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An accurate hydraulic-pneumatic weighing lysimeter for general field use
Agricultural Meteorology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
AN ACCURATE HYDRAULIC-PNEUMATIC W E I G H I N G LYSIMETER FOR G E N E R A L FIELD USE P. D. B E R W I C K A N D C. J. S U M N E R
C.S.L R.O. Division of Meteorological Physics, Aspendale, Vie. (Australia)
(Received May 14, 1967)
SUMMARY
For some time there has been a growing need for a simple and inexpensive, yet accurate and reliable field lysimeter. Such an instrument operating on the hydraulic principle has now been developed; its day to day accuracy is 0.25 mm of rainfall or evaporation while over periods of the order of ten days the zero stability is within about 0.50 or 0.60 mm. The system comprises supporting capsules and a pressure sensing device. It requires no power, is not significantly affected by temperature fluctuations and the design has the advantage of being readily adaptable to quite large loads.
INTRODUCTION The C.S.I.R.O. Division of Meteorological Physics has operated a number of sensitive 6 metric ton lysimeters for the past eight years at Aspendale, Victoria (MCILROY and SUMNER, 1961). The value of the results obtained (MCILROY and ANGUS, 1962, 1963) has demonstrated the desirability of installing a network of these instruments throughout the country. However, the high cost of such precision equipment has precluded its use except in those relatively isolated cases where intensive research is being undertaken. It was decided therefore to design a complete lysimeter installation suitable for general field use at a price low enough to permit its installation in relatively large numbers. Further, the need for simplicity of operation, freedom from maintenance worries, reliability and reproducibility would be kept in mind. After surveying the various types of lysimeters which had been tried (KING et al., 1956; PRUITT and ANGUS, 1960; GLOVERand FORSGATE,1962; LmRY and NIXON, 1962; MAKKINK, 1962; BLOEMAN, 1963; HOLMES, 1963; WINTER, 1963; BLOEMAN, 1964; BEAUMONT, 1965; FORSGATE et al., 1965), a system working on the hydraulic-pneumatic principle was selected as being most likely to achieve the desired results. Additional features were to be an accuracy of 0.25 mm evaporation, a visual indication of weight change Agr. Meteorol., 5 (1968) 5-16
6
1'. D. BERWICK AND t" J, StJMNI-:]¢.
rather than a record al~d an absence of the need for power, whether battery u: mains.
MECHANICAL DESIGN AND INSTALLATION
Briefly the system consists of three flexible pressure units which support the soil container, and a pressure sensor of which two types are described. The installation may be conveniently divided into three parts: (a) The pit and soil container assembly, involving steel fabrication, sheet metal work and fibre glass fabrication. (b) The pressure units and transfer chambers assembly, involving general engineering fabrication. (c) The manometers.
THE PIT AND SOIL CONTAINER
A cross-section of the installation is shown in Fig.1. The outer pit liner supporting the surrounding soil is made of 1.5 mm thick galvanised iron, 1.75 m diameter and 1.4 m deep, and is provided with a pump well and pipe for removing flood water. Corrugated tanks have been avoided since they tend to collapse vertically due to the weight of soil resting on the corrugations. The pit liner is installed on a flat concrete base 7.6 cm thick, poured in an excavation of suitable size. If the water table is likely to rise above the base of the tank at any time when the lysimeter soil pot is removed, concrete of sufficient weight must be cast around the pit base (which is provided with an anchor flange)to prevent floatation. The soil container is made of 6.4 mm thick fibreglass 1.7 m in diameter (providing a clearance of 2.5 cm when placed in the pit), by 1.22 m deep. These dimensions provide an adequate depth for the rooting habits of many crops, while ensuring that its overall weight does not exceed the 5 metric tons limit imposed by the ready availability of mobile cranes of this capacity. Three equally spaced 312 cm tubes moulded into the inside walls of the container house three Z5 cm diameter steel lifting bolts. These pass through the bottom of the container into a galvanised steel supporting frame, fitted with captive nuts, Using a tripod sling and crane the soil container is lowered into position, the bolts withdrawn and the tubes sealed with rubber plugs to exclude dirt and water. A 1 cm thick perforated steel plate mounted on a channel iron frame placed in the soil container supports the soil and leaves a 10 cm deep space underneath in which percolate collects. Percolate can be pumped out periodically via a 1.3 cm bore copper pipe attached to the wall of the container and projecting just above the tim. The container may be back filled with soil or a monolith soil sample inserted in it. Agr. Meteorol., 5 (1968) 5-16
AN ACCURATE HYDRAULIC-PNEUMATIC WEIGHING LYSIMETER
MERCURY
3 RUBBER
SOIL. CONTAINER
PLUG
5- PRESSURE UNITS
7
MANOMETER
PNEUMATIC
--
c~IPE LINES
PIT TANK
3 TRANSFERCHAMBERS
REINFORCED
CONCRETE
Fig. l. Part section through lysimeter installation.
PRESSURE UNITS AND TRANSFER CHAMBERS
The three pressure units supporting the container are located at the vertices of an equilateral triangle. As shown in Fig.2, each comprises an alumiaium base casting, 63.5 cm diameter, to which is clamped a 1.5 mm thick fabric reinforced neoprene sheet. This supports a steel pressure plate (diameter 61 cm) which itself carries the soil container. The base casting is machined at the rim only to accommodate the neoprene sheet and clamp ring, the casting then being impregnated with thermosetting plastic to avoid any possibility of leaks. The pressure plate and clamp ring
CLAMP RING
STEEL PRESSUREPLATE
WOOD SPACER .
.
.
.
.
.
.
.
DIAPHRAGM
.
AIR PIPE
\
BASE CASTING
SILICONE FLUID PRESSURE UNIT
TRANSFER CHAMBER
Fig.2. Section through pressure unit and transfer chamber.
