small diameter tubing is used for natural vacuum in maple sap collection systems

What is Natural Vacuum?

Natural vacuum describes the process of using gravity, rather than a pump, to create a vacuum at each tap hole in the maple trees. The weight of the maple sap in the tubing is ‘naturally’ pulled down by gravity which creates a vacuum in the top of the tubing system where the trees are. The effect is similar to a siphon and can increase maple tree sap productivity to the point where it rivals, or exceeds, the productivity of mechanical vacuum pumps that are typically used in the maple industry.

There are some limitations, but natural vacuum can be a boon to smaller maple farmers. Production can be nearly double compared to the use of buckets or tubing without vacuum. Capital investment, fuel, and maintenance costs are much lower because natural vacuum does not require the use of vacuum pumps and sap releasers.

This article covers some of the basics of maple sap flow, mechanical pumps, and natural vacuum systems. A future article will cover the specifics of the smaller diameter 3/16” tubing system required to optimize for natural vacuum.

Maple Sap Flow

Here in Vermont, we watch the temperatures closely during maple syrup sugaring season. Maple tree sap flows best when the nights are in the twenties and the days are in the forties. The freeze-thaw cycle is what forces the sap to flow up and down the tree.

The pressure inside of the tree compared to the pressure outside of the tree is also important. When the sap is flowing up and down the tree, some sap will also flow out of the tap holes into the buckets or tubing system. A higher pressure differential between the inside of the tree and the outside atmospheric pressure will make more sap flow out of the tap hole.

Due to normal weather conditions, that pressure differential can vary widely from day to day. We’ve all heard the weather reports of ‘an area of low pressure’ or that ‘there will be a high pressure front moving through’. These pressure changes can drastically affect the flow of maple sap from a tree. With a vacuum system, maple farmers can increase the pressure differential in order to increase production of the maple sap.

Sap Buckets and Tubing without Vacuum

The historic rule of thumb when using buckets is that each tap will yield about one quart of maple syrup per season. There is no vacuum with buckets, so farmers are at the whim of Mother Nature. I have looked at an empty bucket hanging on a tree in the beautiful sunshine on a ‘perfect’ sugaring day and been surprised that the sap was barely dripping. Apparently, there was high atmospheric pressure keeping most of the sap in the tree. That same bucket might have been overflowing if a low pressure front had moved in and the day was overcast and drizzling.

maple sap buckets hanging on Vermont trees will make about one quart of maple syrup per bucket

Now that the old style galvanized buckets are no longer used, all commercial operations have moved to tubing systems. While most folks use vacuum pumps, the pumps are not required. Without vacuum, the tubing only helps to collect the sap; it does not increase the pressure differential to increase sap production. Like a bucket operation, this type of tubing system will generally yield about one quart of maple syrup per tap.

Vacuum Pump Systems

With the addition of vacuum pump systems, maple syrup yield is closer to two quarts per tap. With the possibility of doubling production per tap*, it is readily apparent why nearly all modern maple syrup operations use vacuum systems. It’s not that producers are greedy, but the finances of farming push folks to get as much sap from the trees as they can.

(*Note that this is doubling per tap, not total yield. Modern recommendations are for about half the number of taps per tree so there is an argument that using vacuum is better for the trees because there are fewer tap holes.)

The typical vacuum systems include pumps and tubing to transfer vacuum to every tree throughout the sugarbush. At the bottom of the network, sap releasers are used to transfer the sap from the tubing system into the bulk storage tank while maintaining the vacuum further up the lines.

With the size of the factory farms these days in the hundreds of thousands of taps, the pumps can be massive pieces of infrastructure. I was hiking in the woods of Vermont’s Northeast Kingdom and came across the pump house below. It was housed in a shipping container. The power source was two massive generators about half a mile away. The beefy power cables and a propane line ran all the way from the sap processing building that housed the reverse osmosis equipment.

a modern maple sap pump housed in a shipping container deep in the woods

In addition to the pump, a vacuum system requires nearly twice as much mainline tubing to transfer the vacuum throughout the sugarbush. When you see tubing installations, there are often two mainlines running in a pair. The lower one is ‘wet’ for the sap and the upper one is ‘dry’ for the vacuum. The dry lines are used to ensure that the maximum vacuum is transferred to each and every tap hole no matter how far they are from the vacuum pump.

Natural Vacuum Systems

I appreciate the need to use vacuum. I’m also confident that I’ll never use vacuum pumps. My operation is small enough that I can use alternate methods to improve the sap flow without the significant capital investment, associated maintenance costs, or use of fossil fuels.

The alternative method to mechanical vacuum pumps is called natural vacuum. The concept is that when the maple sap tubing is full of sap, gravity will pull the weight of the sap downhill and create a ‘natural’ vacuum at the top of the line. The lines typically end at the collection tank, so no releasers are required.

natural vacuum tubing on a maple syrup farm

To achieve natural vacuum, there are two primary considerations.

The tubing must be small diameter. Tubing with an inside diameter of only 3/16” is used for natural vacuum. This is much smaller than the standard 5/16” tubing used in most maple syrup operations. In terms of cross-sectional area, the 3/16” tubing is only about 1/3 as much as the 5/16” tubing. This makes it feasible for the entire tube to fill with sap so that gravity can pull on it without air getting into the tubing system.

The vertical drop must be significant. Natural vacuum won’t work well on relatively flat land because there isn’t enough sap weight in the lines. To ensure good natural vacuum, you need about 40’ of vertical drop between the bottom end of the tube and the first tree you tap as you go up the hill. Above that lowest tree, the land can be more level because at that point you have already created the vacuum in the tubing system.

Natural Vacuum Can be a Boon to Small Maple Farmers

The vacuum created in a natural vacuum system can actually be higher than with vacuum pumps. It’s an ideal closed system and it can be as high as the laws of physics will allow. However, this doesn’t necessarily mean that you will get more sap from your trees with natural vacuum versus mechanical pumps. The natural vacuum won’t start it’s work until the lines are full of sap. So it may take a while for the system to get filled up and start doing its job.

Based on my experience with natural vacuum in a small-scale operation, I highly recommend exploring natural vacuum. The initial investment and ongoing costs are far less than mechanical pumps. In the next article I will cover the pros and cons of the small diameter 3/16” sap tubing, lessons learned on how to layout the tubing to optimize sap production, and some of the economic benefits of using natural vacuum for small-scale maple syrup operations.