Handbook ofVADOSE ZONE

CHARACTERIZATION&

MONITORINGEdited by

L.G. Wilson, Lorne G. Everett, StephenJ.Cullen

21

Energy-Related Methods: Psychrometers

Todd C. Rasmussen and Shir lee C. Rhodes

INTRODUCTION

The measurement of the direction and magnitude of contamin ant migrat ion in thesubsurface is often hampered by the heterogeneous and complex nature of mostgeologic ,media. An alternat ive to direct measurement of subsurfa ce migrationroutes and rates in the unsaturated zone is to indirectly dete rmine water flow usingthe direction and magnitude of the hydraul ic gradient and the hydraulic conductivityof the medium. The direction and magnilude of fluid flow is governed by thetensorial form of Darcy's law:

(I)

LEWIS PUBLISHERSBou. Raton Ann Afbor london Tokyo

where = 'VHH = z - y, +<t>

and

q = fluid f'lux vector, m-s" ;~( y,) = suction-dependent hydrau lic conductivity tensor. m-s" :

j = hydraul ic gradient . dimensionless;H = tota l hydraulic head. rn;z = elevation head, m;

oJ, = mat ric suction head, m; and<t> = osmotic potential head , m.

The estimation of the hydrau lic gradient requires knowledge of the distribut ion ofthe matric suction and the osmotic potential. The hydraul ic co nductivity tensorfunction can be determined in the laboratory using core segments extracted from thesite of interest , or from field-scale permeability tests using water or air (sec. c.g .•

0-87371-6 10-8/9 5/S0.00 + S.SO(?\ I QQ"i h v r ...w i," I'luhl ich,,"rc

The measurement of rhe direct ion and maenitu de of co nta mina nt mierarinn in rhe

330 HANDBOOK OF VADOSE ZONE CHARACTERIZATION & MONITORING ENERGY·RELATED METHO DS: PSYCHROMETERS 331

The dew-point depression, ~T<l' is the difference belween the ambient and dew­point temperatures. or appr oximately:

may also include , however, the osmotic potential associated with solutes dissolved inthe pore fluid . It is important to note that osmotic potent ials may be significantwhen elevated solute concent ratio ns are present. The por e fluid chemistry can beused, if necessar y. to estimat e the magnitude of the osmot ic potent ial.

The dew-point temperature , To, is the temperature at which the ambient atrno ­phere must be chilled in order for water to condense (i.e. , for Po = pl. An approxi­

mate expression for the dew-point temperat ure is:

Cha pter 28 by E ans and Ra mus en in this boo k). Because the hydr aulic cond uctiv­ity ten sor is a stro ngly nonl inear fun ct ion of the matric suction, it i impor tant toknow th e amb ient mat ric suction so that the ap propriate value o f hydraul ic co nduc­riviry can be determ ined for the sire.

An impo rtant tool for estimating the fluid potent ial , and hence the flow gradient ,is the p ychrornete r. This device measures the potentia l of the water vapor present inthe subsurface atm osphere. If the poteruial o f the water vapor is equal to thepotent ial of the pore fluid . then the psychrometer provides a means for ident ifyingthe por e fluid potenti al. An additional use o f the thermocoupl e p ychrometer is thedeterm inat ion o f the moisture characteristic curve for fluid potent ials greater thanthose read ily a u ained using porous plates, a pproximately 5 bars. The fluid pote ntia lof [ield-collected samples is estimated by allowing samples to equilibra te at specifiedwate r contents, thu s extending the characteristic curve to conditions much drier thannormally obtained from pressure extraction vessels.

(Sa)

(5b)

THEORY OF PSYCHROMETRIC OPERATION

Psychrometers measure the vapor-phase water activity in the subsurface atmo­spher e surrounding lhe sensor. The water activity is related to the relative humidityand the vapor pressure using:

water vapo r acti vity. dimen sionless;relative humidity, percent ;ambi ent water vapor pressure . kPa; andmaximum (saturated) water vapor pressure at ambient tempera­ture. kPa .

where ahp

Po

a h / 106 = p / Po (2)

Tab le 21. 1 presents the relationship between relative humidit y. dew-poin t depre ssiontemperatures, and rnatric suct ions for an ambient temperature of 20·C (273 K).

