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Part F has been divided into Subsections

Jump To: Vacuum Systems, Maintaining Sap Quality, Sap Collection Equipment, Economics of Sap Collection, Research, Proper Tubing Installation, Cleaning Equipment, Energy


Vacuum Systems


Maintaining Sap Quality


Sap Collection Equipment


Economics of Sap Collection




Proper Tubing Installation


Cleaning Equipment


















Measure That Vacuum

Maple Syrup Digest
H. Clay Smith and Arthur H. Rye, Northeastern Forest Experiment Station, Forest Service, U.S.D.A., Burlington, Vermont
Vol. 9, No. 2
July 1970
REF# 100
For vacuum pumping to increase sugar maple sap yields, there must be a vacuum at the taphole. The only way you can be sure that you have the necessary vacuum is to measure it.
One way to do this is to take the cap off the spout and attach a vacuum gage (fig. 1). But this is usually difficult and time consuming.

Figure 1. Measuring vacuum with a hand gage graduated in millimeters of mercury.
Now we have a better way. If shutoff valves are fitted to the tubing system at key points, vacuum gages can be attached quickly and simply for measuring the vacuum.
The key locations vary with each sugarbush. We put valves near the beginning and middle of the tubing lines. We also attach a few valves to the spouts farthest from the vacuum pump. In this way we can determine whether or not vacuum is present throughout the tubing installation.

The valves can be installed on the spout vents this way. Replace the spout cap with a piece of tubing containing a shutoff valve (fig. 2). Use plastic rubber cement to seal the tubing to the metal valve fitting. Keep the valve closed at all times except when vacuum readings are taken. If spouts without vent holes are used, insert a tee in the drop line beneath the spout and attach a valve to this tee. Valves can even be mounted on mainlines; however, in such cases larger valves should be used.
Figure 2. A valve is attached to the spout vent and kept closed until a reading is taken.
To measure the vacuum, attach a vacuum gage to the tubing above the valve while the valve is still closed (fig. 3). After the gage is attached, turn the valve on, wait for the needle on the gage to settle, read the vacuum, turn the valve off, and remove the gage.

Figure 3. When measuring vacuum, the gage is attached to the top of the valve and the valve is turned on.
If you have a good vacuum, you can open a valve slightly and hear a continuous hissing caused by the suction of air from the vacuum pump. When a vacuum is not present, check the entire installation to locate the problem areas. Things to check for include missing caps, separated lines, poor connections, leaking tapholes, broken fittings, and animal damage.
You can buy shutoff valves at most hardware or plumbing stores for approximately $0.50 each. Various types of shutoff valves with nipple thread combinations are available, but we found the ¼ inch valve fitting to be satisfactory.
Several types of vacuum gages are available. Vacuum gages measure vacuum in millimeters of mercury, inches of mercury, or pounds per square inch. They range in price from approximately $3.00 to several hundred dollars, and can be purchased at most hardware or mail order houses. An automobile fuel pump tester (vacuum gage), graduated in inches of mercury, works very well; one costs about $4.00.
Measuring vacuum on a tubing system can be slow and difficult. But the job can be done simply and quickly if shutoff valves are mounted on the spouts at key locations in the tubing lines. Remember, to increase sap yields, a vacuum must be present at the taphole.

The Use Of Vacuum Pumping In Michigan Sugarbushes

Maple Syrup Digest
Melvin R. Koelling, Department of Forestry, Michigan State University, East Lansing, Michigan
Vol. 9, No. 1
February 1970
REF# 102
The introduction of vacuum pumps to plastic tubing for the collection of maple sap is a relatively recent adaptation. Vacuum pumps themselves are not new, however, their use in maple sap collection systems has been rather limited until the past few years. Originally it was believed their advantages to the maple producer were primarily concerned with moving sap through or over areas where natural slope was insufficient to permit gravity flow.
The possibility that vacuum pumps might have additional benefits in sap collection has been the subject of several recent research studies. Results from these studies indicate that in addition to facilitating sap movement in tubing lines, additional increases in sap yields may be obtained from trees subjected to vacuum operated tubing lines. This latter increase is apparently due to the ability of the vacuum pump to pull more sap out of the taphole than would be obtained by gravity alone.
This report discusses the use of vacuum equipped tubing lines during the 1969 sugaring season in Michigan.