Agr. Meteorol., 5 (1968) 5-16
8
.P. D. BERWICK AND ('. .I. SUMNt!R
is flame cut from the same 6.3 mm thick steel plate so avoiding waste ol material and costly machining operations. The annular ring (1.3 cm) of unsupported neoprene sheet formed between the pressure plate and the clamp ring permits the plate to move vertically over a small range while maintaining the same contact area at all pressures. (This small but important point is often overlooked in many hydraulic systems.) A connecting pipe is fitted into the base casting and the pressure unit filled with 2.8 I of silicone fluid; the assembly is then pressure tested by sealing the pipe and loading to 3 metric tons (or 50 °.o in excess of the maximum working load) in a hydraulic press. Loads up to 5 metric tons have been supported by these pressure units without damage. Each pressure unit is connected by a short length of piping to a "transfer chamber" which consists of two spun brass discs 25.5 cm diameter, clamped together, with an " O " ring seal between them, to form an airtight cavity 3.8 cm deep. (To ensure airtight construction of the pneumatic parts of the system, castings have been avoided as they tend to be porous.) From the top of each transfer chamber a 3 mm diameter copper air line leads to a manometer, the purpose of the former being to convert liquid pressure to gas (air) pressure. If this were not done, liquid in the vertical sections of the lines would act as a partial balance to the load on the pressure units, the extent of the balance being governed by the mean density of the liquid column, which is temperature dependent. To integrate such changes over approximately 1.5 m (the depth of the lysimeter pit) in a straightforward manner is not easy, most attempts to do so involving temperature control - - an unnecessarily complicated solution. With equipment of this nature it is essential that it be capable of operating for long periods without servicing, particularly those parts which are inacccessible below the soil container. The most likely cause of failure in this instrument is a leak in the hydraulic-pneumatic system: to avoid this great care has been taken in the design to select impervious materials and to restrict the number of pressure joints to a minimum. The neoprene membrane has operated under toad for a period of two years without detectable deterioration, and the manufacturers of this material expect a life of five or more years under these working conditions. There are two reasons for using air and silicone oil rather than air alone as the sole working fluid. The most important is that air would diffuse through the neoprene relatively quickly and cause the pressure unit to collapse. Secondly, compared to the silicone fluid used, air has a large coefficient of expansion, and the volumetric changes in the pressure units resulting from temperature fluctuations would be such that they could not be accommodated without setting up forces in the neoprene and destroying the calibration ot the instrument. Also, of course, in a hydraulic-pneumatic system (as opposed to one completely hydraulic) it should be remembered that the air in the system is compressible and that when, for example, there is an increase in load on the soil container, the increased internal pressure causes the space occupied by the air to shrink and an appropriate amount of fluid to be displaced from the pressure unit. Conceivably this could give Agr. Meteorol., 5 (1968) 5-16
AN ACCURATEHYDRAULIC-PNEUMATICWEIGHINGLYSIMETER
9
rise to a significant change in the level of fluid in the transfer chambers (which forms part of the balancing head), as well as allow the pressure plate to move outside its range of complete flexibility (see under "Temperature effects", last paragraph). Regarding the first point, a simple calculation shows that an increase of over 12 cm of water on the soil container is needed to give rise to an error in the indicated pressure of 0.025 mm mercury (equivalent to 0.25 mm of water). Regarding the second point, part of the calibration procedure included determining over what vertical range the pressure plate could move before the neoprene exerted any appreciable force: in the present context almost 28 cm of water on the soil container was needed to introduce an error of 0.25 mm of water.
THE MANOMETERS Two types (I and II) are described. Type I is shown in Fig.3. This consists of a rod frame (1) 1.22 m high, mounted in a base plate (2) which is securely bolted to a concrete base after levelling. Three separate mercury manometer systems are mounted on the rod frame which supports three 10 cm diameter steel cisterns (3) provided with air union connections. 1.6 mm bore steel tubes (4) 2.44 m long, wound in a spiral are fitted into the base of the cisterns, the opposite ends being attached to 2.2 cm bore vertical monel metal tubes referred to as float chambers (5). These are mounted together to form an equilateral triangle, on brackets (6) which attach the manometer limb assembly to the frame (1) so that all three can be adjusted vertically as a unit. The three float chambers house floats (7) resting on the mercury surfaces and extending upwards to support a three spoked plate (8), at the centre of which is positioned an index point (9). Mounted on the bracket supporting the manometer limbs is a travelling microscope (10) which is focussed on the index point; the microscope is adjusted manually and capable of measuring to within 0.025 mm (equivalent to 0.127 mm evaporation) over a range of 15 cm. Since temperature will affect the lengths of the mercury columns it is necessary to correct for this; accordingly the sensing bulb (length 2.44 m) of a 5 cm dial indicating thermometer is wound in a spiral in intimate thermal contact with the steel tubes (4). A double walled temperature insulating cover, provided with a window and external hand wheel for setting the microscope, is fitted over the manometer assembly to provide a degree of damping to fluctuating ambient temperatures. Then, knowing the height of the mercury column (about 71 cm for a 6 metric ton soil container) and the temperature change, the necessary correction can be applied to the reading. To minimise undesirable surface tension effects at the mercury menisci, which cannot be kept chemically clean as in a mercury barometer for example, it is necessary to use relatively large diameter float chambers, i.e., 2.2 cm. The spiral tubes permit vertical adjustment of the manometer limbs to suit the weight of the particular soil container being used. They also restrict the rate of
Agr. Meteorol., 5 (1968) 5-16
li
WINDOW COVER WINDO
~,, ~[F~, I
INDICATING DIA THERMOMETER
tE :.OVER
ME
3 - AIR
P
Fig.3. Part section mercury manometer assembly (for further explanations see text). flow of mercury and cause a delay of approximately 10 min between the initiation of a weight change and the ensuing final pressure reading. This damps out oscillations in the manometer due to wind gusts. Fig.4 demonstrates change o f pressure :with time following an abrupt weight change at time t =~ 0 min.