Psychrometers are a generic class of instruments used to measure Ihe relativehumidity that employ a dry bulb thermometer to measure the ambie nt tempe ra ture.and a second. wet bulb thermom eter to measure the lower temperature associa tedwith an evaporating fluid . For most subsurface applications , the amb ient wate rvapor pressure will be near the saturat ion vapor pressure, i.e ., the relative humiditywill generally be greater than 99'70 and the dew-point dep ression will be less than0.2· C . Most liquid-filled thermometers lack sufficient precision to provide measure- :ment s within ± O.I ·C, 'much less the precision ±O.OOI ·C required for rnatricsuction measur ements near I bar . Thermocouple psychrometers are used instead tomeasure the small temperature depres sions associated with relati ve humidities in thesubsurface.

The sat urated vapor pressure , Po' is a function of the ambient temperature, T (K).and can be approximated using:

For water vapor in equilibrium with the liquid phase. the water activity is related tothe flu id pressure by:

T he fluid potential calculated using Equation 4 is generally used to estimate themat ric suction of the liquid water present in pores. The fluid potential in the pores

rlble 21 .1 . Relillon.hlp Between Rel.tlvi Humldliy, Dew-Point Depre••lon, IndMltrlc Sucllon

where

and

Po = exp(19.017 - 5327/T)

(p R T / M) log.(a)

i' fluid potential, Jom' );p densi ty of water, 998.21 kg'm -J at 20·C;R ideal gas constant , 8.314510 J'K- t'mol- I ;

T am bient temperature . K; andM molecula r weight of water. 18.05128 g-mor" .

(3)

(4)

Relillve Humidity(~)

10099.9599.999 .5999895908050302010

1

Dew-Point Deprellion(OC )

0.00000.0080.0160.0810.160.330.821.73.6

1118243359

Mltrlc Sucllon(blrl)

o0 .71.36.8

13.527.269 .1

142301934

1623216931046207

lure . kPa .

The saturated vaoor nressure. n_. is a Funct ion of the amhienr ternnerarur e. T 110 .

measure the small temperature depressions associated with relative humidities in Ihesubsurface.

332 HANDBOOK OF VADOSE ZONE CHARACT ERIZATION & MONI TORING ENERGY·RELATED METHODS; PSYCH ROMETER S 333

Dew-Point Psychrometry

The eff ects of temperature a nd press ure ar e incorporated us ing the co nventio na lpsychrometer equat ion (Campbell, 1979);

Th e psychrometer consta nt , g, va ries with atmospheric pressu re and is relati velyinsensitive to temperature cha nges. The slope o f the sa tu ration vapor pressu re cu rve ,s, and the satu rat ed vapor pre ssure of wat er, Po. are significantly affected by tem ­per ature changes .

TYP ES OF THERMOCOUPLE PSYCHROMETERS

Th erm ocouple psychrometers em ploy a bimetalli c junction (e.g ., copper­co nsta nta n or chromel -constaman), known as a thermo cou ple . Figure 21.1 illus­trates the primary com pone nts of the thermocouple junction. The juncti on producesan elect ric curr ent Ihal is related to the junction temperature. This coupled heat­cu rrent phenomeno n is known as the Pelti er effect . The junction temperature ca n be .found by mea suring the curr ent gen erated by the thermocouple junction , or , con­ver sely, the tem perat ure of the junction can be changed by inducing a currentthrough the juncti on .

T hermoco uple psychrom eters use either the "wet-b ulb" or "dew-po int" method sto determine the water activity of the a tmosphere surrounding th e junction . Bothmethod s use a thermocouple 10 mea sure the ambient tempera tur e . T he wei-bulbpsychrometer mea sures the tempera ture of a "wet " junction (i.c . , a ju nction cov eredwith water ), while the de w-point psychrometer mea sures the temperature at whichsa tura ted vapo r pressure eq uals the ambient vapo r pressu re .

whereg

aT

a = I - aT (s + g) / Po

slope of the sa turation vapor pressure cur ve, Pa 'K- t;psychrom ete r co nsta nt, Pa'K-' ; andwet -bulb depression , K.

~6)

Wet·Bulb Psychrometry

T he wet-bulb temperature is determined either by placing a small drop o f water ona copper-constantan thermocouple junction (Ri chards method), o r by cooling rhethermocouple using the Peltier effect until water co ndenses on it (Spanner method) .Cool ing of the thermocouple is monitored as th e water evaporates . Fo r both meth­ods, the drop in temperatu re on the wet-bulb (i .e., the wet-bulb depression) varieswith the rate o f evaporat ion . T he evaporation rate, in turn , increases as the wate ractivity surro und ing the thermo cou ple decreases .

