Plant, Celt and Environment (1982) 5, 323-327,

TECHNICAL REPORT

Water uptake by some chamber materials

M, DIXON & J. GRACE Department of Forestry and National Resources, The University ofEdinburgh, Mayfield Road, Edinburgh, EH9 3JU

Received 1 September 1981; accepted for publication 13 October 1981

Abstract. The characteristics of water uptake byseveral materials have been studied, Nitrile rubber,silicone rubber and perspex retained substantialquantities of water. Metals took up smaller, but stillsignifiicant, amounts of water, and glass was the leastwater-retentive material used in the tests. The surfacestructure of the material, revealed by scanningelectron microscopy, does not necessarily account forits water uptake characteristics. The results arediscussed in relation to the design and construction ofpsychrometer and porometer chambers.

Key-words: Water uptake; absorption; adsorption; psychrometerchamber; porometer chamber.

IntroductionThere are several techniques in the plant sciences inwhich a knowledge of the water uptake properties ofthe apparatus is crucial. Perhaps the most critical caseis the thermocouple psychrotneter chamber (Millar,1971a,b, 1974; Wiebe et al., 1971), In such a chambera satnple of leaf tissue is allowed to exchange watervapour with a small volume of air until an equilibriumcondition is achieved. At this equilibriutn the waterpotential of the leaf material is related to the humidityof the air in the chamber (Spannet, 1951; Rawlins,1966). Any tendency of the walls of this chamber totake up water will have at least two utidesirableconsequences. Firstly, the time taken to achieveequilibrium may be unduly prolonged—this isextremely inconvenient when attempting to obtain adiurnal course of water potential with a small numberof psychrometer units. Secondly, it is possible thattrue equilibrium may never be obtained as a result ofwall porosity, and thus an absolute error trtay beintroduced.

The other apparati in which water uptake problemscommonly occur include assimilation chambers andporometers (Stigter, Birnie & Lamtners, 1973).

Materials used in the construction of thesechambers include metals, plastics (especially Perspex,known as Plexiglass in North America), and variousrubber compounds as sealitig gaskets. Very littleinformation is available on the rates of water uptakeof these materials under realistie conditions, A recenttechnical report in this journal (Bloom et al., 1980), in

0140-7791/82/0800-0323502,00© 1982 Blackwell Scientific Publications

attempting to assemble data on the subject, reliedheavily on Shepherd (1973), Shepherd does reportmeasurements on such materials, but his results maybe spuriously high as condensation occurred on thesamples; moreover, no indication was given of thekinetics of uptake (Shepherd, 1973),

The present work was undertaken to enlarge uponShepherd's findings and enable a rational choice ofmaterials with which to construct a thermocouplepsychrometer unit.

Materials and methods

Samples of eleven materials (Table 1), each of surfacearea 2-4 cm^, were suspended over silica gel in asealed desiccation jar for 6 d. The resulting dry weightof each sample was then measured to the nearestmicrogram using a Cahn Automatic Electrobalance,Model 4700; and the sample suspended over asaturated aqueous solution of sodium sulphate(Na2SO4- IOH2O) in a sealed jar. The jar was placedin a water bath at 20 C (±0,01 C), Such conditionsproduced a constant humidity of 93",, over thesolution (Weast, 1971) and eliminated the possibilityof condensation on the samples. The weight of eachsample was detennined at intervals until 24 h hadelapsed when the samples were returned to adesiccation jar. The samples were again weighed atintervals until their initial dry weights were restored.

