no way means there is 24" at the taphole.

[DrTimPerkins]

If you simply add the word "well" though the reps statement, you get "vacuum doesn't transfer well through sap" then you're closer to the real situation.

The General explained it well, so I won't expound on the subject.