HORTSCIENCE 42(6):1372–1379. 2007.

Low-cost Shielding to MinimizeRadiation Errors of TemperatureSensors in the FieldJulie M. Tarara1

USDA-ARS, Horticultural Crops Research Unit, 24106 N. Bunn Road,Prosser, WA 99350

Gwen-Alyn HoheiselWashington State University Extension, 1121 Dudley Avenue, Prosser, WA99350

Additional index words. Gill shield, solar, global irradiance, reflectance, albedo, aspiration,thermocouple, relative humidity

Abstract. The importance of shielding temperature sensors from solar radiation isunderstood, but there is a lack of prescriptive advice for plant scientists to buildinexpensive and effective shields for replicated field experiments. Using the generalphysical principles that govern radiation shielding, a number of low-cost, passivelyventilated radiation shields built in-house was assessed for the measurement of airtemperature against the same type of sensor in a meteorological ‘‘standard’’ Gillradiation shield. The base shield material had high albedo (’’0.9) and low emissivity(0.03). Aspirated shields were included for simultaneous measurements of temperatureand relative humidity. Differences in air temperature (DT) between low-cost shields andthe standard Gill were greatest for shields with open bottoms (up to +7.4 8C) and for thosewith poorly perforated sidewalls. Open-bottomed shields were prone to heating fromreflected radiation. Tube-shaped shields appeared to require more than 30% sidewallperforation for convection by ambient wind (up to 4 m�s–1) to offset the midday radiationload of the shield. The smallest daytime DT were between aspirated shields and thestandard Gill, averaging less than ±0.5 8C. Among passively ventilated shields, thesmallest daytime DT consistently were produced by a shield that emulated the stackedplate design of the standard Gill for a total of U.S. $4.00 in materials and 45 minconstruction time. Eighty-nine percent of all daytime DT for the ‘‘homemade Gill’’ shieldwas 1.5 8C or less. The combination of low ambient wind speed (less than 1 m�s–1) and highglobal irradiance (greater than 600 W�m–2) produced the largest DT for all passivelyventilated shields, the magnitude of which varied with shield design; stacked plateconfigurations were more effective shields than were tube-based configurations. Night-time DT were inconsequential for all shields. Cost-effective radiation shielding can beachieved by selecting shield materials and a configuration that minimize daytimeradiation loading on the shield while maximizing the potential for convective transferof that radiation load away from the shield and the sensor it houses.

For automated weather stations, recom-mendations and minimum standards exist forproper shielding of temperature sensors tominimize errors that arise from solar radia-tion striking the sensor (ASAE, 2004;Hubbard et al., 2001; WMO, 2006). Althoughcost is a consideration for weather stations, itdoes not supersede the importance of sensorselection and an attempt at standardization

among installations. By contrast, many hor-ticultural experiments require spatiallyintense sampling of variables like tempera-ture, particularly in replicated field studieswhere there should be enough sensors toquantify the microclimate, for example as afunction of experimental treatment, acrossreplicates, or within plant canopies. Repli-cated arrays of temperature sensors demandlow-cost, effective radiation shielding if theyare to be useful in the many field studies thatare conducted with limited resources.

Proper shielding is critical for measure-ments recorded near the ground where verti-cal gradients in temperature can be large.Although radiation shielding is most impor-tant for measuring air temperature in theopen, it also should be considered for anytemperature sensor that is not completelyshaded during the measurement period. Theeffectiveness of a radiation shield dependsprimarily on the optical properties of thematerials used (Fuchs and Tanner, 1965; Gill,

1979; Lin et al., 2001a) and its ventilation(Gill, 1979, 1983; Lin et al., 2001a, 2001b),because two forms of heat transfer, radiationand convection, generally govern shielddesign. Because solar radiation causes thelargest errors in air temperature measure-ments (Fuchs and Tanner, 1965; Gill, 1979;Hubbard et al., 2001), materials for radiationshields tend to be chosen first for opticalproperties that minimize daytime errors: highsolar reflectance and low emissivity (thermalemittance). Where nighttime errors specifi-cally are of concern (e.g., studies of nocturnalinsect behavior), shield material with highemissivity is preferred (Lin et al., 2001a).

The flat plate and the tube are the twobasic shapes that describe most radiationshields. One shield that has been adoptedwidely in weather station networks is astacked plate design known as the ‘‘Gill’’shield, after its developer (Gill, 1979, 1983).Shield ventilation is achieved by one of twofundamental approaches: passive, or naturalventilation, and forced ventilation. Passiveventilation relies on ambient wind to transferheat away from the shield and the sensor.Passively ventilated shields often suffice formeasurements of air temperature alone, par-ticularly for sensors of small thermal mass(e.g., thermocouples, thermistors). Forcedventilation, more commonly referred to asaspiration, uses a powered fan to draw airacross the sensor at a rate high enough tooffset radiation-induced heating within theshield. Aspirated shields often are requiredfor concurrent measurements of temperatureand relative humidity [(RH); e.g., Fritschenand Gay, 1979; WMO, 2006], for measuringtemperature near the ground, or for measure-ments in confined spaces where the sensormay be decoupled from the air that is to bemeasured. Although it generally is acceptedthat the greatest measurement accuracy isachieved from aspirated sensors (e.g., WMO,2006), the major drawback of aspiratedshields is that they require a power source.

