An improved method for mounting ceramic-moisture sampling cups
and soil-atmosphere sampling tubes in lysimeters is described. The
0.31-m diam by 1.18-m deep lysimeters were constructed from low
pressure polyvinyl chloride irrigation pipe (PVC) and contained soil
1.0-m deep. Tensiometer tubes with attached cups, and open-ended
pyrex gas sampling tubes were installed at three depths, using sealing
ports in the lysimeter walls. The sealing ports were constructed
from half of a PVC pipe compression coupler. This system allows
for inserting the sampling cups and tubes after the soil has been
added and settled. The sampling equipment can also be removed
before the soil is sampled and removed when studies are completed.
Individual sampling units can also be removed for cleaning or repair
during the study without disrupting lysimeter operation. This was
not possible with earlier described lysimeter systems

Robbins, C.W.

Northwest Irrigation & Soils Research Laboratory Publications

Sampling Ports for an Instrumented Lysimeter System

USDA - ARS - NWISRL

Reprinted from the Soil Science Society of America JournalVolume 49, no. 6, November-December 1985677 South Segue Rd.. Madison, WI 53711 USASAMPLING PORTS FOR AN INSTRUMENTEDLYSIMETER SYSTEM'C. W. RoumN52AbstractAn improved method for mounting ceramic-moisture sampling cupsand soil-atmosphere sampling tubes in lysimeters is described. The0.31-m diam by 1.18-m deep lysimeters were constructed from lowpressure polyvinyl chloride irrigation pipe (PVC) and contained soil1.0-m deep. Tensiometer tubes with attached cups, and open-endedpyrex gas sampling tubes were installed at three depths, using seal-ing ports in the lysimeter walls. The sealing ports were constructedfrom half of a PVC pipe compression coupler. This system allowsfor inserting the sampling cups and tubes after the soil has beenadded and settled. The sampling equipment can also be removedbefore the soil is sampled and removed when studies are completed.Individual sampling units can also be removed for cleaning or repairduring the study without disrupting lysimeter operation. This wasnot possible with earlier described lysimeter systems.Additional Index Words: tensiometer, ceramic cup, sail air, soilatmosphere.Robbins, C.W. 1985. Sampling ports for an instrumented lysimetersystem. Soil Sci. Soc. Am. J. 49:1586-1587.AN INSTRUMENTED , continuous weighing lysime-ter system designed for monitoring irrigationwater quality and management effects on soil solutioncomposition was previously described (Robbins andWillardson, 1980). The lysimeters were constructedfrom 0.31-m diam by 1.18-m long PVC low pressureirrigation pipe sections. The upper end was reinforcedwith a 0.12-m polyvinyl chloride (PVC) band and thebottom was a flat end cap cemented in place. Three13-mm chloronated poly -vinyl chloride (hot water pipe)pipes (CPVC) were cemented into the lysimeter sidesat 0.30-, 0.55-, and 0.80-m above the bottom of thelysimeters. A porous ceramic cup was heat sealed intothe inner end of the CPVC pipes and the outer end' Contribution from USDA-ARS, Snake River Conservation Re-search Center, Kimberly, ID 83341. Received 8 Apr. 1985. Ap-proved T June 1985.2 Soil Scientist, USDA-ARS, Snake River Conservation ResearchCenter, Kimberly, ID 83341.was open for inserting the sampling and evacuatingtubes. A drain pipe was placed in the end cap on thebottom and covered with 0.05 m of coarse sand priorto filling the lysimeter with soil. The addition of thesand below the soil gave final placement of the ce-ramic tubes at 0.25-, 0.50-, and 0.75-m above the bot-tom of the added soil. Wooden frames were con-structed and each frame supported four lysimeters.Each lysimeter rested on a small, water-filled tire in-nertube. The butyl innertubes were connected to clearplastic manometer tubes that were calibrated and usedto record weight changes due to irrigation drainageand evapotranspiration.These lysimeters were used for several studies. Atthe end of each study, the soils were sampled by depthand the remaining soil was washed out. Several ce-ramic cups or plastic tubes were broken during sam-pling and cleaning. If the center or lower cups or thebottom drain pipe were broken, the entire lysimeterhad to be replaced. Others who followed the originaldesign have had the same experience.A more recent study was set up that required soil-atmosphere gas sampling in addition to the soil so-lution samples at various depths. Because of the pre-vious equipment and sampling problems and the needfor gas sampling ports new lysimeters were con-structed. The modifications and improvements incor-porated in the new lysimeters are described here.Materials and Mechanical DesignSixteen lysimeters were constructed from 0.31-m diamwhite PVC low pressure irrigation pipe. The pipe was cutin 1.18-m lengths so the final lysimeter would hold a 1.0-mdepth of soil and have