INSTALLATION AND OPERATION
The manometer assembly is mounted in a suitable instrument house which may be located up to 90 m or so from the soil container. At distances greater than Agr. Meteorol., 5 (1968)
5-16
AN ACCURATE HYDRAULIC-PNEUMATIC WEIGHING LYSIMETER
I[
this the connecting pipes contain so much air, that, having regain to the limitations on the size of the transfer chamber (see under "Temperature effects" later), the silicon fluid would flow from the transfer chambers into the air lines once the pressure units were under load. Three 3 mm bore copper connecting pipes are fitted to the manometer cisterns, using special leak proof "O" ring unions. The pipes are buried 30 60 cm below ground level and enter the pit tanks through the wall near the top. Each is connected to the upper outlet of a transfer chamber suitably positioned in the base of the pit, the lower outlet being temporarily left open. Dry air from a commercial pressure cylinder is introduced into each copper pipe at the manometer end, and after purging to eliminate moisture, the pipes are reconnected. The three silicone-filled pressure units are connected to the lower outlets of the transfer chambers with the unions provided. Before lowering the soil container on the pressure units, to avoid mercury overttowing the float chambers, the manometer and microscope assembly are raised to its highest position. In the absence of response from the floats after the load has been in position for at least 15 rain, the manometer/microscope assembly is lowered in slow stages until the index point reaches the desired position. Changes in weight of the soil container will now cause the mercury level in the manometers (and therefore the floats carrying the index) to rise or fall, the displacement being observed by the travelling microscope and read off in millimetres on the scale provided. Often in the early setting up stage it will be found that the weight of the soil containers is not evenly distributed over the pressure units; this causes the individual manometer floats to take up different levels so that the three spoked plate they support is not horizontal. To overcome this each manometer cistern is provided with indivi-
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.
.
.
.
I
'
'
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0-550
~
i
..i~i--.
,
/11
/
Travelling 0.540 microscope (Inches) 0 "530
/
/
./
/" 0.520 0
I . . . . 5 - r i m e - Minutes
I I0
,
,
i
Fig.4. Artificial d a m p i n g . Pressure vs. time, after a d d i n g 28 lb. (
!
15
0.22 inches rainfall) at time
t=0
Agr. Meteorol., 5 (1968) 5-16
12
P . D . BERWICK AND C. J. SUMNER
dual means of vertical adjustment. In subsequent service, if non-uniform evaporation causes the floats to fall by somewhat different amounts, the index point nevertheless continues to indicate the correct mean level. The reason lies in the geometry of the equilateral triangle, the medians of which interact at a point 2/a.a from the vertices where a is the length of the median. (It will be recalled that both pressure units and float chambers are arranged in the form o| equilateral triangles.) Assume that a weight W is applied to the soil container: each pressure unit is then subjected to a pressure change proportional to W/3 and each float rises by an amount also proportional to W/3--say k. W/a. Clearly, in this case the index also rises by the same amount, k. W/.a. Suppose now W is applied to one pressure unit only: only one float rises but this time by an amount k W. However, because of the property of the equilateral triangle just referred to, the index rises by kW/3--as before. Strictly the argument applies only to small vertical displacements, but in practice all displacements turn out to be small.
THE TYPE II SENSOR
Essentially this is an ordinary m a n o m e t e r (HANLEY, 1964) the " U " section of which is replaced by a horizontal capillary tube (see Fig.5), The manometer is filled with mercury and an air bubble introduced into the capillary tube: the application of pressure, AP, to one arm of the manometer causes a displacement of the bubble, d, given by: AP ---- 2d (b2/a 2) cm of mercury, where a = radius of manometer and b == radius of capillary tube (both being in cm).
1
Pressure
atmosphere
inlet
~
Mercury ~r d . ~
~."11zri
i i...."
Air bubble
Fig.5. TypeII manometer (not to scale). A g r . M e t e o r o l . , 5 (1968) 5-16
AN ACCURATEHYDRAULIC-PNEUMATICWEIGHINGLYSIMETER
13
In the present manometer a ~ 9.63 ram, b = 1.398 mm. Thus 0.25 m m of evaporation causes a bubble displacement of approximately 1 ram. The advantage of this type of sensor over the first described is that its cost is extremely l o w - - o n the other hand it is relatively fragile. In setting up the manometers (of which there are t h r e e - - o n e per pressure unit) a high degree of cleanliness must be maintained. The inside of the manometer must be thoroughly scoured with chromic acid and fresh clean mercury employed. In order that the system shall function with adequate accuracy and reproducibility, the angle of contact between the mercury and the walls of the manometer arms must be kept constant: in practice this means a minimum. To achieve this the manometer arms are wetted with a few drops of 9 8 ~ of sulphuric acid (HANLEY, 1964). As can be seen from the diagram, one arm of the manometer is made to take a pressure hose fitting and the other a ground glass stopper which permits ventilation to the atmosphere. The stopper outlet faces downwards to assist in keeping dirt out of the system, and by means of a long narrow bore tube (not shown) restricts the diffusion of outside air into the space above the mercury. This is essential, as the concentrated acid, being hygroscopic, will otherwise rapidly absorb water, discolour and lose its effectiveness. The shaping of the neck between the capillary tube and the manometer is designed to facilitate the trapping of an air bubble in the former, as well as prevent its loss should an unexpectedly large pressure change drive the bubble out of one end of the capillary tube. To get the bubble back into its capillary tube the manometer is tilted by hand, when mercury will flow from one arm to the other around the bubble, which remains in the neck. On restoring the manometer to its normal horizontal position the bubble will move back into the capillary. The presence of the neck also causes a bubble to form automatically as soon as mercury is poured into the system. An interesting feature of this type of manometer is that it is completely unaffected by ambient temperature fluctuations. Although the lengths of the columns of mercury in the manometer arms vary with temperature their masses do not. In other words, a temperature change does not cause a flow of mercury and the position of the bubble therefore remains unaltered.