The surface of each sample was photographedunder the scanning electron microscope, after sputter-eoating the non-conducting materials with gold(Polaron, Model E5000),

Results and discussion

Water uptake rates and absolute levels of waterretained are shown in Fig, 1, Some materials take upconsiderable amounts of water and require manyhours to reach an equilibrium, so would not besuitable for the construction of chambers. Plastic andrubber materials are particularly undesirable, thoughwhen they must be used, as in sealing rings onpsychrometer chambers, it should be noted that someare better than others, Viton, for its low uptake level,and silicone rubber for its rapid rate of water loss maybe suitable for this purpose but ordinary O-rings ofnitrile rubber are especially poor, Viton is more

323

324 M. DIXON & J, GRACE

Table t. List of materials used in tests, with full specificationwhere appropriate

Material Specification

Butadiene-acrylonitrileFluoroearbon rubberSilicone rubberPolytetrafluoroethyleneAcrylic plasticGla,ssStainless SteelBrass, polishedBrass, tarnishedNickelNickel-chrome

Walker's mcdium-nitrile (80 LR.H.D.)Walker's 'Viton' (80 LR.H.D.)Walker's silicone rubber (70 LR.H.D.)Walker's 'Fluolion' PTFE•Perspex' sheetCover Slip, BS 53836 17BS 18/8Hard rolledHard rolledElectroplated on brass, 1 5 /mi thiekElectroplated on brass, 30 /im thick

practical than silicone rubber as an O-ring material,as the latter is too compressible and creates variationin sealing pressure, which in turn affects instrumentsensitivity (Wiebe et al, 1971). Polytetra-fluoroethylene (PTFE) took up very little water, butO-rings made of it are too hard and do not readilymake a seal.

The relatively low uptake levels and retentionperiods of the metallic and electroplated samples aswell as the PTFE sample show them to be quitesuitable as chamber wall materials. Especiallyunsuitable is brass, as the tarnished brass achieved thehighest level of water uptake of the metals and thelongest retention period. The relationship betweenpolished (Plate 1,B) and tarnished brass (Plate I,A)can probably be taken as the general case whencomparing oxidized and polished surfaces of the samemetal. The porous surface of the stainless steel sample(Plate l.F) may account for its poor performancecompared to the electro-plated brass samples (Fig, 2c,d). The diffusion of water vapour into pores cannotitself account for observed gains in weight as water-saturated air is less dense than dry air. Rather it isnecessary to postulate the adsorption of watermolecules onto the much-increased wall area pro-vided by a porous system.

500

400 -

300-

200

100

Within the metals, the rougher surfaces tended todisplay the highest levels of uptake: within the plasticsand rubbers, silicone rubber and Viton have quiterough surfaces (Fig, 3) yet relatively low levels ofwater uptake. It is presumed that water is taken upinto intermolecular lacunae which are not visibleunder SEM (Barrie & Platt, 1963),

The absolute levels of uptake reported here are lessthan those given by Shepherd (1973), who exposedsimilar materials for 24 h over distilled water. Forexample, his level of water uptake for PTFE is almosttwenty times more than that found in this study. Thisdiscrepancy is probably due to the condensationwhich he admits occurred in his experitnent, requiringthat the samples be blotted before weighing.Condensation was prevented in the present study bythe use of an incompletely saturated atmosphere.

There is strong evidence to suggest that many of thedifficulties involved in the use of thermocouplepsychrorneters arise due to water uptake by the wallsof the chamber. In particular, the long equilibrationtime observed by most workers is not onlyinconvenient but may introduce absolute errors dueto metabolic changes which necessarily occur withinthe tissue over several hours following excision.Equilibration times exceeding 5 h are frequentlyrequired, even when using calibration solutions (Barrs& Slatyer, 1965; Lambert & van Schilfgaarde, 1965),Moreover, Lambert and van Schilfgaarde (1965) andMillar (1971a) report different equilibration times inpsychrometer chambers made frotn differentmaterials, Millar found that brass and stainless steelchambers equilibrated more rapidly than those ofpolythene and teflon, whilst epoxy-resin, rubber andperspex chambers took especially long to equilibrate.The long equilibration time has led some authors toadopt a special chamber geometery, so that thesensing thermocouple is shielded from the walls; inthis way a satisfactory reading can sometimes beobtained before a true equilibrium has occurred (Peck,1969; Millar, 1974),

e

90 100

Figure 1. The characteristics of water uptake and loss for the materials listed in Table 1, Symbols represent nitrile rubber ( x ), Perspex (O),silicone rubber ( • ) , Viton (O), tarni,shed brass (#), stainless steel (A), brass (A), nickel (T), Fluolion PTFE (D), nickel-chrome (^), glass( • ) . Note the difference in scaling factors on the axes of the two parts of the ligure. The arrows indicates the time when the samples wereremoved from the hydrated atmosphere. , ,

WATER UPTAKE BY MATERIALS 325

Figure 2. Finestrtictureof the surfaces of the materials tested: (a) tarnished brass; (b) polished brass; (c) nickel-chrome; (d) nickel; (e) glass;(0 stainless steel.