Field Installations

A source of vacuum for a tubing network may be obtained by any one of several types of pumping units. Two types are most commonly used, these being wet and dry pumps. Wet type units consist of a small centrifugal pump which forces sap through a well jet containing a Venturi tube. The centrifugal pump operates from a small recirculating reservoir of sap. Sap from the field line attached to an outlet on the Venturi tube continually replenishes sap in the reservoir.
Dry type units consist of a compressor type pump which evacuates a reservoir or chamber to which the field tubing line is attached. Provision must be made to periodically release the vacuum and empty the reservoir. Most commonly this results in an intermittent vacuum as opposed to a continuous vacuum on tubing lines equipped with a wet type pumping unit.
Vacuum pumps were introduced in Michigan sugarbushes in 1968. A small operation employing a wet type pump operating on approximately 750 tapholes was used in the northern portion of the state in that year.
In 1969 four producers used vacuum pumps on a portion of their tubing networks. Approximately 6000 tapholes were involved in the four operations. All pumps used were of the wet type, however, several different models of pumps with varying size Venturi tubes were employed. Two of the vacuum pumps were powered by electric motors and the remaining two were gasoline powered. Two of these installations used aerial tubing systems, one used a ground line, and the remaining installation had a combination of aerial and ground lines.


The producers found that vacuum pumps were helpful in collecting sap. The greatest single advantage attributed to vacuum pumps was the increase in total seasonal sap yields which were obtained. All producers reported increases in yield with one producer noting a seasonal increase of approximately 80 percent.
This additional sap was obtained primarily during weeping or near weeping flows when temperatures and tree pressure conditions favorable for sap flow were marginal, however, increases in sap yield were also noted during good flow days. It was noted that on several marginal flow days, sap could be obtained from vacuum taps when comparable nonvacuum taps were not producing.
A second advantage cited for vacuum pumping systems is that unfavorable slope or lack of slope is not a major problem in using a tubing system for sap collection. Any slope present should be used if possible, but where a favorable slope is not present the use of vacuum will assist in sap movement over level or nearly level areas. With a tight vacuum system it is possible to move sap uphill through tubing lines for relatively short distances.
Other advantages related to slope include keeping the tubing lines relatively free of sap thereby reducing collection problems due to sap freezing in the tubing lines. This is particularly important in flat areas. The use of vacuum pumps can assist in incorporating into a tubing system areas where tubing previously could not be used due to a lack of adequate slope for gravity flow. By including these areas previous collection costs in emptying containers, opening up trails, and operating gathering equipment in such areas can be reduced or eliminated.
A third major advantage given is that vacuum pumps can help cover up some problems resulting from a poorly designed and installed tubing system. Such problems include leaky connections, overloaded lines (too many taps) and improperly graded lines. This problem should not exist but where it does vacuum pumping is advantageous.

Problems Of Operations

Although operators involved in vacuum pump usage believed the advantages outweighed the disadvantages, some problems were encountered. All of these can be overcome and none are believed to be a deterrent to use of pumps. Some major obstacles were:
1. Power is required to operate the pump. This may not be a serious problem particularly if electricity is present in or near the sugar bush. If electricity is not available a gasoline or other type of engine will be required. Servicing and maintenance will present minor problems in the use of such engines.
2. Leaks in the tubing lines, primarily at connections. This is a rather serious problem that must be overcome if pumping is to be successful. These leaks may occur at every connection in the system and occasionally may be difficult to detect. Care in drilling the tapholes and connecting all fittings and tubing together will help in minimizing leakage. A good ear and the use of a small vacuum gauge is of great value in locating leaks.
3. Freezing may be a problem in certain types of operations. In the use of wet type pumps, a reservoir of sap must be maintained to furnish the circulating liquid for the pump. This should be protected from freezing to enable continuous usage of the pump when desired. With dry type pumps, freezing may not be a serious problem. Where electricity is available the use of heat tapes or heating lamps that are thermostatically controlled has afforded good protection.
4. Periodic maintenance is required on the entire tubing system. This is also necessary on nonvacuum tubing systems, however, connection leaks, animal damage, or defective operation of the pumping unit are potentially more costly in vacuum installations.