Using fundamental principles of shielddesign from the environmental biophysicsand meteorological literature (e.g., Fritschenand Gay, 1979; Fuchs and Tanner, 1965; Gill,1979, 1983; Lin et al., 2001a), the objectiveof this research was to demonstrate theseprinciples for the horticultural scientist usingseveral designs of inexpensive and easilyconstructed radiation shields for two typesof temperature sensors that are commonlydeployed in field experiments: 1) the thermo-couple, a cheap, easy-to-use sensor of smallthermal mass; and 2) a combined temperature-RH sensor with moderate thermal mass. Pas-sively ventilated designs were intendedto cost less than U.S. $5.00 in materials andrequire less than 1 h to construct. Shield‘‘success’’ was defined as shielded sensorsconsistently producing daytime air tempera-ture values that were within �1 �C ofthose from identical sensors in ‘‘standard’’passively ventilated Gill shields. Shielded–aspirated sensors were compared with thosein standard Gill shields to provide informa-tion to scientists who are able to deploy

Received for publication 6 Dec. 2007. Acceptedfor publication 18 Mar. 2007.We thank John Ferguson and Patrick Humberstadfor technical support and Carolyn Scagel for help-ful editorial comments.Mention of a trademark, proprietary product, orvendor does not constitute a guarantee or warrantyof the product by the U.S. Department of Agricul-ture and does not imply its approval to theexclusion of other products or vendors that alsomay be suitable.1To whom reprint requests should be addressed;e-mail jtarara@wsu.edu

1372 HORTSCIENCE VOL. 42(6) OCTOBER 2007

powered systems and who choose a forcedventilation approach, particularly for simul-taneous measurements of temperature and RH.

Materials and Methods

The experiment was conducted nearProsser, WA (lat. 46�18#N, long. 119�45#W). All temperature and temperature-RH sensors were mounted 2 m above a bare,dry soil surface, the average midday albedoof which was 0.21, within the range that onewould expect for a soil of low organic matter(Campbell and Norman, 1998; Monteith andUnsworth, 1990). Sensors were installed in agrid with a separation distance of 2 m. Eachtype of radiation shield was replicated threetimes in a completely randomized design.The nearest building structure was �30 m tothe north of the sensor arrays. There were nostructures or trees within tens of meters to thesouth and west of the experiment. The nearestirrigated surface (lawn) was �40 m west ofthe plot. Prevailing winds were south tosouthwest.

Air temperature measured independentlyof RH was by type T thermocouple (copper-constantan; 0.5-mm diameter; part no. PR-T-24-SLE; Omega Engineering, Stamford,CT). All thermocouple junctions (2-mmlong) were manufactured in-house. Severalcentimeters (�12 to 18) of thermocouple leadwire were coiled inside the shields to mini-mize errors associated with conductionof heat along the lead to the thermocouplejunction. Combined temperature-RH sensorswere manufactured by Vaisala (modelHMP45C; Campbell Scientific, Logan, UT).A passively ventilated Gill shield (model41002; R.M. Young, Traverse City, MI)was used as the standard of comparison forboth the thermocouples and the temperature-RH sensors. Wind speed and direction weremeasured by a three-cup anemometer andwind vane (Wind Sentinel; Met One, GrantsPass, OR). Global irradiance (Rs) was mea-sured by a pyranometer (model 8-48; EppleyLaboratories, Newport, RI). Sensor signalswere scanned at 5-s intervals and averagedevery 15 min by datalogger (CR-10X; Camp-bell Scientific). A solid-state thermocouplemultiplexer (AM-25T; Campbell Scientific)was used to switch among thermocouplesignals. The data acquisition system washoused in an insulated ice chest that wasplaced in a nearby travel trailer to minimizethermal gradients across the measurementpanel. A control for signal error attributableto any thermal gradients across the panel wasprovided by several thermocouples wiredinto input channels across the measurementpanel with their junctions buried �0.6 mbelow the bare ground surface. Measure-ments from these thermocouples agreed towithin ±0.15 �C.

An 8-mm thick ‘‘foil bubble wrap’’ or‘‘foil bubble insulation’’ (polyethylene bubble-pack aluminum foil-faced insulation; Reflec-tix, Markleville, IN) that can be obtained atany hardware outlet was used to constructseveral designs of passively ventilated

radiation shield. The material’s surface emis-sivity (0.03) was determined by the manu-facturer according to ASTM Standard C1371(ASTM International, West Conshohocken,PA). Because the surface is highly specularrather than a diffuse reflector, spectrometerestimates (Fieldspec-Full Range; AnalyticalSpectral Devices, Boulder, CO) of its albedo(shortwave reflectance) were suspect, so weestimated the albedo of the foil surface in thefield using a pair of inverted pyranometers(Eppley model 8-48; 0.285 to 2.8 mm) thathad been recalibrated by the manufacturerdiffering in output by 0.4% (�3 W�m–2)under irradiance greater than 700 W�m–2.Albedo was estimated as the ratio betweenreflected and incident irradiance for measure-ments recorded between 1100 and 1300 HR

(local standard time) under overcast skies tominimize errors resulting from specularreflectance. The approximate albedo of thereflective surface was 0.88. If painted flatwhite, the bubble wrap surface had an aver-age albedo of 0.72.