The equilibrium temperature attained by the wet bu lb depen ds on facto rs tha tinfluence heat flow toward th e junction and vapo r f low away from the j unction ,Heat fluxes to the wet junction result from co nduct io n thro ugh the co nnecti ngwire s, from radiation to the bu lb from the walls of the sample chamber, and fromconductive and convective heat gain from the surrounding air. T hese heat tluxesareco ntro lled by the dimensions of the sample chamber and the rmocouple co mpo nents.Cooling at the wet junction is proportional to the product of the latent heat o fvaporization and the evaporation ra re, a fun ction of the water acti vity within thechamber and the diffusivity of water in air. These fact ors generally increase withtemperature. The diffusivity o f water ill air a lso decreases with atmospheric pres­sure.

SampleChamber

Figure 21.1. Sc he matic diagram of Peltier junction and psychrome ter cnarnber,

The equ rnbrtum temperature auatned by the wet bu lb depends on racto rs ma tinfluence heat flow toward th e junction and vapo r f low away from the j unction .H ea t fluxes to th e weI iu ncnon result from co n d uc t io n rhr o ueh the co nnect ina

An improved psychrometer technique uses a feedback loop to co ntrol the Peltiercooling rat e, th ereby rnainta ining the temperatu re of the measuring junction at thedew point (i.e., the temperature at which the ambient vap or p ress ure eq uals thesaturated va por pressure, p = Po). When held at the dew-point temperature, a wetthe rmocouple junctio n neither loses water through evapora tio n nor gai ns waterthrough co ndensation because hea t flow to the wet junction fro m its surroundi ngs isexac tly o ffset by adj ustments in the cooling c urrent . The measurement is indepen­dent of the rate of heat flow to the wet junc tion an d , th us, no water vapor d iff usio noccurs . This meth od eli minates mos t of the temperature and pressure dep ende nceassociated with wet-bulb measurements. The lowest matric suction that ca n berelia bly measured in the laborato ry using this device is a pp roxi mat ely 1 bar un deridea l cond ition s, with a lo wer limi t of approximat ely 3 to 5 bars for field applica ­tions.

Chilled-Mirror Psychrometry

T he chilted-mirro r psychrometer improves the accuracy of the water activity mea­su rement by employing a mi rror that is chilled to near th e dew-poin t temperat ur e.Slight changes in the tempe rat ure of the mirror surface results ill conde nsation onthe surface of th e mirr o r whe n the mirror surface drops to the dew-poin t tempe ra­lure. The induced conde nsation causes a change in the light reflect ion propert ies ofthe surface. Monito ring of the change in light reflection from the mirror can then beused to determine the dew -point temperature in the laboratory from 1.5 to over 2500bars . .

USGS Four·Wlre Current Psychrometer

While psychrometers a re no rmally installed near the ground surface a t depths ofless tha n a few meters, psychrometers have been installed 10 a depth of over 350 m indeep unsaturated volcanic roc ks (Monrazer et aI. , 1988). A ne w me thod suit able fo rmo nitoring in deep unsatu rat ed media (u p to 60001) has bee n developed by Merri llInstr um ents and J.P. Rou sseau of the U.S . Geological Sur vey. Denver. The deviceuses a 4-wi re ther mocouple .psychrometer in current mod e rather than in the voltage

. .......u., I U ... U4 .. ......... H ' , .,'" , "" "' ..... , "" V ' ] "";7H~e; " 14 ,]0 ..... "' .. ........ I ~ A V V . V ....... . UIloI~ I' • 1,,10. uuu ....

ideal condit ion s, with a lo wer limi t o f app roximat ely 3 to 5 ba rs for field applica ­tions .

334 HANDBOOK OF VADOSE ZONE CHARACTERIZATION & MONITORING ENERGY·RELATED METHODS: PSYCHROMETERS 335

PSYCHROMETER CALIBRATION

mode to co mpensate for lo ng line lengt hs , These psychrometer s are able 10 meas uremanic suctions bet ween 0 .5 to 80 bars .

where O. is the solute os molali ty, obtained from sta nda rd handbooks of chemistryand physics . Because the osmotic potential. 4>. is a function of temperature , calibra­tion values should be reca lculated if the reference so lution varie s from 20°C.