Installation Suggestions

The use of vacuum pumps has given rise to some suggestions regarding their installation and maintenance which may be helpful to other would be users. Some of these are as follows:
1. Where possible use the minimum amount of tubing which will satisfactorily collect the sap from all trees to be tapped.
2. Keep the tubing system as simple and direct as possible to facilitate vacuum transmission and sap movement. When possible design the system so it would flow by gravity alone in event the vacuum pump were not operating.
3. Minimize the number of connections and fittings in the system. This, may be facilitated by placing tubing in the same location in successive years. A properly designed and coded tubing system will assist in this respect.
4. Do not use high heat sources to soften the tubing when installations are made. This will tend to enlarge the tubing and may be a source of leakage around connections.
5. A vacuum pump which supplies a constant vacuum as opposed to an intermittent type unit should yield greater increases in sap yield.
6. When using wet type pumps, impeller pumps will give longer maintenance free service than roller type pumps due to sugar accumulations.


The use of vacuum pumps is a relatively new approach to sap collection. The basic idea appears to be one that can greatly help the maple syrup producer reduce his sap collection costs and at the same time help stabilize annual production. Increases in sap yield of up to 100 percent can be expected, particularly during poor seasons.
The use of vacuum pumps appears to be an area where technology is ahead of completely reliable equipment. The development of complete, self contained units which the producer can simply connect a field line and use would be of considerable value. Information on the capacity of various size lines and pumps is also needed. In spite of these problems and some minor difficulties in installation and operation, the use of vacuum pumps offers considerable potential for increasing sap yields, assuring an annual crop and improving the overall economic position of maple producers.

Sugar Maple Sap Volume Increases As Vacuum Level Is Increased

USDA Forest Service
Russell S. Walters and H. Clay Smith*, Research Foresters U.S. Department of Agriculture, Forest Service Northeastern Forest Experiment Station
Research Note NE-213
REF# 291
Abstract. Maple sap yields collected by using plastic tubing with a vacuum pump increased as the vacuum level was increased. Sap volumes collected at the 10- and 15-inch mercury vacuum levels were statistically significantly higher than volumes collected at the 5-inch level. Although the 15-inch vacuum yielded more sap than the 10-inch vacuum, the difference was not statistically significant. An efficient vacuum system should have a vacuum level of at least 10 inches of mercury at the taphole.
Sugar maple sap yields are known to be greater when sap is collected on a hillside by gravity in a closed plastic pipeline than when a vented system is used (Blum 1967). This is because a natural vacuum is created by the weight of the sap flowing through the closed tubing line. A vacuum pump connected to an unvented tubing system can also increase sap yields. However, we did not know how much vacuum is needed to get the greatest sap yield.
Collecting sugar maple sap at three different levels of artificial vacuum – 5, 10, and 15 inches of mercury – we found greater yields from the higher vacuum levels.