In-house-constructed radiation shieldsrepresented simple to complex variations ofthe basic shield shapes of plates and tubes(Figs. 1–3; Table 1). All passively ventilatedshields were constructed from the same basematerial (i.e., reflective foil insulation) so thatdesigns could be used to demonstrate advan-tages and disadvantages of various physicalconfigurations. Among passively ventilatedshields, the ‘‘cone’’ represents the simplestplate configuration; the ‘‘double cone’’ is thesimplest possible stacked plate (Fig. 1). The‘‘hanging tube’’ (Fig. 2) represents a simplevertically oriented tube shield with a 1:3diameter:height ratio. Its solid sidewalls couldbe expected to demonstrate a ‘‘chimney’’

effect for rising warm air. The ‘‘rocket’’ andthe ‘‘pagoda’’ are hybrid designs of a verti-cally oriented tube with top or bottom plates.The pagoda relies on natural ventilation fromthe bottom of the shield and from perfora-tions that comprise �10% of the sidewallsurface. Because of the closed-bottomapproach to the rocket, perforation was nearthe top and bottom of the tube, representing�30% of sidewall surface area. The ‘‘hand-made Gill’’ included six plates spacedslightly farther apart than the plates on thecommercial Gill for ease of construction (Fig.1). The length of the handmade Gill was two-thirds that of the commercial shield becausethe in-house version did not need to accom-modate a sensor package as long as theVaisala temperature–RH probe. Since theexperiment was conducted, the 12-plate com-mercial Gill has been supplanted by themanufacturer with a 10-plate version (model41003; R.M. Young).

The approximate time required to con-struct a given shield was recorded only afterthe first shield of the finalized design wasassembled. The same individual constructedall shields. Estimates of materials cost foreach shield were all-inclusive except for thecost of the subminiature thermocouple con-nector, which is independent of shield design.The complete set of materials used forpassively ventilated shields was foil bubblewrap, hot-melt glue, rigid foam board, cableties, and foil tape.

Aspirated shields (Fig. 3) incorporatedthin-walled polyvinyl chloride (PVC)pipe and DC-powered brushless fans andwere designed to support a thermocoupleor a temperature-RH sensor with supplemen-tal thermocouple. The PVC cylinder was

Fig. 1. Schematic diagrams of plate-shaped, passively ventilated radiation shields. The commercial Gillshield housed both a thermocouple and a temperature–relative humidity sensor. All in-house builtshields housed a thermocouple, the location of which is denoted by the dashed line (lead wire) andblack dot (junction).

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MISCELLANEOUS

wrapped externally with the foil bubble wrapmaterial before deployment to the field. Thetemperature-RH sensor was mounted hori-zontally within the shield midway betweentop and bottom of the tube. Shields wereoriented with the air intake pointed north toavoid direct beam radiation striking thesensor. Three models of 12-V DC fans wereused to match air flow rates with shield sizeand sensor type (low or moderate thermalmass). The thermocouple (‘‘aspirated TC’’)was aspirated at a mean air speed of 1.8 m�s–1

(0.09-A fan, model JF0413SIM; JamecoElectronics, Belmont, CA). The low-speedtemperature-RH shield (‘‘aspirated T-RH’’)was aspirated at a mean air speed of 1.2 m�s–1

(0.09-A fan, SUNON model KD1204KBX-8;Jameco Electronics). The high-speed (‘‘super-aspirated T-RH’’) temperature-RH shield wasaspirated at a mean air speed of 4.4 m�s–1

(0.28-A fan, NMB model 3610KL-04W-B40;Jameco Electronics). Air speed at the entranceof the tube was measured by hot-wire ane-mometer (model TA-5; Airflow TechnicalProducts, Netcong, NJ).

Because of input channel limitations,three replicates of eight sensor–shield com-binations could be assessed simultaneously.Therefore, short-duration (2 to 4 d) daytimetests were conducted to narrow the range ofshields to eight for inclusion in an 11-d runthat covered all periods of the day beginningon day of year 235, 2005. Aspirated TC andthree passively ventilated shields with sidewalls (Fig. 2) were included in the short testsduring midday hours (periods 3 to 5). Datafor all other shields were acquired during the11-d run. Each day was divided into sevenperiods relevant to the diurnal cycle of solarradiation (Fig. 4). Days with rain or overcastskies were eliminated from the analysis.