TEMPERATURE EFFECTS When this project was initiated it was realised that the core of the problem would be the elimination of all significant temperature effects. Those relating to each of the types of sensor have already been discussed. Two others will now be dealt with. The first refers to the effect on the areas of the supporting plates resting on the pressure units. A simple calculation shows that a change of almost 8°C is needed to produce a pressure change equivalent to 0.25 m m of evaporation; and since in many places this is greater than the annual temperature variation at a depth of 1.4 m - - t h e bottom of a lysimeter p i t - - t h e effect is insignificant. In any case such errors as do Agr. Meteorol., 5 (1968) 5-16
!4
r,. I). BERWICK AND ('. J. SUMNF I,:
arise can be nullified completely by changes of head occurring simultaneously in the rest of the system. Consider a rise in temperature: the supporting plates expand and the pressure (load per unit area) falls -as indicated by a drop in level of the mercury in the manometers. At the same time, however, and independently of this, the air in the transfer chambers expands, causing the level of the silicone fluid thereit~ to fall and the mercury in the manometers to rise-- by an amount sutticient to maintain a constant head. There are thus two effects, the one opposing the other. By using transfer chambers of an appropriate diameter the net change can be made equal to zero. The second relates to the air in the system which, having a relatively large expansion coefficient, could conceivably cause excess bowing of the neoprene diaphragm. However, the volume of air is sufficiently small so that the pressure units themselves, which are completely flexible over a small range, need to expand or contract only slightly to accommodate the changes in volume. This is true provided the pressure plate and clamp ring are coptanar. The volume changes do not affect the lengths of the balancing columns of mercury, since these lengths--other things being equal--are a function only of the pressure, i.e., the weight of the soil container.
RESULTS OF TESTS
Employing a soil container filled with water (weight 3 metric tons), a number
14.5
I
[,
I
14.0
I
I
50
/
Travelling
,/'+
13'5 microscope
(millimetres) 13.0
/ 12.O 0
I IO
I
I
I
20
:50
40
Weight
added
(Ibs)
,,| 60
I/ Ibs ~ O' 25rnrn RAINFALL (APPROXIMATELY) Fig.6. C a l i b r a t i o n c u r v e .
Agr. Meteorol., 5 (1968)
5-16
15
AN ACCURATEHYDRAULIC-PNEUMATICWEIGHINGLYSIMETER 7 , Water (mm) .............. Temperature (°C)
6 Mercury leveJ 5 ',equivalent mm 4 of woter loss or ~ i n by 3 lysimeter ) 2
20
"....
15
..'..
.....
Temp. I0 °C
....,,.
5 0
I
I
30
10
June
I
I
I
I
30
t0
20
duly
August
The clrdinote representing equiuolent
millimetres
0
microscope
readings is marked in
of water for convenience
Fig.7. Z e r o stability test.
of calibration checks were carried out on the constancy and linearity of the calibration. This was achieved by adding various known weights to the container and plotting the values against the indicated pressure. Fig.6 is an example of one test. A check was also made to see if the calibration varied with the absolute weight of the soil container. Three metric tons of lead ingots were added to the water-filled container, bringing the weight up to 5 metric tons, but there was no change in the calibration. Finally the same container, again filled with water, was fitted with a metal cover to prevent evaporation, and subject to a seven weeks zero stability test. Air temperatures both inside and outside the manometer cover, together with pressure readings, were read on a daily basis. There was no evidence of any drift (see Fig.7). This set of data was obtained using Type I manometer.
CONCLUSION From the results of the various tests it appears that the objective has been achieved. The instrument has been patented and is in commercial production; detailed design plans and installation instructions may be had on application to the authors.
ACKNOWLEDGEMENTS The authors wish to thank Mr. E. L. Deacon for his helpful comments from time to time and also to add their appreciation of the efforts of Mr. R. O. Simm who built the first prototype, and who, in the early stages when teething troubles were being overcome, always provided ready and willing assistance. Agr. Meteorol., 5 (1968) 5-16
16
P. I). BERWICK AND (L J. SUMNI:I~
REFERENCES
BEAUMONT,R. T., 1965. Mr. Hood pressure pillow snow gauge. J. Appl. Meteorol., 4 (5) : 626-631. BLOEMAN,G. W., 1963. A device for weighing large lysimeters with high. accuracy, lnst. CultuurtecJ~. Waterhuish. Wageningen, Nota, 184 : 18 pp. BLOEMAN,G. W., 1964. Hydraulic device for weighing large lysimeters. Trans. Am. Soc. Agr. Engrs., 7 (3) : 297-299. FORSGATE, J. A., HOSEGOOD, P. H. and McCuLLOCH, J. S. G., 1965. Design and installation of semienclosed hydraulic lysimeters. Agr. Meteorol., 2 (1) : 43-52. GLOVER, J. and FORSGATE,J. A., 1962. Measurement of evapotranspiration from large tanks of soil. Nature, 195 (4848) : 1330. HANLEY, H. J. M., 1964. A differential mercury manometer. J. Sci. Instr., 41 (7) : 486. HOLMES, R. M., 1963. Note on hot water bottle lysimeter. Can. J. Soil Sci., 43 : t86-188. KING, K. M., TANNER,C. B. and SouMI, V. E., 1956. A floating lysimeter and its evaporation recorder. Trans. Am. Geophys. Union, 37 (6) : 738-742. Liaav, F. J. and NIXON, P. R., 1962. Portable lysimeter adaptable to a wide range of site situations. Intern. Assoc. Sci. HydroL, Comm. Evaporation Publ., 62 : 153-158. MAKKINK, G. F., 1962. Five years of lysimeter research (Vijfjaren onderzoek). J. Agr. ScL, 11 0963) : 429~J,31 (abstract). MCILROY, I. C. and SUMNER,J. C., 1961. A sensitive high capacity balance for continuous automatic weighing in the field. J. Agr. Eng. Res., 6 (4) : 252-258. MC1LROY, 1. C. and ANGUS, D. E., 1962. The Aspendale multiple weighed lysimeter installation; Australia Commonwealth Sci. Ind. Res. Org., Div. Meteorol. Phys., Tech. Paper, 14 : 27 pp. MCILROY, I. C. and ANGUS, D. E., 1963. Grass, water and soil evaporation at Aspendale. Agr. Meteorol., 1 (3) : 201-224. PRUITT, W. O. and ANGUS, D. E,, 1960. Large weighing lysimeter for measuring evapotranspiration. Trans. Am. Soc. Agr. Engrs.,3 (2) : 13-18. ROSE, C. W., BYRNE, G. F. and BEo6, J. E., 1966. An accurate hydraulic lysimeter with remote weight recording. C.S.LR.O., Div. Land Res. Tech. Paper, (1966) 27 : 31 pp. WINTER, E. J., 1963. A new type of lysimeter. J. Hort. Sci., 38 (2) : 160-168.