A significant pro blem associated with th e use of th is technique lies in the inabili ty

Regular cali bration of thermocou ple psychrometers is necessa ry due to the possi­bil ity of corrosion of the metals used to create the thermoco uple junction . Anotherrea son fo r regular ca libration is the possib le accumulat ion of so luble salts on thejunct ion which affects the measu red wet -bulb and dew -point temperatures .

Psych rometers ca n be ca librated usin g a sa lt so lutio n of kn own wat er activ ity. Thesail so lut io ns a re used 10 create an osm oti c potential which main tain s a pre scribedwate r va po r activi ty in the atmosphere above the sa il so lution . One po ssib le designfor the ca libration chamber uses a closed , thermally-insu lated chamber partiallyfilled with a reference solutio n . The psychrometers 10 be ca libra ted are pla ced in theair space above the so lutio n (Figure 21.2) , assuring that the sa lt solut ion does notco me into direct con tact with the psychrometer o r psychrometer ca ble. Sufficienttime should be allowed for the contents of the cha m ber to co me into thermalequi lib rium, e.g., o vernigh t fo r a IOO-m L solutio n placed in a 500-m L cha mber(Rhodes, 1993).

Two methods using osmotic potentials are used for estab lishing the known wateracti vities . The fir st mel hod employs sail solut io ns (e .g .• NaCl or KCI) o f variableconcentrations to impose a ran ge of calibration po int s. Table 21.2 ·presents a list ofNaCI an d KC I salt so lut io ns and their associated relat ive humidi ties and mat ricsuctions . The osmotic po tential associated wit h various sol ution s can be calcula tedusing Rao ull's la w an d Kelvin's law (Rasmussen and Evans, 1986):

KCt

o4 ,589

13,217.521.926.330.534,939 ,2437

o4.59.0

13.517.922,42703 1,636 240,945.6

NaCIKCt

10099.799.399.098 ,798 ,498 ,197,B97 ,497,196.8

RelatlYe Humidity Matrlc Sucllon('lit) (ba,.)

NaCI

10099.799.399.098.798 ,498 ,097 .797,497 ,096 .7

Salt Solutions Uaeful lor P.ychrometrlc Callbrallon (at 20·C)

0,00,10,20,30.40.50.6070 80,91,0

to maint ain a co nsta nt sa il so lution co ncentration. Eva porat ion of the salt solut io nmay cause an unacceptable increase in the sa lt concen tra tion , a nd, hence , osmoticpotent ial. wh ile condensatio n ma y cause a decrease in the salt solution concentra­tion .

Saturated salt so lut io ns arc used to avoid the problems associ ated with changingsalt solution con centrations (Rhode s. 1993). In this met hod . a surplus of salt isadded to a volume of water so thaI if evaporation or co ndensation were to occ ur,the n salt would preci pitate o r disso lve to ma intain a consta nt co ncentration . Tabl e2 1.3 present s a list of se lected salts, with the relativ e humidities a nd water potentia lsgiven by their sa tura ted solutio ns a t 20°C.

For both methods. the psych rometer(s) are placed in at least IWO , an d p refe rablyfou r, different sa il so lutions that represent the ran ge of matric suc tions at the site o finterest. After allowing fo r the temperatu re a nd vapor pre ssure within the chamberto equilibrate, readings of psyc hrometer o utp ut a re ob tai ned , and the psych ro met ersare moved to a no the r chamber which conta ins a differen t f lu id pot ential. Thepsychrometer reading an d chamber temperature should be recorded for each so lu ­tion once the reading becomes steady. Once the calibra tio n data ha ve been collected,the observed read ings should be compa red to the theoretica l va lues specified by th emanufacturer. If the psychrometer departs from th e theoretical value beyond the

Tab le 21.2 .

SaltConcentration

(molality)

(7a)

(7b)If> = RT / V log.(a)

55.556a =

55.556 + 0 ,

and

98 2795 6992 11390 142M ln84 23679 31575 389

Table 21 .3 . Saturated Salt Solution. U.eful for Psychrometer Calibration (at 20·C)

Relative Humidity Matrlc Suction('lit) (bar.)

Pb(NOJl/Na/S03'7H/OK1HPO.ZnSO..7H/OK/CrO.KBrNH.CINaCI03

Saturated Sa lt

CalballonConlalne'

Figure 21 .2 . Chamber for psychrometer calibration us ing sa il so lutions .

Riven by their sa tura ted solutions a t 20°C.