A difference in air temperature (DT) isdefined as that between the given sensor type(thermocouple or temperature-RH sensor) inan experimental radiation shield and the airtemperature measured by the same sensortype in a commercial Gill shield. Positivevalues of DT indicate that air temperature inthe experimental shield was higher than that inthe commercial Gill shield; conversely, neg-ative values of DT indicate that air tempera-ture in the experimental shield was lower thanthat in the commercial Gill shield. Differencesbetween the in-house-built shields and thecommercial Gill, as well as differencesbetween the two sensors within the commer-cial Gill, were analyzed with a mixed model(Proc Mixed). A repeated statement was usedto account for the autocorrelation in time-series data and least squared means wereadjusted by Bonferroni (SAS Version 9.1;SAS Institute, Cary, NC). Significant effectsof solar radiation, wind speed, and theirinteraction were tested with a general linearmodel (Proc GLM). Mean values across threereplicates are reported throughout.

Results

Within the commercial Gill shield, thethermocouple and temperature-RH sensor

Fig. 3. Schematic diagrams of aspirated radiation shields. Aspirated thermocouple is the aspirated shieldfor a single thermocouple. Aspirated temperature–relative humidity (T-RH) is the low-velocity (1.2m�s–1) aspirated shield for a T-RH sensor; superaspirated T-RH is the high-velocity (4.4 m�s–1)aspirated shield for a T-RH sensor. The dashed outline depicts the position of the T-RH sensor alongthe central axis of the tube. Aspirated T-RH and superaspirated T-RH shields also included athermocouple, the location of which is indicated by a single dashed line (lead wire) and black dot(junction).

Fig. 2. Schematic diagrams of tube-shaped, passively ventilated radiation shields. The ‘‘pagoda’’ shieldhad �10% sidewall perforation and the ‘‘rocket’’ had �30% sidewall perforation. All in-house builtshields housed a thermocouple, the location of which is denoted by the dashed line (lead wire) andblack dot (junction).

1374 HORTSCIENCE VOL. 42(6) OCTOBER 2007

did not significantly differ from each otherand agreed on average (n = 2112, 15-minaverages) to within 0.04 �C, well within theaccuracy of a Type T thermocouple, with amaximum divergence of ±0.8 �C. Amongpassively ventilated shields, significant dif-ferences in air temperature between the in-house shields and the commercial Gill wereapparent only during the day (periods 2 to 5)with almost no significant differences at night(Figs. 5 and 6). For the simplest plate design,the cone, DT across the data set (n = 2112),ranged between –1.2 and +5.4 �C. Of allobserved DT for the cone, 63% were 1.5 �C orless, but only 37% of daytime DT (periods 2to 6) and 5% of midday DT (periods 3 to 5)fell below this value. The DT was affectedsignificantly by wind speed (P < 0.030), Rs

(P < 0.0001), and the interaction between thetwo driving forces (P < 0.0001), increasinglinearly with Rs (Fig. 7A). However, DTincreased with wind speed up to �1 m�s–1

and then declined (Fig. 7A). The simplest

stacked plate design, the double cone, had arange of DT across the data set (n = 2112)between –1.2 and +2.6 �C with 46% ofdaytime DT (periods 2 to 6) and 23% ofmidday DT (periods 3 to 5) 1.5 �C or less. TheDT of the double cone was significantlyaffected by wind speed (P < 0.0001), Rs

(P < 0.0001), and the interaction betweenthe two (P < 0.0001). The presence of ashielded bottom in the double cone wasevident in a logarithmic response of DT toRs, with an asymptote above�200 W�m–2, incontrast to DT in the open-bottomed cone,with its linear response to Rs across the entirerange of values during the study (0 to �850W�m–2). Because data were retained only fordays with predominantly clear skies, Rs wasless than 200 W�m–2 only early and late in theday at solar elevation angles 15� or less.

Air temperature in the handmade Gillshield consistently was higher than that inthe commercial Gill only between �800 and1400 HR (Figs. 5 and 6), a period that is

critical to many biological studies. Over 11 d,DT was 1.5 �C or less for 89% of daytime(periods 2 to 6) and 87% midday (periods 3to 5) measurements. Across the entire datasetfor the handmade Gill (n = 2112), DT wasbetween –0.7 and +2.2 �C; 93% of allobserved DT were 1.5 �C or less and two-thirds of all DT were 1.0 �C or less. The DT ofthe handmade Gill was affected significantlyby wind (P < 0.0001), Rs (P < 0.0001), andthe interaction between the two (P < 0.0001).Like with the double cone, DT of the home-made Gill had a logarithmic response to Rs

approaching an asymptote of �200 W�m–2.Again, this corresponded to early morningand evening, or solar elevation angles 15� orless. The largest DT occurred at wind speeds�1 m�s–1 regardless of Rs. The responsesurface of the homemade Gill shield (Fig.7B) closely resembled that of the cone (Fig.7A), but with smaller DT.