It is also observed, when calibrating psychrometerchambers, that the calibration line does not passthrough the otigin; i,e, for pure water the unitdevelops a finite voltage output which maycorrespond to several bars. Most workers accept thisand incorporate it into the calibration, claiming it tobe fully reproducible as long as a standardequilibration titne is employed (Wiebe et al., 1971),However, we have found rather poor reproducibilitywith aluminium chambers and think it is the result ofwater uptake on the walls. The niches and channels inmetallic walls behave as tortuous extensions of thechatriber volume which in an extreme case could beinfinite. In a completely dry chamber, they take a longtime to equilibrate with the tnain chamber volutneand in a wet chamber they become an unwantedsource of water vapour so that the final equilibriuirthumidity is exeessively high.

It is useful to compare the mass of water in thewalls with that in the chamber volume and in the leafsample. As an exatnple, a chamber designed to acceptleaf discs typically may have a volume of 0.5 cm^which at equilibrium contains air tiearly saturatedwith water vapour—about 4 /(g of water. Its exposedwall area might be about 3 ctn^ with perhaps afurther 0.2 ctn"̂ of exposed rubber sealitig ritig.Reference to Fig, 1 shows that the total surface areasurrounding the volutrte has the capacity to take up

far more water vapour than the 4 /(g present in thechamber air. It follows that the equilibrationcharacteristics of the whole unit will be mainlydetermined by those of the wall material. The watercontent of a leaf disc cut to fit on the floor of thischamber might be about 0.1 g, probably adequate toprovide enough water to saturate the whole systemwith vapour without itself suffering much decline inwater potential. The leaf discs themselves maycontribute to the variability in equilibrationcharacteristics: the cuticle and epicuticle may take upwater vapour despite their hydrophobic properties(Millar, 1974) and the cuticular resistance will be afactor affecting equilibriutn (Peck, 1969), The recenthistory of the chamber will also influence equilibra-tion characteristics; the water stored from one samplebeitig carried over to the next. Cleaning with watertnay not be advisable for the same reasons.

Errors in porotneters, arising from water uptake bysurfaces, have also been acknowledged, Perspex wallsare said to be responsible for drift phenomena(Morrow & Slatyer, 1971) and polypropylene isrecomtnended by Stigter, Birnie & Lammers (1973)on the basis of trials with instruments made of severalmaterials. Gander & Tanner (1976) studied wateruptake by the walls and sensor materials ofporometers exposed to much lower values of humiditythan in the present study. They found that acrylic

326 M, DIXON & J. GRACE

Figure 3. Fine structure of the surfaces of plastics and rubbers; (a) Fluolion PTFE; (b) silicone rubber; (e) Perspex; (d) nitrile rubber- (elViton. ' '

plastic was particularly unsuitable, though acceptableif waxed. Stainless steel was preferred over brassbecause it does not corrode. The magnitudes of wateruptake in their study are in broad agreement withthose we have found. Attention has been given to therubber sealing ring in the chambers of diffusionporometers (Stiles, 1970), Hack (1980) showed thatwater retention by this ring could be a serious sourceof error in the measurement of leaf diffusiveresistance. However, the rubber in such sealing ringsis usually in the form of foam, in which the waterretention properties are exacerbated by the system ofinterconnected pores.

We conclude that water uptake by wall materials inporometers nevertheless deserves careful considera-tion, and tests on specific materials ought to be madebefore actual construction is undertaken.

Although a comprehensive list of materials isoutside the scope of this investigation, these resultsdemonstrate the importance of the characteristics ofwater uptake by surfaces employed as chamber wallmaterials.