Agr. Meteorol., 5 (1968) 5-16
AN ACCURATE HYDRAULIC-PNEUMATIC W E I G H I N G LYSIMETER FOR G E N E R A L FIELD USE P. D. B E R W I C K A N D C. J. S U M N E R
C.S.L R.O. Division of Meteorological Physics, Aspendale, Vie. (Australia)
(Received May 14, 1967)
SUMMARY
For some time there has been a growing need for a simple and inexpensive, yet accurate and reliable field lysimeter. Such an instrument operating on the hydraulic principle has now been developed; its day to day accuracy is 0.25 mm of rainfall or evaporation while over periods of the order of ten days the zero stability is within about 0.50 or 0.60 mm. The system comprises supporting capsules and a pressure sensing device. It requires no power, is not significantly affected by temperature fluctuations and the design has the advantage of being readily adaptable to quite large loads.
INTRODUCTION The C.S.I.R.O. Division of Meteorological Physics has operated a number of sensitive 6 metric ton lysimeters for the past eight years at Aspendale, Victoria (MCILROY and SUMNER, 1961). The value of the results obtained (MCILROY and ANGUS, 1962, 1963) has demonstrated the desirability of installing a network of these instruments throughout the country. However, the high cost of such precision equipment has precluded its use except in those relatively isolated cases where intensive research is being undertaken. It was decided therefore to design a complete lysimeter installation suitable for general field use at a price low enough to permit its installation in relatively large numbers. Further, the need for simplicity of operation, freedom from maintenance worries, reliability and reproducibility would be kept in mind. After surveying the various types of lysimeters which had been tried (KING et al., 1956; PRUITT and ANGUS, 1960; GLOVERand FORSGATE,1962; LmRY and NIXON, 1962; MAKKINK, 1962; BLOEMAN, 1963; HOLMES, 1963; WINTER, 1963; BLOEMAN, 1964; BEAUMONT, 1965; FORSGATE et al., 1965), a system working on the hydraulic-pneumatic principle was selected as being most likely to achieve the desired results. Additional features were to be an accuracy of 0.25 mm evaporation, a visual indication of weight change Agr. Meteorol., 5 (1968) 5-16
6
1'. D. BERWICK AND t" J, StJMNI-:]¢.
rather than a record al~d an absence of the need for power, whether battery u: mains.
MECHANICAL DESIGN AND INSTALLATION
Briefly the system consists of three flexible pressure units which support the soil container, and a pressure sensor of which two types are described. The installation may be conveniently divided into three parts: (a) The pit and soil container assembly, involving steel fabrication, sheet metal work and fibre glass fabrication. (b) The pressure units and transfer chambers assembly, involving general engineering fabrication. (c) The manometers.
THE PIT AND SOIL CONTAINER
A cross-section of the installation is shown in Fig.1. The outer pit liner supporting the surrounding soil is made of 1.5 mm thick galvanised iron, 1.75 m diameter and 1.4 m deep, and is provided with a pump well and pipe for removing flood water. Corrugated tanks have been avoided since they tend to collapse vertically due to the weight of soil resting on the corrugations. The pit liner is installed on a flat concrete base 7.6 cm thick, poured in an excavation of suitable size. If the water table is likely to rise above the base of the tank at any time when the lysimeter soil pot is removed, concrete of sufficient weight must be cast around the pit base (which is provided with an anchor flange)to prevent floatation. The soil container is made of 6.4 mm thick fibreglass 1.7 m in diameter (providing a clearance of 2.5 cm when placed in the pit), by 1.22 m deep. These dimensions provide an adequate depth for the rooting habits of many crops, while ensuring that its overall weight does not exceed the 5 metric tons limit imposed by the ready availability of mobile cranes of this capacity. Three equally spaced 312 cm tubes moulded into the inside walls of the container house three Z5 cm diameter steel lifting bolts. These pass through the bottom of the container into a galvanised steel supporting frame, fitted with captive nuts, Using a tripod sling and crane the soil container is lowered into position, the bolts withdrawn and the tubes sealed with rubber plugs to exclude dirt and water. A 1 cm thick perforated steel plate mounted on a channel iron frame placed in the soil container supports the soil and leaves a 10 cm deep space underneath in which percolate collects. Percolate can be pumped out periodically via a 1.3 cm bore copper pipe attached to the wall of the container and projecting just above the tim. The container may be back filled with soil or a monolith soil sample inserted in it. Agr. Meteorol., 5 (1968) 5-16
AN ACCURATE HYDRAULIC-PNEUMATIC WEIGHING LYSIMETER
MERCURY
3 RUBBER
SOIL. CONTAINER
PLUG
5- PRESSURE UNITS
7
MANOMETER
PNEUMATIC
--
c~IPE LINES
PIT TANK
3 TRANSFERCHAMBERS
REINFORCED
CONCRETE
Fig. l. Part section through lysimeter installation.
PRESSURE UNITS AND TRANSFER CHAMBERS
The three pressure units supporting the container are located at the vertices of an equilateral triangle. As shown in Fig.2, each comprises an alumiaium base casting, 63.5 cm diameter, to which is clamped a 1.5 mm thick fabric reinforced neoprene sheet. This supports a steel pressure plate (diameter 61 cm) which itself carries the soil container. The base casting is machined at the rim only to accommodate the neoprene sheet and clamp ring, the casting then being impregnated with thermosetting plastic to avoid any possibility of leaks. The pressure plate and clamp ring
CLAMP RING
STEEL PRESSUREPLATE
WOOD SPACER .
.
.
.
.
.
.
.
DIAPHRAGM
.
AIR PIPE
\
BASE CASTING
SILICONE FLUID PRESSURE UNIT
TRANSFER CHAMBER
Fig.2. Section through pressure unit and transfer chamber.