Passively ventilated shields with sidewalls (Fig. 2) produced the largest middayDT (periods 3 to 5) among all in-house-builtshields with average performance in thegeneral increasing order: hanging tube <pagoda < rocket. Across daytime data (n =82), air temperatures in all three tube-basedshields were higher than those in the com-mercial Gill, with DT ranges of +1.4 to+3.4 �C for the rocket, +2.4 to +7.2 �C forthe pagoda, and +2.0 to +7.4 �C for thehanging tube. The DT of the hanging tubewas affected significantly only by solar radi-ation (P < 0.001), whereas convective heattransfer apparently was ineffective (P < 0.395for wind speed). For example, at ambientwind speeds between 2.5 and 3 m�s–1, DT wasbetween +4 and +5 �C. By contrast, atambient wind speeds above 1.5 m�s–1, DTfor the rocket generally was below +2 �C.Nonetheless, at low ambient wind speed andhigh solar radiation, midday DT for the rocketconsistently was greater than 2.5 �C, indicat-ing that �30% sidewall ventilation in thiscombination tube-and-plate design was lesseffective than a stacked plate configuration.Under similar conditions, DT of the home-made Gill never exceeded +1.8 �C. The DTfor the rocket was affected significantly byRs (P < 0.001) and the interaction between

Table 1. Shape, geometric dimensions, approximate construction time, materials cost, and shield material of 10 radiation shields included in the experiment sortedby class of ventilation.

Ventilation class/type Base shapeDimensions

(ht. · diam.) (cm)Constructiontime (min)

Materialscost ($U.S.) Primary materials

PassiveCone Single plate 5 · 11.5 10 0.70 Reflective foilDouble cone Stacked plates 8 · 14 20 2.00 Reflective foil, foam spacersHandmade Gill Stacked plates 10.5 · 13 45 4.00 Reflective foil, foam spacersCommercial Gill Stacked plates 16 · 12 NA 180.00z White thermoplasticPagoda Tube (vertical), top-stacked plates 17.5 · 12 30 2.00 Reflective foil, foam spacersRocket Tube (vertical), top- and bottom-stacked plates 20.5 · 13 30 2.50 Reflective foilHanging tube Tube (vertical) 26.5 · 9 5 2.00 Reflective foil

AspiratedTCy Tube (horizontal) 11.2 · 4.5 45 12.00 PVC pipe, reflective foil, fanT–RHx Tube (horizontal) 32.5 · 8.3 120 21.00 PVC pipe, reflective foil, fanSuper T–RHx Tube (horizontal) 32.3 · 10 120 26.00 PVC pipe, reflective foil, fan

zRetail price.yTC = thermocouple.xT-RH = temperature–relative humidity sensor.

Fig. 4. Exemplary diurnal curve of global irradiance (Rs; solid line) and solar elevation angle (SEL; dashedline) with the seven periods into which each 24 h was divided for analysis of air temperaturemeasurements. Data were recorded on day of year 236, 2005.

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Rs and wind speed (P < 0.0005). The DT ofthe pagoda was not significantly affected bywind speed (0 to 3.0 m�s–1), but it wassignificantly affected by Rs (P < 0.0001)and the interaction between Rs and wind(P < 0.0001). Only 16% of midday values(periods 3 to 5) for the pagoda fell below+1.5 �C. Common to all passively ventilatedshields, the largest DT occurred at low wind

speed (i.e., 1 m�s–1 or less) and high Rs

(i.e., greater than 600 W�m–2). Based on DTvalues under clear skies and a range ofambient wind speed, the experimental setof passively ventilated shields could beassigned a general rank in increasing orderof effectiveness during the day of: hangingtube < pagoda < rocket < cone < double cone< handmade Gill.

Among aspirated sensors, aspirated TCwas not significantly different from the ther-mocouple in the commercial Gill shield atany period of the day with a range in DT (n =2112) between –1.3 and +0.8 �C (Figs. 5 and8). Of all observed DT for aspirated TC, 95%were 0.5 �C or less, as were 86% of middayvalues (periods 3 to 5). Air temperature in theaspirated TC shield tended to be slightlylower than that in the commercial Gill dur-ing the evening and at night (Fig. 8) whenambient wind speeds generally were at theirlowest and the Gill was least effectivelyventilated. There was a significant interaction(P < 0.0001) between wind speed and Rs onDT, apparent at wind speeds above �3 m�s–1

and Rs greater than 600 W�m–2 when airtemperatures in the aspirated TC shield con-sistently were higher than those in the com-mercial Gill (Fig. 9A). Higher ambient windspeeds apparently offset a greater fraction ofthe total radiation load of the commercial Gillshield than could be dissipated by the fixedair speed of the aspirated TC shield. Airtemperature in the low speed aspiratedT-RH shield (Fig. 5) tended to be higher thanthat in commercial Gill, although not alwayssignificantly so (n = 98), with a minimum DTof +0.05 �C and a maximum DT of +1.5 �Cduring the middle of the day (periods 3 to 5)for global irradiance values up to �900W�m–2. Wind speeds were in a fairly narrowrange (0.7 to 3.0 m�s–1) during the short-duration tests of the low-speed aspiratedT-RH shield. Air temperature in the super-aspirated T-RH shield (Figs. 5 and 8) wasrarely significantly different from that in thecommercial Gill with a range of DT (n =2112) between –1.6 and +1.24 �C. Solarradiation and wind speed, as well as theirinteraction, significantly affected DT (P <0.0001; Fig. 9B). Air temperature in thesuperaspirated T-RH shield tended to be lessthan that in the commercial Gill at low windspeed and low Rs but slightly higher than thatin the commercial Gill at higher Rs whenambient wind speed also was high enough toventilate the Gill shield effectively.