336 HANDBOOK OF VADOSE ZONE CHARACTER IZATION & MONITORING ENERGY -RELATED METHODS : PSYCHROMETERS 337

tolerance specified by the manufacturer; then either the psychr ometer sho uld bedi scarded or a new calibration cu rve should be produced from the read ings a nd usedto convert measurement s to water pot ential.

Because thermocouple psychrometers are usually calibrated with salt solutions ofknown water potential. the acc ura cy of a sample measurement depends upon rep ro ­du cing conditions similar to those under which the calibration was performed .Because ca libratio n condit ion s (e .g .• identical barometric p ressur e) ca n rarely beduplicated exactly, Equation 6 can be employed to correct fo r temperature andpres su re dependencies if the wet-bulb tech nique is employed.

FIELD INSTALLATION OF PSYC,",ROMETERS

II is extremely important that on ly the smallest pos sible installat ion hole su r­ro unding the psychrometer be used . A small installatio n hole wit h mini mal bac kfill ­ing should be used to minimize disturbance to the geologic medium surrounding theprobe . The hole sho uld be backfilled with a silica flour or nati ve geologi c med ia tominimize vo id spaces. Clays of low permeability should not be used as backfillma terial fo r two reasons. First, clays require lo ng ' t imes to equil ibrate wit h 'theambient matric suc tion . Second, clays may be of suc h low per meability that theypre vent equilibration of water vapor between the psychrometer and the surroundingporous medium . For these reasons, sealing materials such as grouts and bentoniteclays should be avoi ded du rin g borehole completion to minimize abso rption orrelease of wate r in the region surrounding the monitori ng interval.

Figu re 21.3 A dem o nstrates one poss ible field ins ta llation strategy. A small di am­eter hole is placed vertically to the desi red de pt h. A dri lling method should be usedthat does not employ water or air (e.g ., a percussion tool or auger may be suitable).Psychrometers should be emplaced in duplicate pa irs for the purpose of eva luatingthe mag nitude o f instrument dri ft. A la rge weigh t can be attac hed to the rherrnocou­'p le leads near the psychrometer pairs to help lower the probes to the desired depth ,and to provide a lar ge thermal mass to minimize thermal transients . The hole is then

backfilled with a silica flour moistened to reproduce the anticipated rnatric suctionat the site of interest.

Another pos sible field installat ion strategy is to use inflatable pa ckers. In thisappl ication, a monitoring interval is flanked by two packers to isolate the zone ofinterest (Figure 21.3B). The packers are expanded to sea l the interval using com­pres sed gas lines connected to a com presso r or bouled gas on the su rface . Becausethe expa nding gas coo ls adiabatically when the packers are inflated , a large thermalperturbat io n will be obs erved and must be allowed to dis sipate prior to obt aining areliable reading. The advantage' of using inflatable packers lies in the ab ility toreposit io n the psychrometers so that grea ter spatial co verage can be obtained usin gthe same instrument s .

INTERPRETATION OF PSYCHROMETER MEASUREMENTS

As de scribed above. psychrometric measurements can be made using either: (a)the dew-point method , (b) the wet-bu lb meth od , or (c) a co mbina tion of both .Figure 21.4 pre sent s representative outputs for a sample at 25 bars and 32.S' C. Itcan be noted that the output reading in microvolts shows a more pr o no unceddeflection using the dew-point method than for the wet-bulb met hod . The deflectio nto a sta ble micro volt reading is the value used to calcula te the mat ric suction .

For some observations, however, no clear deflection is obser ved . Instead , a moregradual drift in reading is obser ved over tim e. This drift is especia lly apparent in thewet-bu lb method , and when the dry -bulb technique is used nea r the upper or lowe rlimit of its range. Under these conditions , utilizat io n of both modes provides redun­dant estimates of matric suction , along with an indi cation of measurement accuracy.In general , beuer accuracy is indicated if both techniques provide consistent matricsuction estimates .

Figure 21.5 presents results of laboratory psychrometric readings for lOS rockcore segments obtained from the Apache Leap Tuff Site in cent ra l Arizona (Ras ­mussen et aI., 1990). The cylindrical core segments (measuring I cm in length and I

A

Hole backflted- w!Ih moIslen«!

aillta !Iou,

B

PacIlor In(la lion Lllleand Psychtomeler Cable

~Upp"' Pack",

PsyclllOJ1'l<tI'" _ OIed Inlerval

L""",Padll"

to ' :I'

Figure 21.3. Field installat ion 01 thermocouple ps ychrometer using (A) bac kfill and (8)inflatable packe rs .