Agr. Meteorol., 5 (1968) 5-16
8
.P. D. BERWICK AND ('. .I. SUMNt!R
is flame cut from the same 6.3 mm thick steel plate so avoiding waste ol material and costly machining operations. The annular ring (1.3 cm) of unsupported neoprene sheet formed between the pressure plate and the clamp ring permits the plate to move vertically over a small range while maintaining the same contact area at all pressures. (This small but important point is often overlooked in many hydraulic systems.) A connecting pipe is fitted into the base casting and the pressure unit filled with 2.8 I of silicone fluid; the assembly is then pressure tested by sealing the pipe and loading to 3 metric tons (or 50 °.o in excess of the maximum working load) in a hydraulic press. Loads up to 5 metric tons have been supported by these pressure units without damage. Each pressure unit is connected by a short length of piping to a "transfer chamber" which consists of two spun brass discs 25.5 cm diameter, clamped together, with an " O " ring seal between them, to form an airtight cavity 3.8 cm deep. (To ensure airtight construction of the pneumatic parts of the system, castings have been avoided as they tend to be porous.) From the top of each transfer chamber a 3 mm diameter copper air line leads to a manometer, the purpose of the former being to convert liquid pressure to gas (air) pressure. If this were not done, liquid in the vertical sections of the lines would act as a partial balance to the load on the pressure units, the extent of the balance being governed by the mean density of the liquid column, which is temperature dependent. To integrate such changes over approximately 1.5 m (the depth of the lysimeter pit) in a straightforward manner is not easy, most attempts to do so involving temperature control - - an unnecessarily complicated solution. With equipment of this nature it is essential that it be capable of operating for long periods without servicing, particularly those parts which are inacccessible below the soil container. The most likely cause of failure in this instrument is a leak in the hydraulic-pneumatic system: to avoid this great care has been taken in the design to select impervious materials and to restrict the number of pressure joints to a minimum. The neoprene membrane has operated under toad for a period of two years without detectable deterioration, and the manufacturers of this material expect a life of five or more years under these working conditions. There are two reasons for using air and silicone oil rather than air alone as the sole working fluid. The most important is that air would diffuse through the neoprene relatively quickly and cause the pressure unit to collapse. Secondly, compared to the silicone fluid used, air has a large coefficient of expansion, and the volumetric changes in the pressure units resulting from temperature fluctuations would be such that they could not be accommodated without setting up forces in the neoprene and destroying the calibration ot the instrument. Also, of course, in a hydraulic-pneumatic system (as opposed to one completely hydraulic) it should be remembered that the air in the system is compressible and that when, for example, there is an increase in load on the soil container, the increased internal pressure causes the space occupied by the air to shrink and an appropriate amount of fluid to be displaced from the pressure unit. Conceivably this could give Agr. Meteorol., 5 (1968) 5-16
AN ACCURATEHYDRAULIC-PNEUMATICWEIGHINGLYSIMETER
9
rise to a significant change in the level of fluid in the transfer chambers (which forms part of the balancing head), as well as allow the pressure plate to move outside its range of complete flexibility (see under "Temperature effects", last paragraph). Regarding the first point, a simple calculation shows that an increase of over 12 cm of water on the soil container is needed to give rise to an error in the indicated pressure of 0.025 mm mercury (equivalent to 0.25 mm of water). Regarding the second point, part of the calibration procedure included determining over what vertical range the pressure plate could move before the neoprene exerted any appreciable force: in the present context almost 28 cm of water on the soil container was needed to introduce an error of 0.25 mm of water.
THE MANOMETERS Two types (I and II) are described. Type I is shown in Fig.3. This consists of a rod frame (1) 1.22 m high, mounted in a base plate (2) which is securely bolted to a concrete base after levelling. Three separate mercury manometer systems are mounted on the rod frame which supports three 10 cm diameter steel cisterns (3) provided with air union connections. 1.6 mm bore steel tubes (4) 2.44 m long, wound in a spiral are fitted into the base of the cisterns, the opposite ends being attached to 2.2 cm bore vertical monel metal tubes referred to as float chambers (5). These are mounted together to form an equilateral triangle, on brackets (6) which attach the manometer limb assembly to the frame (1) so that all three can be adjusted vertically as a unit. The three float chambers house floats (7) resting on the mercury surfaces and extending upwards to support a three spoked plate (8), at the centre of which is positioned an index point (9). Mounted on the bracket supporting the manometer limbs is a travelling microscope (10) which is focussed on the index point; the microscope is adjusted manually and capable of measuring to within 0.025 mm (equivalent to 0.127 mm evaporation) over a range of 15 cm. Since temperature will affect the lengths of the mercury columns it is necessary to correct for this; accordingly the sensing bulb (length 2.44 m) of a 5 cm dial indicating thermometer is wound in a spiral in intimate thermal contact with the steel tubes (4). A double walled temperature insulating cover, provided with a window and external hand wheel for setting the microscope, is fitted over the manometer assembly to provide a degree of damping to fluctuating ambient temperatures. Then, knowing the height of the mercury column (about 71 cm for a 6 metric ton soil container) and the temperature change, the necessary correction can be applied to the reading. To minimise undesirable surface tension effects at the mercury menisci, which cannot be kept chemically clean as in a mercury barometer for example, it is necessary to use relatively large diameter float chambers, i.e., 2.2 cm. The spiral tubes permit vertical adjustment of the manometer limbs to suit the weight of the particular soil container being used. They also restrict the rate of
Agr. Meteorol., 5 (1968) 5-16
li
WINDOW COVER WINDO
~,, ~[F~, I
INDICATING DIA THERMOMETER
tE :.OVER
ME
3 - AIR
P
Fig.3. Part section mercury manometer assembly (for further explanations see text). flow of mercury and cause a delay of approximately 10 min between the initiation of a weight change and the ensuing final pressure reading. This damps out oscillations in the manometer due to wind gusts. Fig.4 demonstrates change o f pressure :with time following an abrupt weight change at time t =~ 0 min.