Discussion

There is a legitimate need among fieldbiologists, including horticulturists, for in-expensive and effective radiation shieldsbecause of the large number of sensors thatmay be deployed in a replicated field exper-iment. The commercial Gill shield madeof injected thermoplastic is appropriate forweather stations and for a reference meteo-rological mast in an experiment, but at a costof U.S. $180, it is too expensive for manyhorticulturists, agronomists, ecologists, andothers to consider for replicated plots.Stacked plate shields exist for some self-logging temperature and T–RH sensors (e.g.,Watchdog, Spectrum Technologies, Plain-field, IL; Hobo, Onset, Pocasset, MA) atapproximately one-third the price of a stan-dard Gill shield. Other authors have investi-gated inexpensive shields for deploymentaround livestock shelters, where commercial

Fig. 5. Mean difference in measured air temperature (DT) between sensors in an experimental radiationshield and the same type of sensor in a standard commercial Gill shield for each of seven periods intowhich the day was divided. Symbols are plotted at the midpoint of the period (HR). Error bars represent±1 SE.

Fig. 6. Difference in measured air temperature (DT) between sensors in a passively ventilated experimentalradiation shield and the same type of sensor in a standard commercial Gill shield over three exemplarydays with clear skies and maximum global irradiance of 800 to 830 W�m–2. Vertical lines demarcateday (periods 2 to 6) and night (periods 1, 7).

1376 HORTSCIENCE VOL. 42(6) OCTOBER 2007

Gill shields are too fragile to withstandcurious animals (Wheeler et al., 2002).Requirements for robustness in horticulturalfield experiments generally are less stringent,so more attention can be paid to findinghighly reflective, inexpensive materials andto designing the shield to best meet estab-lished physical principles.

There are many common materials fromwhich radiation shields may be constructed.Strictly in terms of optical properties, the

following surface coatings were recommen-ded in order of preference: aluminizedMylar (DuPont, Wilmington, DE), flat whitepaint, and clear plastic coatings on the uppersurface of polished aluminum foil (Fuchsand Tanner, 1965). The ‘‘foil bubble insu-lation’’ used in the present study could bedesirable because of its optical and thermalproperties and low cost (�U.S. $5.50 perm2). The surface has high shortwave reflec-tance (�0.9) and low emissivity (0.03),

advantageous for minimizing daytime errorsin the measurement of air temperature. Thelow emissivity of the material minimizeslongwave radiation transfer from the innersurface of the shield to the sensor. It hasbeen suggested that a difference in temper-ature of +5 �C between shield inner surfaceand the sensor may result in an error in airtemperature measurement of up to 1.4 �Cduring the day (Lin et al., 2001a). As aninsulative material, the foil bubble wrap haslow thermal conductivity (0.035 W�m–1

�C–1); therefore, one would expect heat fromthe external surface of the shield to beconducted poorly to the inner surface ofthe shield, a desirable attribute for single-walled shields.

In a study of passively ventilated shieldsfor weather stations, including a commercialGill shield, solar radiation entering the shieldsincreased linearly as the albedo of the under-lying surface increased (Hubbard et al.,2001). Over a highly reflective surface ofsimulated snow, in the absence of wind,temperature errors in excess of +5 �C werereported for air temperatures measured in theGill shield, �1.6 times the error that wasrecorded for temperatures measured overgrass under the same experimental conditions(Gill, 1983). These observations are relevantto horticultural applications where the surfacecould be a natural or plastic mulch of rela-tively high albedo. In the present study,shields with open bottoms (tube, pagoda,rocket, cone) tended toward higher daytimeDT. A potential disadvantage of a materiallike the foil bubble wrap used here lieswith its optical properties; surfaces exhibitingspecular reflectance can allow multiple reflec-tions of solar radiation inside the shieldand intense reflected radiation to strike thesensor. One might address such an issue bypainting the interior surface of the materialflat white.

The daytime energy balance of a shieldis dominated by incoming shortwave (i.e.,solar) radiation and outgoing convective heattransfer (Lin et al., 2001a). Passively venti-lated radiation shields cannot completelyblock solar radiation without simultaneouslyimpeding air flow, which decouples thesensor from the air that is to be measuredand in turn leads to inaccurate estimates ofactual air temperature. General guidelines(WMO, 2006) suggest a deviation of themicroclimate inside a shield from the sur-rounding air mass at ambient wind speedsless than 1 m�s–1. Among passively ventilatedshield configurations, vertically orientedtubes provide excellent shielding from directbeam solar radiation at many solar elevationangles (i.e., times of the day) but at a cost ofthe solid wall compromising convective heattransfer away from the shield and the sensor.With its high potential for ventilation, thestacked plate design does not eliminate errorsof this nature but relegates them to lowersolar elevation angles where solar radiationalso is less intense. In the standard Gilldesign, solar elevation angle had only a smalleffect on errors in temperature measurement,

Fig. 7. Response surface of the difference in measured air temperature (DT) between sensors in anexperimental radiation shield and the same type of sensor in a standard commercial Gill shield againstthe two main driving forces of DT, solar radiation [global irradiance (Rs)] and wind speed. (A) Coneshield, the simplest plate configuration, with an open bottom. (B) Handmade Gill shield, a low-costmockup of a standard commercial Gill shield.