Figure 21 .4.

.,+--l-...,....-..,..J.....L.--~-,...J..........,

•

Thermocouple psychrometer response curves lor (1) dew-poim. (2)weI-bulb, and (3) comb ined modes Osmotic potential is 25 bars suct ionand temperature is 32.8'C.

that does not employ water or air (e.g ., a percussion too l or auger may be suitable).P<:v rh"nm,.tlPrC' C' hnu l A h.....""nl~ 4"1 i n 4"lunlj "'111' ~ rIo"'llo ;r ~ r ,h ., f' u~ lu t : '" ..

In general , Deller accuracy IS mcncarec II nom iecnmques provice consrstenr marnesuction estimates.

'.

Extractor Data •

HANDBOOK OF VADOSE ZONE CHARACTERIZATION & MONITORING 339

--II

... ... -DIS TANCE FflOO,I E" ' RAIOCt ,....,

ENERGY·RELATED METHO DS: PSYCHROMETERS

' 00o ·o

,.

..

figure 21.6. Observed matric suctions in Kartchner Caverns, southern Arizona. Circlesindicate mean observed matric suclion as a function or distance from theentrance. Vertical bars indicate one standard deviation errors about mean.

Psychrometer Data

, .

.II\ . .····il· . I ..:. I .'.i•••:;1'. '.'1:-: ... ;.,...... .. ..:.: ..".

.I. • . •..., •.. :'.. -. "'.I.I• • •• ~··I. ,... .." -. .. ..

.. """.. ..• • h : • • :II : .. . -

;" "1·,1.11, ," I·- .. ..:1.... ...... " •..... . .. ..e. .. I I' I .··. .. I

.. .. ::!: ! ..":-::: ::::::.:: :..:.:..e•••~ -....-.. .

1000

'00 '

Iz0f= 10U::><J)

Uit\(

"

338

PSYCHROMETRY ERRORS

100908030 40 50 6Q 70

RELATIVE SATURATION (percenl)2010o

o1-l----,r----.---~--.----r--__.--__.__--+_-.......- .....,

f igure 21.5. laboratory moisture characteristic (drying) curves tor 105 core segmentsIrom the Apache leap Tull Site, cent ral Arizona. Extraction andpsychrometer curves show all observations (O.X) and mean (lines).

Accurate determinations o f matric suc tions in the subsurface require an und er­sta nding of the potential so urces of transient and systematic erro rs. Errors from thefo llo wing ca uses sho uld be co nsidered :

cm in d iameter) were first satura ted in a sterile, deion ized so lut io n contai ning 0.001M CaSO•. The segments were then allowed to dry on a precision balance 10 reach atarget relat ive sat urati o n (between 70 and 10070 satura tion in 10070 increments). Thesegments were then transferred to a psychrom eter chamber, allowed 10 eq uilib rate ,and the mat ric suctio n was determined . Also shown for co mpa rison a re mo isturechar ac te rist ic curves (dry ing cu rves) obt a ined for 105 cores using an extracto r vesseltechnique. A reaso nab ly good agreem ent ca n be ob served by extra po lating betweenthe two sets of data .

An inte rest ing field application of the rmocouple psychrom eters was conduc ted atKartchner Caverns in southern Ari zo na . Rela tive humidities very near saturatio nwith in the cave were monitored to de termine the magnitude a nd d irect ion o f wate rvapor mo vement resulting from the enlar gement of the cave ent ran ce. Figur e 21.6pre sen ts the interpreted rnatric suctions within the ca ve atmos phere . It is apparentfrom the figu re that the IwO station s clos est to the entrance ar e signifi ca ntly dri erthan more d istant statio ns . If it can be assumed that vapo r flu x within the morerem ote sectio ns of the cave is small and that the ca ve atmosphere is in equilibriumwith the sub sur face med ia sur ro und ing the cave, then the mat ric suctio n of the rocksurrounding the cave can be est imated to be approximately S bars . No appa renttrend ca n be discerned in the cave mat ric potential beyond the first two stati ons.

• temperature fluctuat ion s with tim e,• temperalUre gradient s ,• vapor pressure gradient s. an d• wet junct ion characteristics .