INSTALLATION AND OPERATION
The manometer assembly is mounted in a suitable instrument house which may be located up to 90 m or so from the soil container. At distances greater than Agr. Meteorol., 5 (1968)
5-16
AN ACCURATE HYDRAULIC-PNEUMATIC WEIGHING LYSIMETER
I[
this the connecting pipes contain so much air, that, having regain to the limitations on the size of the transfer chamber (see under "Temperature effects" later), the silicon fluid would flow from the transfer chambers into the air lines once the pressure units were under load. Three 3 mm bore copper connecting pipes are fitted to the manometer cisterns, using special leak proof "O" ring unions. The pipes are buried 30 60 cm below ground level and enter the pit tanks through the wall near the top. Each is connected to the upper outlet of a transfer chamber suitably positioned in the base of the pit, the lower outlet being temporarily left open. Dry air from a commercial pressure cylinder is introduced into each copper pipe at the manometer end, and after purging to eliminate moisture, the pipes are reconnected. The three silicone-filled pressure units are connected to the lower outlets of the transfer chambers with the unions provided. Before lowering the soil container on the pressure units, to avoid mercury overttowing the float chambers, the manometer and microscope assembly are raised to its highest position. In the absence of response from the floats after the load has been in position for at least 15 rain, the manometer/microscope assembly is lowered in slow stages until the index point reaches the desired position. Changes in weight of the soil container will now cause the mercury level in the manometers (and therefore the floats carrying the index) to rise or fall, the displacement being observed by the travelling microscope and read off in millimetres on the scale provided. Often in the early setting up stage it will be found that the weight of the soil containers is not evenly distributed over the pressure units; this causes the individual manometer floats to take up different levels so that the three spoked plate they support is not horizontal. To overcome this each manometer cistern is provided with indivi-
0 "560
.
.
.
.
I
'
'
.f'/"
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~
i
..i~i--.
,
/11
/
Travelling 0.540 microscope (Inches) 0 "530
/
/
./
/" 0.520 0
I . . . . 5 - r i m e - Minutes
I I0
,
,
i
Fig.4. Artificial d a m p i n g . Pressure vs. time, after a d d i n g 28 lb. (
!
15
0.22 inches rainfall) at time
t=0
Agr. Meteorol., 5 (1968) 5-16
12
P . D . BERWICK AND C. J. SUMNER
dual means of vertical adjustment. In subsequent service, if non-uniform evaporation causes the floats to fall by somewhat different amounts, the index point nevertheless continues to indicate the correct mean level. The reason lies in the geometry of the equilateral triangle, the medians of which interact at a point 2/a.a from the vertices where a is the length of the median. (It will be recalled that both pressure units and float chambers are arranged in the form o| equilateral triangles.) Assume that a weight W is applied to the soil container: each pressure unit is then subjected to a pressure change proportional to W/3 and each float rises by an amount also proportional to W/3--say k. W/a. Clearly, in this case the index also rises by the same amount, k. W/.a. Suppose now W is applied to one pressure unit only: only one float rises but this time by an amount k W. However, because of the property of the equilateral triangle just referred to, the index rises by kW/3--as before. Strictly the argument applies only to small vertical displacements, but in practice all displacements turn out to be small.
THE TYPE II SENSOR
Essentially this is an ordinary m a n o m e t e r (HANLEY, 1964) the " U " section of which is replaced by a horizontal capillary tube (see Fig.5), The manometer is filled with mercury and an air bubble introduced into the capillary tube: the application of pressure, AP, to one arm of the manometer causes a displacement of the bubble, d, given by: AP ---- 2d (b2/a 2) cm of mercury, where a = radius of manometer and b == radius of capillary tube (both being in cm).
1
Pressure
atmosphere
inlet
~
Mercury ~r d . ~
~."11zri
i i...."
Air bubble
Fig.5. TypeII manometer (not to scale). A g r . M e t e o r o l . , 5 (1968) 5-16
AN ACCURATEHYDRAULIC-PNEUMATICWEIGHINGLYSIMETER
13
In the present manometer a ~ 9.63 ram, b = 1.398 mm. Thus 0.25 m m of evaporation causes a bubble displacement of approximately 1 ram. The advantage of this type of sensor over the first described is that its cost is extremely l o w - - o n the other hand it is relatively fragile. In setting up the manometers (of which there are t h r e e - - o n e per pressure unit) a high degree of cleanliness must be maintained. The inside of the manometer must be thoroughly scoured with chromic acid and fresh clean mercury employed. In order that the system shall function with adequate accuracy and reproducibility, the angle of contact between the mercury and the walls of the manometer arms must be kept constant: in practice this means a minimum. To achieve this the manometer arms are wetted with a few drops of 9 8 ~ of sulphuric acid (HANLEY, 1964). As can be seen from the diagram, one arm of the manometer is made to take a pressure hose fitting and the other a ground glass stopper which permits ventilation to the atmosphere. The stopper outlet faces downwards to assist in keeping dirt out of the system, and by means of a long narrow bore tube (not shown) restricts the diffusion of outside air into the space above the mercury. This is essential, as the concentrated acid, being hygroscopic, will otherwise rapidly absorb water, discolour and lose its effectiveness. The shaping of the neck between the capillary tube and the manometer is designed to facilitate the trapping of an air bubble in the former, as well as prevent its loss should an unexpectedly large pressure change drive the bubble out of one end of the capillary tube. To get the bubble back into its capillary tube the manometer is tilted by hand, when mercury will flow from one arm to the other around the bubble, which remains in the neck. On restoring the manometer to its normal horizontal position the bubble will move back into the capillary. The presence of the neck also causes a bubble to form automatically as soon as mercury is poured into the system. An interesting feature of this type of manometer is that it is completely unaffected by ambient temperature fluctuations. Although the lengths of the columns of mercury in the manometer arms vary with temperature their masses do not. In other words, a temperature change does not cause a flow of mercury and the position of the bubble therefore remains unaltered.