sufficient freeboard for irrigation waterponding. The perforated drain pipes were constructed andmounted in the end caps as previously described (Robbinsand Willardson, 1980). The end caps were not cemented tothe bottom of the PVC pipe as was done on previous lysi-meters since a water table was not allowed in the lysimeters.Two vertical columns of 30-mm diam holes at 0.25, 0.50,and 0.75 m from the soil surface were made at 90° to eachother. An adapter and seal system were made for each holeby cutting the center section (approximately 45 mm) froma PVC pipe compression coupler (Fig. 1A). This left ap-proximately 12 mm of the center section on each end be-yond the compression nut threads. One half of each couplerwas then cemented into a hole in the side of the lysimetersFig. 1. (A) PVC pipe coupler showing cut locations for making twosample port seals. (B) complete water extraction equipment (a)ceramic am, (b) PVC tube from tensiometer, (c) lysimeter wall,(d) coupler body, (e) compression nut, (f) rubber seal, (g) plea-iglass tubing, (b) hose clamp. (i) plastic tubing, a) robber stopper,and (k) sample bottle).(Fig. 1 B). The sampling tubes were then inserted into thesoil columns and sealed in place through the ports.The coupler had been designed for splicing 12.5-mm (0.5in.) iron pipe size (IPS), PVC water pipe that had a 22-mmoutside diam. A 22-mm outside diam teniometer tube andceramic cup was cut to an overall length of 0.23 m. Afterthe lysimeters were filled with soil and compacted to thedesired bulk density, the tensiometer tubes were pushed intothe soil and the rubber seals were tightened into place withthe compression nuts. A tensiometer tube was inserted 0.15m into the soil at each depth. The water sampling bottle andtubes were then added (Fig. 1B). A 22-mm hole was made0.15-m deep into the soil from the side at each of the re-maining ports and a pyrex boiler gauge, thick walled tubewas placed into the hole. The boiler gauge tube was 19-mmoutside diam and 0.23-m long. To make a tight fit, a gasketwas made around the glass tube by wrapping it with blackelectrical tape. After securing the glass tube in place, theouter end was plugged with a 13-mm rubber septum. Theinner end was left open for soil gasses to diffuse into thetube.CommentsWater samples were extracted from the tensiometertubes as previously described (Robbins and Willard-son, 1980) and the gas samples were either taken fromthe glass tubes with a hypodermic needle on a 20-mLsyringe or with a needle coupled to a sampling wandon a portable gas chromatograph.By making the ceramic cup tubes and the glass gassampling tubes removable, the soil could be added,compacted, irrigated, and settled in the lysimeterswithout chance of sampling tube damage and unnec-essary stress on the lysimeter walls. Short dummy plugswere put into the ports during soil addition. More uni-form soil settling was obtained under these conditionsthan when the ceramic tubes were in place during soiladdition. During the study, if a ceramic cup crackedor was plugged, or if a glass tube became plugged, theycould be removed, repaired, or cleaned, and replacedimmediately. If soil-ceramic cup contact was not com-plete, contact was easily made by loosening thecompression nut, gently pushing the tensiometer tubeagainst the soil and tightening the compression nut.On several occasions, drawing air from the glass tubeswhen the soil was near saturation drew soil or waterinto the glass tubes. By having glass tubes the contentcould be visually inspected without removing the sep-turns. Visual inspection is important since moistureor wet soil will plug and sometimes damage the port-able gas chromatograph.Once a lysimeter study was completed, both sets oftubes were removed and soil samples by depth weretaken without damaging the tubes, which were not inthe way for sampling and cleaning. After removing thesamples the remaining soil was washed out afterknocking the bottom off by gently pounding aroundits edge since it was not cemented to the pipe on thisset of lysimeters. As long as the drainage tubes werekept clean and open, the water table in the bottomnever became deeper than the end plugs were high.These changes made the lysimeter much easier to fillwith soil at the beginning, and easier to sample andclean at the end of each study. These lysimeters haveeach been used three times and four of them havebeen used four times. These compression couplerscome in a wide variety of sizes and`could be used forsampling or monitoring equipment with diameters dif-ferent than those used in this study.ReferencesRobbins, C.W. and L.S. Willardson. 1980. An instrumented lysi-meter system for monitoring salt and water movement. Trans.Am. Soc. Agric. Eng. 23:109-111.

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Sampling Ports for an Instrumented Lysimeter System

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