Labo rat o ry psychrometers ar e designed to measu re the wate r pot eru ial o f soi lsamples by sea ling th e th ermocou ple j ust ab ove or within th e sample . O ne designallows th e so il sa mples 10 be packed into several sta inless steel cups . which arerot ated to meet the thermocouple. These a re filled either with a Richards-typejunction mod ified with the addit ion of a ceramic bead or with a Pel tler -rype wetjunction . Measurements made with thi s eq uipment may be sign ifica ntly affected bytemperature flu ctuation with time, part icularl y when the sample is co nsiderablysma ller than th e di men sions o f the cha mber. The accuracy is greatest when thesa mp le co mp letely surro unds th e jun ction. The effect of temperature flu ctuationscan be minimized by enclosing the device in a constant temperature bath or p lac ingthe device in co ntact with a large heat sink to reduce the rat e of temperaturechanges.

Field psychr ometers utili ze thermocouples protected by cups or bulbs of vario ussizes, usually ceramic or fine mesh , to provide a protected spa ce within the in situen vironment . Field measurement s are o ften subject to the rmal gradients. Potentialerror s due to nonisothermal med ia can be elimi nated if the psychrometer is orientedso th at its axis of symmetry is perpendicul ar to the temperature gradient, or if a

• temperatu re fluctuation s with tim e ,

'" HA NDBOOK OF VADOSE ZONE CHARACTERIZATION & MON ITORING ENEAGY·AELAT~O METHODS: PSVCHROMETERS ""therm al mass (c.g., a ..'cight) is platcd Rear the device to minimize ther mal tra n­sients.

Th erm ocouple psychrOlllctcr devices 31 t eur renrly available th at measure bothwet-bulb and de.... -pcimremcerannes in the Ileld . Field units should be emplaced induplicate 10 allow multiple de vices 10 be used for assur ing accur acy. Also , theuevc es should be It lutally removed and tecaflbrared 10 deter mine "'hettler corro­sion or cc nta minaticn of the bimetallic juncnon ma y ha ve occurred . Psychromet crsshould be emplaced in suc h a manner as 10 assure minimal void space 5uHouodi ngthe device, and lncU ill malnial should be minimized by USinl an installa tion h<xcthat closely "1llChes lhe dimensions of the- psychrome-le-r. Also . ps~ch r orne-Ie-J$ pro­recred using ce-ramic bulbs may re-q uire' prolon ged e-quilibr.u ion limes compared 10fine- lIIe-sh psy~-hrome-h~rs.

While vapor prC'Ssule-vadienu may be caused by C'Xlrancous sources or sinks for....ale-r "apor, rhis prob lem is rare ...·ilh Pd riC'r ps)'chronM:te-u and can be reduced ifsamples a re- nO'i signifICant ly smalltt lhan their SImple- chambers. 8oI:h labof aloryInd field psychrOR1Clct nM:ilSure-me-nIS may be' arrC'Cl C'd by ""-C'{ junction characln is­tics . Idn l psychromclC'r cond ilions assume- Iree C"'aporation h om a sphe-rk al ....C'{junction $l.lt face. Yn actual size and sha pe'of th e- ",C'{ a re-a depends upon us ""e-lli necharacle-fislks, amone OI~r factors, and can be' com amin alC'd ...-ilh Uk'. SuchchangC'S ca n be' e-l imina lC'd by tinning or rttaliblaling .

[)Cviations from idnl psychro tl'lC'tC'f Ope'rati ng co nditions may eaese signiflcanlloss o f psychrOR1Cltr k' nsili,·ity, and subs(anlial e-rrors ca n be'introduced if lC'fnpe'ra ·lure- diHtrmcn bC'iwC'C'n the- reference junctio n and the liquid phase arc not ro n­Holled 10 wilh in :t o.OO I-C (Ra....lins and Campbdl, 1986). Thus. lhere- is a crilicllneed 10 l«uU.lely measure- temperatures; a small temperature diHC'fence errorresuh s in I la rge en or in the calculat C'd Iluid pot emial.

Cautio n sho uld be exercised when inlcr prC'fing psythromet rk re-wings co ll«lednear I sile rontainin& vo lalile liquids. The: psythro mC'{ rk equ ation I$$UmC'S lhalwaler is Ihe first liquid 10 co ndense from the soil atmo sphere . For siles whC'fe otherv<Mlti le liqu ids arc prek'nt in iuHkiC'nl qua nlil y 10 . t hieve partial preliSules neartheir vapo r· phaSe' sa luralion pressure, lhe first liquid 10 ccedense may be' a liquidot her lhan wate r. For exam ple. if a benzene-saturated sue is suspcdC'd. and the WC'{ ·bu lb depre ssion is smaliC'r or Iht dew.potrutemperature is higher for the benzene CISthan for water, then the benzene may condense pr ior 10 water . If lhis po$sibility isco nfi rmed, then the- vapor pressure of benzene can be dete-rmined , bUI no informa­lion about water UHlllrion is possible .