Study Methods

In late February 1971, 15 trees located in a northwestern Vermont sugarbush were selected and tapped. Three tapholes were drilled in each tree. All the tapholes were drilled to the same dimension: 7/16 inch in diameter and 3 inches deep inside the bark. A gasoline-powered tapper was used to make the holes.
Each of the three tapholes per tree was assigned, at random, to one of three different vacuum levels – 5, 10, or 15 inches of mercury. Each taphole was connected by 5/16-inch plastic tubing to a separate 55-gallon steel drum, in which the sap from the taphole was collected (fig. 1).
Figure 1. (not available) Sap from each taphole was collected in individually sealed 55-gallon steel drums.
In each system, vacuum was developed in a central tank by a vacuum pump and transferred through plastic tubing to the individual collection drums located at each tree. The desired vacuum levels were maintained in each system by regulating pressurecontrol valves. Vacuum levels were monitored by gages located at the ends of the tubing systems farthest from the pumps.
Vacuum was applied to the systems for approximately 130 hours during, the sugaring season from March 19 to April 18. At the end of the sugaring season, the sap in each drum was measured to the nearest 1/2 liter.


The results of this study indicated that the average sap-yield volumes per taphole varied positively with the vacuum levels. The greater the vacuum level, the greater the average sap yield. The average volumes were 37.8, 76.8, and 82.7 liters (40.0, 81.2, and 87.4 quarts) for the 5-, 10-, and 15-inch vacuum levels respectively. The sap volumes collected at the 10- and 15-inch vacuum levels were statistically significantly greater than the amount of sap collected at the 5-inch vacuum level. Sap volumes between the 10- and 15-inch vacuum levels were not statistically significantly different. Significant differences were determined at the 1-percent level.
These results are illustrated graphically in figure 2. The range of total per-taphole sap yields for each vacuum level is represented by the unshaded boxes. The average per taphole yield is indicated by a crossbar, and the shaded boxes represent ± 2 standard errors of the mean (Dice and Leraas 1936). A summary of the basic data is presented in table 1.

Figure 2. Average sap yield per tap for each vacuum level is indicated by the crossbar. The shaded areas indicate ± 2 standard errors of the means. The total length of the unshaded boxes represents the total range of individual taphole yields.
Table 1. Summary on analyses for comparing yields and vacuum levels


It has long been known that maple sap will exude from a taphole or wound in dormant sugar maples when the internal pressure of the tree becomes greater than the external, or atmospheric, pressure (Jones and others 1903; Marvin 1958). This is a function of changes in temperature. Cold nights induce a negative pressure within a tree, resulting in moisture being absorbed through the roots. Warm days provide a positive internal pressure, forcing sap from the tree at tapholes or wounds. The volume of flow is not related to the temperature rise on the day of the flow, but rather to the length of the preceding cooling period.
A theory developed by Sauter** suggests that the mechanism of the sap-flow phenomenon is due to gas (mainly CO2) expanding and contracting in response to temperature changes. Under cooling conditions, CO2 contracts and dissolves in the sap, causing negative tree pressure. During warm daylight hours, CO2 is produced by living cells. Also, warming of tree drives the CO2 out of sap solution. The gas accumulates in spaces of the fibrous tissue and expands, creating positive tree pressure on the vessel, causing sap to flow out through the taphole.
Maple sap cannot be pulled or sucked from a taphole. Rather, it is forced out by a positive pressure differential when the pressure inside the tree is greater than the atmospheric pressure outside. Applying vacuum to a taphole has the effect of artificially reducing the external pressure, thus creating the pressure differential necessary to allow sap to flow from the tree.
In each sap season there are a number of days when conditions are almost right for a sap run. That is, the tree’s internal pressure is about equal to the atmospheric pressure. This situation also occurs for a period at the beginning and end of each sap run. It is during these times that a sap flow can be induced or prolonged by vacuum pumping to create a pressure differential. During periods of good sap-flow conditions, even heavier flows may be induced by vacuum.
The use of vacuum has raised questions about its effect on the composition of the sap collected. Maple sap is a very complex solution of sucrose, other sugars, minerals, and organic compounds. A large amount-about 98 percent of the dissolved solids-is sucrose. For the most part, these solutes diffuse from the living ray and parenchyma cells into the sap in the vessels. Because of the complexity of the constituents in maple sap and the mechanism whereby sap enters the vessels, concern developed that vacuum might possibly increase the amount of water in the sap, resulting in diluted sap.
Maybe various sap constituents respond selectively to the exertion of vacuum at the taphole, and maybe sap composition is altered by extracting more or less of the individual solutes. Vacuum may even cause the extraction of substances that do not normally appear in sap flowing from the taphole. Laing and others (1971) considered these possibilities and concluded that, under normal conditions of gravity flow, only a small volume of the available vessel sap reaches the taphole, and that vacuum pumping (as high as 25 inches of mercury) simply removes additional unaltered vessel sap.

