CampHamp
03-23-2014, 03:16 PM
I want to figure out when to automatically shut down my pumps with a thermostat. I also wonder if turning off vacuum and going to gravity would help keep the sap flowing longer into a freeze.
I know that standing water at 1 atmospheric pressure freezes at 0C or 32F. I thought that sap in my lines would freeze around there. Wrong!
The sap in my lines has kept running below -2C or 28.4F and it seems to vary (not much experience catching this observation yet). I expect what causes some fluctuation in this include (1) it is a flowing liquid (not standing still), (2) there are sugars and other solutes in the water, (3) the rate of temp change - a faster drop would probably get to a lower temp before frozen, (4) the sap comes out of the tree warmer sometimes than others (the trees might be warmed deeply by the sun, for example), (5) vacuum? and (6) wind would make heat exchange on the tubing occur sooner.
I found this chart that shows the freezing point as sucrose it increased in water:
9364 [source (http://www.chem.purdue.edu/gchelp/solutions/freeze.html)]
So if my sap is at 2% (usual for me), then I should expect -4C before it freezes [correction: molarity is not the same as percentage or Brix. Sugar content will not change freezing point much at 2%]. Also, since the liquid is flowing through the tube it should even fall to lower temps before freezing, I would expect.
So, when does sap freeze in tubing (highest or lowest temps you've seen)? Does tubing always freeze before the trees stop flowing? What variables have you witnessed that change tubing freeze temps?
I apply vacuum as well (15-23"Hg) using a diaphram pump. I couldn't find info about what the effect of vacuum is on freeze points. I expect the effect is small but may cause a slightly lower freezing point. Any info on this would also be interesting for this thread, especially if it would run much longer if we switched to gravity (removed vac) at the end of the day.
I know that standing water at 1 atmospheric pressure freezes at 0C or 32F. I thought that sap in my lines would freeze around there. Wrong!
The sap in my lines has kept running below -2C or 28.4F and it seems to vary (not much experience catching this observation yet). I expect what causes some fluctuation in this include (1) it is a flowing liquid (not standing still), (2) there are sugars and other solutes in the water, (3) the rate of temp change - a faster drop would probably get to a lower temp before frozen, (4) the sap comes out of the tree warmer sometimes than others (the trees might be warmed deeply by the sun, for example), (5) vacuum? and (6) wind would make heat exchange on the tubing occur sooner.
I found this chart that shows the freezing point as sucrose it increased in water:
9364 [source (http://www.chem.purdue.edu/gchelp/solutions/freeze.html)]
So if my sap is at 2% (usual for me), then I should expect -4C before it freezes [correction: molarity is not the same as percentage or Brix. Sugar content will not change freezing point much at 2%]. Also, since the liquid is flowing through the tube it should even fall to lower temps before freezing, I would expect.
So, when does sap freeze in tubing (highest or lowest temps you've seen)? Does tubing always freeze before the trees stop flowing? What variables have you witnessed that change tubing freeze temps?
I apply vacuum as well (15-23"Hg) using a diaphram pump. I couldn't find info about what the effect of vacuum is on freeze points. I expect the effect is small but may cause a slightly lower freezing point. Any info on this would also be interesting for this thread, especially if it would run much longer if we switched to gravity (removed vac) at the end of the day.