Acknowledgments

Scanning electron micrographs were made by DrChristopher Jeffree, Botany Department, Edinburgh

University, Electroplating was by courtesy ofPrecision Machining Ltd,, Dalkeith, Scotland, O-ringswere supplied by James Walker and Company Litnited,Lion Works, Woking, Surrey, England, We thank ourcolleagues in this Department for valuable suggestionsmade in the course of this work. The experiments werecarried out during the tenure of a Research Grant fromthe Natural Environmental Research Council.

References

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Barrs, H.D, & Slatyer, R.O. (1965) Experience with three vapourmethods for measuring water potential in plants. Une.seo AridZotie Research. 25, 369 383,

Bloom, A.J., Mooney, H.A., Bjorkman, O. & Berry, J. (1980)Materials and methods for carbon dioxide and water exchangeanalysis, Planl. Cetl and Envirottment. 3, 371 376.

Gandar, P.W. & Tanner, C.B. (1976) Water vapour sorption by thewalls and sensor of stomatal diffusion porometers. AgronomyJournat. 68, 245-249.

Hack, H.R.B. (1980) The uptake and release of water vapour by thefoam seal of a diffusion porometer as a source of bias, Ptant.Celt and Environment. 3, 53 57.

Lambert, J,R. & Schilfgaarde, J, van (1965) A method ofdetermining the water potential of intact plants. Soil Science.100, 19,

Millar, B,D. (1971a) Improved thermocouple psychrometer for thiemeasurement of plant and soil water potential. L Thermocouple

WATER UPTAKE BY MATERIALS 327

psychrometry and an improved instrument design. Journat ofExperimentat Botany. 22, 875-890.

Millar, B.D. (1971b) Improved thermocouple psychrometer for the,.:, measurement of plant and soil water potential. 11. Operation' >- and calibration. Journat of Experimentat Botany. 22,

891 905.Millar, B,D. (1974) Improved thermocouple psychrometer for the

measurement of plant and soil water potential. III.•̂ Equilibration, Jotirnal of Experimentat Botany. 25, 1070-1084.Morrow, P.A. & Slatyer, R.O. (1971) Leaf resistance measurements

with diffusion porometers; precautions in calibration and use.Agricultural Meteorology. 8, 223- 233,

Peck, A.J. (1969) Theory of the Spanner psychrometer, 2. Sampleeffects and equilibration. Agricultural Meteorology. 6, 1 11 124.

Rawlins, S.L. (1966) Theory for thermocouple psychrometers usedto measure water potential in soil and plant samples.Agricutturat Meteorotogy. 3, 293-310.

Shepherd, W. (1973) Moisture absorption by some instrumentmaterials. Ttie Review of Scientific Instruments. 44, 234.

Spanner, D.C, (1951) The Peltier effect and its use in themeasurement of suction pressure. Journal of ExperintcntalBotany. 2, 145 168.

Stigter, C,J,, Birnie, J, & Lammers, B, (1973) Leaf diffusionresistance to water vapour and its direct measurement. II,Design calibration and pertinent theory of an improved leafdiffusion resistanee meter, Mededetingen van detMndtioidwhogesehoot te Wcigeningeit. 73,(15), 1-55.

Stiles, W. (1970) A diffusive resistance porometer for field use, I,Construction. Jourttat of Apptied Ecotogy. 7, 617 622.

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Note added in proofSince this work was completed our attention has beendrawn to numerous other materials. In particular,there are a variety of plastics currently available whichmay prove to be suitable chamber materials. We haverecently tested three of these.

Under the satne test conditions as those statedabove, TPX (polymethylpentene) (Mitsui & Co, Ltd)reached a maximum water uptake level of

3,42 /(g cm ^ in 7 h. High density polyethylene(Polypenco Ltd) reached a maxitnum water uptakelevel of 7.64 ^igcm"^ in 5 h. Cross-linkedpolystyrene (Q.200.5) (Polypenco Ltd) continued totake up water to a maximum of 56,25 //g cm^- afterabout 15 h.

Further infonnation on the kinetics of wateruptake by these materials is available from theauthors.