Conclusions and Recommendations

We found that the higher the vacuum level generated within an unvented tubing system, the greater the sap yield will be. A vacuum level of at least 10 inches of mercury should be maintained at the taphole. The sap volume yielded by both the 10- and 15-inch vacuum levels was significantly greater than that yielded by the 5-inch level. However, the 15-inch volume was not significantly greater than that of the 10-inch level.
Further, results of this study and from other research strongly emphasized the importance of maintaining high vacuum at each taphole. To check the effectiveness of the vacuum system, age readings should be made at the ends of the pipeline system farthest from the pump. There is, of course, a loss of vacuum through friction in long tubing lines. This means that a vacuum level higher than 10 to 15 inches should be maintained at the pump, perhaps as high as 20 to 25 inches, in order to achieve the desired level at the taphole. Maintaining high vacuum requires constant inspection and maintenance of the tubing system.
*At the time this study was made, both authors were at the Northeastern Forest Experiment Statio’s laboratory at Burlington, Vermont. H. Clay Smith is now at the Experiment Station’s laboratory at Parsons, West Virginia.
**Sauter, Jorg. 1971. A new hypothesis for the mechanism of sap flow in sugar maple. Botany Seminar, University of Vermont, 10 May 1971.

Literature Cited

Blum, Barton M. 1967.
Plastic tubing for collecting maple sap: Comparison of suspended vented and unvented installations. USDA For. Serv. Res. Pap. NE-90. 13 p.
Dice, Lee R., and Harold J. Leraas.
1936. A Graphic Method For Comparing Several Sets Of Measurements. Univ. Mich. Lab. Vertebrate Genet. Contrib. 3. 3 p.
Jones, C. H., A. W. Edson, and W. J. Morse.
1903. The Maple Sap Flow. Vt. Agric. Exp. Stn. Bull. 103. 184 p.
Laing, F. M., J. W. Marvin, Mariafranca Morselli, David W. Racusen, E. L. Arnold, and Elizabeth G. Malcolm.
1971. Effect Of High-Vacuum Pumping On Volume Yields And Composition Of Maple Sap. Vt. Agric. Exp. Stn. Res. Rep. MP 65. 11 p.
Marvin, J. W.
1958. The Physiology Of Maple Sap Flow. In The Physiology Of Forest Trees: 95- 124. Ronald Press, New York.

Pipeline Pressure Testing To Detect Vacuum Leaks

Maple Syrup Digest
Dale E. Minick, President, Sugar Camp, Inc.
Vol. 25, No.3
October 1985
REF# 181
Anyone widely traveling about the maple producing regions and talking to producers using plastic tube networks and vacuum pumps for sap collection is soon struck by the wide variations in user satisfaction and success experienced with this technique. We’ve heard claims of sap yield increases ranging from 0 to 700 percent.
After awhile, the curious starts asking why this vast difference and the foolish starts seriously looking for answers. This article is a summarization of the “whys” we found over the past 8 or 10 years in hundreds of sugar bushes, doing private research, and from the review of published data. The findings and the solution will amaze you.