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most noticeable at ambient wind speedsbelow 0.4 m�s–1; there was little or no effectof solar elevation angle when wind speedsexceeded 1.2 m�s–1 (Gill, 1983).

Our simplest plate shield, the cone, max-imizes ventilation efficiency through its openbottom, but at a cost of substantial error atmany solar elevation angles and from radia-tion that may be reflected from the underly-ing surface. The pagoda, an open-bottomedshield with a nearly solid sidewall, had aresponse surface (DT versus Rs and wind)most closely resembling that of the cone,although DT was driven by a different mech-anism. There was no significant effect ofwind on the DT of the pagoda, contrary tothe significant effect of wind on DT in thecone. In the pagoda, a minimally perforatedsidewall (10% perforation near the top ofthe tube) obstructed wind, thus reducing theeffectiveness of convective heat transfer,whereas in the cone, even low wind speedsallowed forced convection of heat away fromthe sensor and shield. The pagoda demon-strated both the drawback inherent in verticaltube shields and the drawback of an open orunshielded bottom.

The double cone is the simplest alterna-tive design to address the problem of shield-ing the sensor from below. Compared withthe commercial Gill, midday DT of thedouble cone may be higher than is acceptablefor many experiments. Midday DT of thedouble cone were less than 1 �C larger thanthose produced by the handmade Gill shield,which may be acceptable in applications withless stringent accuracy requirements or inplots where the shield is partially shaded inthe middle of the day. The handmade Gillcosts U.S. $2.00 more in materials andrequires 25 min longer to construct than the

double cone. The handmade Gill could notsupplant aspirated shields in heat transferstudies, where measurement accuracies of±0.1 �C may be required, but for studies ofgrowth and development, where measure-ment accuracy of ±1 �C may be sought(e.g., Perrier, 1971), a shield design similarto the handmade Gill described here mayoffer a cost-effective choice for passivelyventilated shielding. Tanner (1990) sug-gested that it would be unrealistic to expectaccuracies better than ±1 �C for air temper-atures measured in naturally ventilatedshields under full sunlight and winds below3 to 4 m�s–1.

That nighttime air temperatures were notsignificantly different between the commer-cial Gill and any experimental shield con-firms the generally held understanding thatdaytime shortwave-driven errors in the mea-surement of air temperature are most impor-tant in field studies. Longwave radiationgoverns the radiation balance of the shieldat night, but nocturnal errors in temperaturemeasurement tend to be smaller than daytimeerrors. Air temperature in all of the experi-mental shield designs was lower than that inthe commercial Gill at night, predominantlythe result of the shields’ longwave radiationbalance given consistently low wind speeds.Longwave radiation exchange is of second-ary importance during the day because it isemitted from sources whose temperatures aresimilar to that of the temperature sensoritself.

The goal of aspiration is to balance theeffect of radiation load on the shield throughconvection regardless of ambient wind speedso that the shielded sensor remains coupled toambient air and returns an accurate measure-ment of air temperature. Errors in the mea-

surement of air temperature are inverselyproportional to air speed in the shield becauseenergy transfer away from the shield isdominated by convection. Standards forweather stations (e.g., ASAE, 2004) do notspecify minimum aspiration rates, but guide-lines published for measuring environmentalvariables in plant growth chambers suggestshielding temperature sensors with a reflec-tive material and aspirating them at 3 m�s–1 orgreater (ASAE, 2002). Mounting a fan atop aGill shield (Crescenti et al., 1989) resulted inunacceptable errors in measurements of airtemperature because the fan battery andmounting assembly self-heated and becausethe fan drew air across only the top two platesrather than from the bottom to the top.The aspirated TC and superaspirated T-RHshields in the present study, given theirsimilar and frequently lower values of airtemperature relative to those in the commer-cial Gill, indicate sufficient ventilation rateswere achieved to offset the radiation loads ofthose shields. Elsewhere, when wind speedinside a modeled radiation shield was 0.7m�s–1, which corresponded to an ambientwind speed of 2.4 m�s–1, all expected mea-sured air temperatures were within ±0.5 �Cof actual air temperature (Lin et al., 2001a).At very low wind speed (0.5 m�s–1) and highirradiance (greater than 900 W�m–2), Gill(1979) recorded a temperature error of +1.5�C in the developmental version of what nowhas become the commercial Gill shield.Where power is available, sufficiently aspi-rated radiation shields remain the ideal forfield experiments.

