How high is high enough

[Sapboy]

Im in the process of changing a pipeline setup from lower vac. to high vac. and it got me thinking. How high can you go with vacuum and still see higher sap yields? I did a little looking and i havent been able to find an answer so I though it might be fun to throw it out on here. Its widely known these days that you can expect to get 5 to 7 percent more sap for every additional inch of vacuum you can get but I would think that at some point that rate of increase would start to level off...right? Have there been any studies done at extremely high vac. levels (above 35 or 40) to know? Are they're any producers out there running above 30" now?

[DrTimPerkins]

Have there been any studies done at extremely high vac. levels (above 35 or 40) to know?

Yes, definitely a "fun" question. Just be sure that when you get to 35-40" Hg you stop to avoid harming the tree. Most gauges stop around 29-30" Hg though.....so you'll have to get the really expensive gauges to know when you're above that. Perhaps somebody can tell you were you can find them. Or you could call Goodrich's to see if they carry them.

[Sapboy]

Thanks Dr. Tim! Im sure I wont get vacuum that high. If I can get 28" to 30" id be very content. When I put my first pipline system in my dealer told me at that time if I could get 20" id be doing pretty good. Now we are all trying to get to the upper 20s. Just wondering if we're all going to try to be going for 35" plus in another 10 or 15 years.

[Thompson's Tree Farm]

My understanding was that according to the laws of physics, you could not get over a bit past 29 inches and that is at sea level.

[DrTimPerkins]

My understanding was that according to the laws of physics, you could not get over a bit past 29 inches and that is at sea level.

Yes....you are correct. Sorry Sapboy...I couldn't resist playing with you a little.....you're not going to ever hit that 35-40 range....not even if you're sugaring in deep space.

"Hg is a "relative scale" as used in the maple industry is relative to standard air pressure. Vacuum is the pressure BELOW atmospheric pressure, or a "partial pressure." The actual "standard atmospheric pressure" on the absolute scale is 29.92" Hg, which in maple we consider to be zero (on our relative scale). So when we get to 0" Hg on this absolute scale (which is -29.92" Hg on the absolute scale....although we typically ignore the negative....so "high" vacuum is actually low partial pressure), there are no molecules left to remove....it is a "pure" vacuum, and no amount of pumping will reduce it below that point. In actuality, no vacuum pump on earth, regardless of how good it is, is capable of achieving pure vacuum.....and certainly not those used in the maple industry. To even get close you need to use some real exotic machinery....like mercury distillation vacuum (like they use in electron microscopes).

As far as the sap yield relationship with vacuum....it is linear across the range (at least as far as we have measured), but there is no reason to believe it levels off. This is because the sap exudation mechanism is almost purely a physical process. In short, the more you suck (the higher the vacuum -- thus the lower the pressure in the taphole), the more sap you get.....at least until you hit the limit (which is approximately -29.92" Hg at sea level under standard temperature on our relative scale). Given that most sugarmakers are somewhat above sea level, and not using pumps capable of pulling what (scientifically) is called "high vacuum", you're never going to get to that. The standard gauges used in maple aren't terribly precise or accurate (typically they are +/- 2%, so can be off by 0.6"Hg either way), so people may think they're pulling 29-30", but really aren't. If you are.....you've got one of those "special" gauges I mentioned (which don't really exist), or the calibration is just way off.

[Sapboy]

Well thats a bit embarassing. Now that I think about it that does makes sense. But good news for me. Once I get my vacuum levels caught up to everyone else at least I wont be able to fall behind in that area anymore.

[DrTimPerkins]

Well thats a bit embarassing. Now that I think about it that does makes sense. But good news for me. Once I get my vacuum levels caught up to everyone else at least I wont be able to fall behind in that area anymore.

No need.....you are quite far from the first to ask this question. Vacuum is one of those strange things....with the absolute and the relative scale and high vacuum being a very low (negative) number. And then when we get above standard air pressure we switch units to PSI. It is confusing to a lot of folks.

I've got a long list of embarrassing/stupid things I've done......about 52 yrs worth so far.....and more to come I'm sure. Just laugh it off and move along.

[sjdoyon]

Oh boy, I can see the dealers advertising their vacuum pumps as the closest to "Pure Vacuum" as you can get, requiring us all to run out and get new pumps.

When I read the 35-40 HG comment, I thought you'd next throw in the high vacuum would cause your maple trees to explode.



Yes....you are correct. Sorry Sapboy...I couldn't resist playing with you a little.....you're not going to ever hit that 35-40 range....not even if you're sugaring in deep space.

"Hg is a "relative scale" as used in the maple industry is relative to standard air pressure. Vacuum is the pressure BELOW atmospheric pressure, or a "partial pressure." The actual "standard atmospheric pressure" on the absolute scale is 29.92" Hg, which in maple we consider to be zero (on our relative scale). So when we get to 0" Hg on this absolute scale (which is -29.92" Hg on the absolute scale....although we typically ignore the negative....so "high" vacuum is actually low partial pressure), there are no molecules left to remove....it is a "pure" vacuum, and no amount of pumping will reduce it below that point. In actuality, no vacuum pump on earth, regardless of how good it is, is capable of achieving pure vacuum.....and certainly not those used in the maple industry. To even get close you need to use some real exotic machinery....like mercury distillation vacuum (like they use in electron microscopes).

As far as the sap yield relationship with vacuum....it is linear across the range (at least as far as we have measured), but there is no reason to believe it levels off. This is because the sap exudation mechanism is almost purely a physical process. In short, the more you suck (the higher the vacuum -- thus the lower the pressure in the taphole), the more sap you get.....at least until you hit the limit (which is approximately -29.92" Hg at sea level under standard temperature on our relative scale). Given that most sugarmakers are somewhat above sea level, and not using pumps capable of pulling what (scientifically) is called "high vacuum", you're never going to get to that. The standard gauges used in maple aren't terribly precise or accurate (typically they are +/- 2%, so can be off by 0.6"Hg either way), so people may think they're pulling 29-30", but really aren't. If you are.....you've got one of those "special" gauges I mentioned (which don't really exist), or the calibration is just way off.

[jrgagne99]

Dr. Tim,
How's about doing some research where you create >30 inHg delta-P across the bark by enclosing the entire tree or two inside a pressurized dome (like a futuristic Lunar or Martian habitat dome) and then pulling hard vac at the tap! :-) While you're at it, you could do some large-scale climate control in the dome and create the ideal temperature conditions to boot! That would be some interesting research!

But seriously, do you think the linear trend with delta-P would continue past 30 inHg?

[DrTimPerkins]

But seriously, do you think the linear trend with delta-P would continue past 30 inHg?

Yes, I suspect that it would. We have used pressure to perfuse xylem tissue on several occasions, so it is quite possible to either pull the water through the vascular tissue with vacuum or push it through with pressure (which really is the same thing if we think of it on an absolute scale). The resistance to water movement through the system is relatively constant. So if the resistance stays the same and the driving force increases, flow would increase. You would have to separate the roots from the stem somehow, with the roots surrounded by water under pressure...and then pull a vacuum on the taphole in the stem. This would achieve a pressure gradient higher than you could get with vacuum alone.

[maple flats]

Maybe then a big producer might only need run maybe 100 taps and a really huge one maybe 250 taps. Of course he would need to have forced watering to the roots to get the gains needed.
While such may technically be achievable, I would think that we would at some point be removing too much sweet and make it impossible for the tree to open it's buds. It seems like we would thus kill our golden cow.

[PerryW]

Thanks for the clarification Doc. I had to look at my calendar to see if it was April Fool's Day.