The Principle Problem

The major reason for poor pipeline performance is a lack of understanding on the part of the installer of the following previously unreported fact:
Air, Flows up to Ninety (90) Times More Freely
Through Pipes and Orifices Than Sap Does.
Please go back and read this statement at least one more time. This fact is extremely important to you as ignoring it prevents you from consistently increasing sap yield and causes you to waste a lot of your time, money and effort. Your pipeline is affected in two ways. For example:
1. Under a vacuum of 20″ hg, a hole as little as .050″ in diameter will leak a volume of air into your pipeline equal to the total volume of sap flow from 1,000 taps near peak flow times. A series of smaller holes has the same effect.
2. Once inside your pipeline that air will flow 90 times more freely than the sap and will therefore be pumped from your network first at the expense of improved sap flow. Those bubbles you see in your lines are passing through the sap, not pushing the sap ahead as much as they appear to be, because of the vast flow friction difference of air and sap.
In a worst case scenario, depending on the location of the leak or leaks, your pump could pump air almost exclusively all season long with sap flows being no more than they would have been under gravity flow, while still maintaining a vacuum gauge reading at the pump that really impresses your producer neighbors who are not as skillful as you are.
Considering this important new information and the current price of “Vacuum” in gallon cans, it should be obvious that the tiniest of leaks are more important than once thought and that you cannot afford to “Put in a bigger pump so you don’t have to worry about a few leaks, nor bother looking for them.” You’re kidding yourself if you believe that. More sap is the true test of your system, not high gauge readings. Your pipeline must be as tight as possible to increase sap yields significantly and avoid pumping air across the bush all season long. Waiting for the right breeze would make more sense and be a whole lot cheaper.

Finding Leaks–A Simple Solution

The obvious patiently waits to be stumbled upon by the weary. We stumbled on this technique while doing a tube washing experiment. We found that it doesn’t pay to suck on your pipeline while trying to find vacuum leaks, it’s better to pressurize it! Fill your network with sap, or better yet water before the season starts, and maintain the liquid at a steady 20 PSI with a pump. Then go for a walk throughout the bush looking for the easily seen “fountains” that stream from every leak point. It’s much easier to see these fountains than it is to hear the nearly inaudible hisses that develop under vacuum, especially on windy days. You’ll be absolutely amazed at how leak tight you thought your system was before you left. Nearly all leaks can be found this way, large and small, with perhaps hollow trees being an exception. We’ve found a couple of dozen sources of leaks during our travels but that is the basis for another article.
Sapsucker owners can use this method by placing a pressure relief valve set at 20 PSI in the vacuum mainline on a tee and attaching it to the pumps outlet tube end. Pumping sap, or water, into the network from the storage tank with a short pickup hose attached to the suction inlet tube end and routing the pressure relief valve overflow back to the storage tank completes the installation. At most a ten minute job. Turn on the pump and go for your walk. Take a friend or two along, it will speed things up.
Closed system gravity users and producers using other types of vacuum systems can use this method by attaching a relief valve set at 20 PSI to most any type of water pump and installing it as described above.


This very simple straightforward method of pipeline leak detection can save producers countless hours and many start up frustrations that usually occur at a time when they can be least afforded. Most important of all though, better sap yields can be realized and that is what we all set out to achieve in the first place. How much more sap you get is the true test of any pipeline system, and unfortunately there’s no gauge available for that.

Vacuum Transfer System

Maple Syrup Digest
Russell S. Walters, Research Forester, USDA Forest Service Northeastern Forest Experiment Station Burlington, Vermont 05402
Vol. 21, No. 2
July 1981
REF# 159
Installing a transfer tank in a vacuum sap collection system may increase sap yields if the vacuum pump is a long distance from the sugarbush. Vacuum loss due to friction inside a pipe increases with distance. The vacuum transfer system reduces such loss. During two maple sap flow seasons in northern Vermont, larger volumes of sap were collected from a pipeline with a vacuum transfer tank than from a similar pipeline without a transfer tank. The pipeline with the transfer tank maintained higher vacuum levels at the taphole.

What we did

We conducted this study in a typical even-aged sugarbush in northern Vermont. The trees ranged from 16 to 28 inches in diameter. In 1973, we tapped 111 study trees and the next year 135. Three tapholes were drilled in each tree in late February of both years.
Each of the three tapholes per tree was connected to one of three parallel pipeline systems. The systems were identical except in the method by which sap was collected from them: gravity, vacuum pump, or vacuum pump with a transfer tank. The plastic pipelines were installed according to recommended procedures (Fig. 1).