A furlher tomplk arion resulu when 1...·0 vapo rs form an azeoucpe upon cc nden­sauo n. An azeotrope is a mixture of liquids whose components do nOl Ulldergodilutio n or conte-nlri lion as the mixture is distilled . For the previous example usingbenzene , the condensate may consisl of a mixtu re of benzene and water. with th e­benzene cOOl prising l pp roximalely 900;'. o f the mtsture by we-igh!. As lhe mixturee- vapo rates d uring a te'S1 readin l , lhe- co ncemralions of both components re- mainco nsla nl, alleri nll he psychometr ic relalionship which assume-s lhe presence or pur e­water. This binar y az:eol ropic system is furthe-r comp licaled if more lhan IWOvolalilecOmponenlS are prUent.

It should be emp hl.li izC'd lha l lhe cave-a ls de-scribed he-re do nol preclude Ihe- usc ofpsychro meters al contaminl lC'd sitC'S. While- the methods may not provide quantita ­tive- evide-nce- reaardinl fluid potentials, temporal chanlC'S in re-ad ings ca n providecrucial informa lion relardinl d irttlion and magni tude of milration or waler andcontaminann .

ADDITIONAL INFO RMATION

An excenem, although sc mewhar da ted , summary of thermocouple psychrometr ythcory and app lications can be found in Brown and Van Heveren (1972). Ra...·linsand Campbell (1986) also pl ovide an excellent OvC'rvie...· o f thermoco uple psyt hro­meny,

REFERENCES

B. .....'n, R.W.• and B.P. Van Ha'·~fn . · p~)-""'O<nflr7 ,n Wa,,, Rd alions Rf~arc" ,- P'o­c«d i nl~ 01 lhe Symposi\lm on Thnmocoupk Psych,onwlffS . Utah Sl:alf Uni' n .ily.Ma,"" 17- 19. 1971 , l Oian. UT. 1972.

Campbell. 0 .5., - lmpo-oved Tb...- rnocouple Plycl"ometns for Measu' nM nl of Soil WalerPOIn llial in a Tfn,pc,aIUrfGl"adwnl.- J. oj PlIYJies. E.: Sri. " UI",~"IJ , 12; I- j 119791.

Monlu n , P., E_P. Wf'Cks. F. Thami,. O. t1ammerlm'isln . S.N. Ya,d. and P.8 . Hofr" tlln,MMoniiOfiJ\& lhe \'ado5or Zoac: in F, ar;t urfd Tuff,- G,04''Id If"'t' MO#!. Rt ... . ' ;72-'8(1911).

Rasmussm. T.e., and 0 .0 . E.-an~. Unuturllfd Row and Transport Tluou.... Frl(lurfdRod - Relatfd 10Hiah·u-,'d Waslf RtpOSiIOf>CS , Phase II, NUREG/ CR-46j " U.S. Nu·clear RCl ulatory Cornrniuion, WasbiJ\&ton, DC. 19".

RiUlIIlIUnI, T.e. , 0 .0. EVlIos. P.J. Sh«ls. and J.H. 8WlfOfd, Unsatu,atfd Franurfd RockCbranerizallon Mtthods and I)"la Seu at tbeApacht Leap Turr Site. NUREG/C R-H96.1)9 PII. , U.S. Nuclea. Reaulalory Commiuion , Wa~hi....OJl, DC, 1990.

Ra...lins. S.L., and 0 .5. Campbell. MWlle. POIcnt ial: Thnrnocoupk PsychrO<nfl ry,- in A.Klute. Ed.. MerlrodJ oj Soil AM/, sis , Pa,t I, 2nd Edilion, 19&6, lIP. j97-61' .

Rhodes, S.C.. -h1oisturf CharlCle. islicCU"'·C'$ fOf Apache Leap Tuff; Temperatu.e Effectsand HystCIC'$is. Superior, Arizonat Unpublhl>cd MS tl>csis . Drpa,tmtnl of Hyd.oloar andWale. Rewu.ccs, The Univerlit y of A"lona, TUQOfl , AZ, 199) .