TEMPERATURE EFFECTS When this project was initiated it was realised that the core of the problem would be the elimination of all significant temperature effects. Those relating to each of the types of sensor have already been discussed. Two others will now be dealt with. The first refers to the effect on the areas of the supporting plates resting on the pressure units. A simple calculation shows that a change of almost 8°C is needed to produce a pressure change equivalent to 0.25 m m of evaporation; and since in many places this is greater than the annual temperature variation at a depth of 1.4 m - - t h e bottom of a lysimeter p i t - - t h e effect is insignificant. In any case such errors as do Agr. Meteorol., 5 (1968) 5-16
!4
r,. I). BERWICK AND ('. J. SUMNF I,:
arise can be nullified completely by changes of head occurring simultaneously in the rest of the system. Consider a rise in temperature: the supporting plates expand and the pressure (load per unit area) falls -as indicated by a drop in level of the mercury in the manometers. At the same time, however, and independently of this, the air in the transfer chambers expands, causing the level of the silicone fluid thereit~ to fall and the mercury in the manometers to rise-- by an amount sutticient to maintain a constant head. There are thus two effects, the one opposing the other. By using transfer chambers of an appropriate diameter the net change can be made equal to zero. The second relates to the air in the system which, having a relatively large expansion coefficient, could conceivably cause excess bowing of the neoprene diaphragm. However, the volume of air is sufficiently small so that the pressure units themselves, which are completely flexible over a small range, need to expand or contract only slightly to accommodate the changes in volume. This is true provided the pressure plate and clamp ring are coptanar. The volume changes do not affect the lengths of the balancing columns of mercury, since these lengths--other things being equal--are a function only of the pressure, i.e., the weight of the soil container.
RESULTS OF TESTS
Employing a soil container filled with water (weight 3 metric tons), a number
14.5
I
[,
I
14.0
I
I
50
/
Travelling
,/'+
13'5 microscope
(millimetres) 13.0
/ 12.O 0
I IO
I
I
I
20
:50
40
Weight
added
(Ibs)
,,| 60
I/ Ibs ~ O' 25rnrn RAINFALL (APPROXIMATELY) Fig.6. C a l i b r a t i o n c u r v e .
Agr. Meteorol., 5 (1968)
5-16
15
AN ACCURATEHYDRAULIC-PNEUMATICWEIGHINGLYSIMETER 7 , Water (mm) .............. Temperature (°C)
6 Mercury leveJ 5 ',equivalent mm 4 of woter loss or ~ i n by 3 lysimeter ) 2
20
"....
15
..'..
.....
Temp. I0 °C
....,,.
5 0
I
I
30
10
June
I
I
I
I
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t0
20
duly
August
The clrdinote representing equiuolent
millimetres
0
microscope
readings is marked in
of water for convenience
Fig.7. Z e r o stability test.
of calibration checks were carried out on the constancy and linearity of the calibration. This was achieved by adding various known weights to the container and plotting the values against the indicated pressure. Fig.6 is an example of one test. A check was also made to see if the calibration varied with the absolute weight of the soil container. Three metric tons of lead ingots were added to the water-filled container, bringing the weight up to 5 metric tons, but there was no change in the calibration. Finally the same container, again filled with water, was fitted with a metal cover to prevent evaporation, and subject to a seven weeks zero stability test. Air temperatures both inside and outside the manometer cover, together with pressure readings, were read on a daily basis. There was no evidence of any drift (see Fig.7). This set of data was obtained using Type I manometer.
CONCLUSION From the results of the various tests it appears that the objective has been achieved. The instrument has been patented and is in commercial production; detailed design plans and installation instructions may be had on application to the authors.
ACKNOWLEDGEMENTS The authors wish to thank Mr. E. L. Deacon for his helpful comments from time to time and also to add their appreciation of the efforts of Mr. R. O. Simm who built the first prototype, and who, in the early stages when teething troubles were being overcome, always provided ready and willing assistance. Agr. Meteorol., 5 (1968) 5-16
16
P. I). BERWICK AND (L J. SUMNI:I~
REFERENCES
BEAUMONT,R. T., 1965. Mr. Hood pressure pillow snow gauge. J. Appl. Meteorol., 4 (5) : 626-631. BLOEMAN,G. W., 1963. A device for weighing large lysimeters with high. accuracy, lnst. CultuurtecJ~. Waterhuish. Wageningen, Nota, 184 : 18 pp. BLOEMAN,G. W., 1964. Hydraulic device for weighing large lysimeters. Trans. Am. Soc. Agr. Engrs., 7 (3) : 297-299. FORSGATE, J. A., HOSEGOOD, P. H. and McCuLLOCH, J. S. G., 1965. Design and installation of semienclosed hydraulic lysimeters. Agr. Meteorol., 2 (1) : 43-52. GLOVER, J. and FORSGATE,J. A., 1962. Measurement of evapotranspiration from large tanks of soil. Nature, 195 (4848) : 1330. HANLEY, H. J. M., 1964. A differential mercury manometer. J. Sci. Instr., 41 (7) : 486. HOLMES, R. M., 1963. Note on hot water bottle lysimeter. Can. J. Soil Sci., 43 : t86-188. KING, K. M., TANNER,C. B. and SouMI, V. E., 1956. A floating lysimeter and its evaporation recorder. Trans. Am. Geophys. Union, 37 (6) : 738-742. Liaav, F. J. and NIXON, P. R., 1962. Portable lysimeter adaptable to a wide range of site situations. Intern. Assoc. Sci. HydroL, Comm. Evaporation Publ., 62 : 153-158. MAKKINK, G. F., 1962. Five years of lysimeter research (Vijfjaren onderzoek). J. Agr. ScL, 11 0963) : 429~J,31 (abstract). MCILROY, I. C. and SUMNER,J. C., 1961. A sensitive high capacity balance for continuous automatic weighing in the field. J. Agr. Eng. Res., 6 (4) : 252-258. MC1LROY, 1. C. and ANGUS, D. E., 1962. The Aspendale multiple weighed lysimeter installation; Australia Commonwealth Sci. Ind. Res. Org., Div. Meteorol. Phys., Tech. Paper, 14 : 27 pp. MCILROY, I. C. and ANGUS, D. E., 1963. Grass, water and soil evaporation at Aspendale. Agr. Meteorol., 1 (3) : 201-224. PRUITT, W. O. and ANGUS, D. E,, 1960. Large weighing lysimeter for measuring evapotranspiration. Trans. Am. Soc. Agr. Engrs.,3 (2) : 13-18. ROSE, C. W., BYRNE, G. F. and BEo6, J. E., 1966. An accurate hydraulic lysimeter with remote weight recording. C.S.LR.O., Div. Land Res. Tech. Paper, (1966) 27 : 31 pp. WINTER, E. J., 1963. A new type of lysimeter. J. Hort. Sci., 38 (2) : 160-168.
Agr. Meteorol., 5 (1968) 5-16