Figure 1. The sap collection networks were constructed of 5/16 inch plastic tubing as unvented, aerial-line systems with droplines at least 18 inches long. The small lateral lines were connected to 1/2 inch main lines.
In the gravity system, the sap simply flowed downhill through the pipeline to the collection tank. The second pipeline was connected to a 40-gallon steel tank, where vacuum was created by a compressor-type vacuum pump. This tank was emptied by a water pump that was controlled by a float switch inside the tank (Fig. 2). The third system – the vacuum-transfer system (Fig. 3) differed from the vacuum system only in that a second tank was placed in the sugarbush (Fig. 4). Conduit lines from this tank branched out to different parts of the sugarbush. This system places the source of the vacuum closer to the trees. The slope from the transfer tank to the vacuum pump averaged about 6%. Two pipelines connected the transfer tank to the vacuum tank: the pipe at the bottom carried sap, while the one at the top evacuated gases, which created a vacuum.

Figure 2. Schematic of a sap collection vacuum tank. The float switch inside the tank automatically controls the water pump for sap removal.

Figure 3. Schematic illustrates how the vacuum-transfer tank is connected to the sap collection vacuum tank.

Figure 4. The vacuum-transfer tank is in the sugarbush. Sap flows from the trees on the left into the tank. The upper pipe on the right removes air from the tank, creating a vacuum, and sap travels out through the lower pipe.
The pumps were controlled automatically by a thermostat so they would run when the air temperature was above 30°F. Vacuum gauges were installed on each vacuum tank, the transfer tank, and each pipeline at the point farthest from the vacuum pump.


The volume of sap per taphole collected by the vacuum-transfer System during each season was greater than that collected by the other two systems (Table 1). In 1973, the amount collected by vacuum transfer was 27% more than was collected by vacuum only, and 109% more than was collected by gravity. The amount collected by vacuum alone was 65% more than was collected by gravity. All of these differences were statistically significant. In 1974, the transfer system yielded 17% more sap than vacuum alone; this difference was not significant. The transfer tank and the vacuum systems produced 29% and 23% more sap, respectively, than the gravity system. These differences were significant.
Table 1. Average sap volume per taphole collected by three tubing systems, 1973 and 1974, in gallons.

All pairs within the same year differ significantly except those underscored.
The vacuum level created and maintained in the vacuum tank by the pumps was very consistent, about 22.5 inches of mercury (Hg). At the transfer tank, the vacuum level was generally somewhat less, about 18 inches of Hg. The readings at the tapholes farthest from the pumps during the sap flows averaged about 17 inches of Hg for the transfer system and 14 for the vacuum system. The highest natural vacuum level that was recorded for the gravity system was 4 inches of Hg. However, quite often readings were zero, indicating no natural vacuum.


Vacuum pumping can often induce sap flow even when conditions are not quite right for a natural flow to occur. Vacuum can also increase the sap flow rate from the taphole during normal flow periods. In addition, applied vacuum helps sap to flow quickly through the pipeline; this reduces the possibility of back pressure buildup caused by pipeline overload, which inhibits sap yield.
In the vacuum-transfer system, the two separate pipes from the transfer tank to the sap collection tank evacuated air from the system more efficiently than the single pipe in the vacuum only system. Thus the vacuum level at the taphole in the transfer system was higher, usually by about 3 inches.
In our study, the distance between the transfer tank and the pump was only about 175 yards. If this distance had been longer, presumably, the advantage shown for the transfer system would have been greater. In a sugarbush where the trees are as much as twice that distance or more from the vacuum pump and the collection tank, the transfer-tank system would be more effective and is recommended.
For a more detailed account of this study, see Walters, R.S., 1978, “Vacuum Transfer System Increases Sugar Maple Sap Yield.” U. S. Department of Agriculture, Forest Service Research Note, NE-264.
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