Conclusion

Because any solar shield will impedeventilation, investigators must balance solarshielding with the potential for sufficientconvective heat transfer away from the shieldand its sensor. A potentially effective radia-tion shield designed around stacked plateslike the commercial Gill shield (U.S. $180)can be constructed from a readily available,inexpensive, lightweight, and highly reflec-tive material at a cost of U.S. $4.00 inmaterials and 45-min assembly time. Eighty-seven percent of all midday measurements ofair temperature in this ‘‘homemade Gill’’were 1.5 �C or less above those recorded incommercial Gill shields; two-thirds of allobserved DT were 1.0 �C or less. Regardlessof configuration among the in-house pas-sively ventilated shields, the combination oflow ambient wind speed (less than 1 m�s–1)and high global irradiance (greater than 600W�m–2) produced the largest DT, the magni-tude of which varied with shield design;stacked plate configurations had lower DTthan tube-based configurations. NighttimeDT were inconsequential for all shields.Open-bottomed radiation shields are notadvised, particularly over reflective surfaces.The smallest daytime DT were betweenaspirated shields and the commercial Gill,averaging less than ±0.5 �C. Our resultssuggest that thermocouples in small aspirated

Fig. 8. Difference in measured air temperature (DT) between sensors in aspirated radiation shields and thesame type of sensor in a standard commercial Gill shield over three exemplary days with clear skiesand maximum global irradiance of 800 to 830 W�m–2. Vertical lines demarcate day (periods 2 to 6) andnight (periods 1, 7).

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shields may be in equilibrium with ambientair at aspiration rates above 1 m�s–1, butcombined temperature-RH sensors inhigher-volume shields require higher aspira-tion rates (e.g., 4 m�s–1) to minimize diver-

gence of measured air temperature fromactual air temperature. One’s ultimate choiceof materials and design for radiation shieldsrests largely on the required accuracy of thetemperature measurements and on the resour-

ces available for achieving the desired levelof replication.

Literature Cited

ASAE. 2002. Guidelines for measuring and report-ing environmental parameters for plant experi-ments in growth chambers. ASAE EP411.4.American Society of Agricultural Engineers,St. Joseph, MI.

ASAE. 2004. Measurement and reporting practicesfor automatic agricultural weather stations.ASAE Standard EP505. American Society ofAgricultural Engineers, St. Joseph, MI.

Campbell, G.S. and J.M. Norman. 1998. An intro-duction to environmental biophysics. Springer,New York.

Crescenti, G.H., R.E. Payne, and R.A. Weller.1989. Improved meteorological measurementsfrom buoys and ships (IMET): Preliminarycomparison of solar radiation air temperatureshields. WHOI-89-46. Woods Hole Oceano-graphic Institute, Woods Hole, MA.

Fritschen, L.J. and L.W. Gay. 1979. Environmentalinstrumentation. Springer-Verlag, New York.

Fuchs, M. and C.B. Tanner. 1965. Radiationshields for air temperature thermometers.J. Appl. Meteorol. 4:544–547.

Gill, G.C. 1979. Development of a small ruggedradiation shield for air temperature measure-ments on drifting buoys. Report to NOAA DataBuoy Office for Development Contract #01-7-038-827 (IF). Bay, St. Louis, MO.

Gill, G.C. 1983. Comparison testing of selectednaturally ventilated solar radiation shields.Report to NOAA Data Buoy Office for Devel-opment Contract #NA-82-0A-A-266. Bay, St.Louis, MO.

Hubbard, K.G., X. Lin, and E.A. Walter-Shea.2001. The effectiveness of the ASOS, MMTS,Gill, and CRS air temperature radiationshields. J. Atmos. Oceanic Technol. 18:851–864.

Lin, X., K.G. Hubbard, and E.A. Walter-Shea.2001a. Radiation loading model for eval-uating air temperature errors with a non-aspirated radiation shield. Trans. ASAE 44:1299–1306.

Lin, X., K.G. Hubbard, E.A. Walter-Shea, andJ.R. Brandle. 2001b. Some perspectives onrecent in situ air temperature observations:Modelling the microclimate inside the radia-tion shields. J. Atmos. Oceanic Technol.18:1470–1484.

Monteith, J.L. and M.H. Unsworth. 1990. Princi-ples of environmental physics. Edward Arnold,London.

Perrier, A. 1971. Leaf temperature measurement,p. 632–671. In: Z. Sestak, J. Catsky, and P.G.Jarvis (eds.). Plant photosynthetic production:Manual of methods. W. Junk, The Hague.

Tanner, B.D. 1990. Automated weather stations.Remote Sensing Reviews. 5:73–98.

Wheeler, E.F., J.L. Zajaczkowski, and R.E. Graves.2002. Effect of solar shielding on portabledatalogger temperature readings. Trans. ASAE19:473–481.

WMO. 2006. Guide to meteorological instrumentsand methods of observation. WMO-No.8.World Meteorological Organization, Geneva.

Fig. 9. Response surface of the difference in measured air temperature (DT) between sensors in anexperimental radiation shield and the same type of sensor in a standard commercial Gill shield againstthe two main driving forces of DT, solar radiation [global irradiance (Rs)] and wind speed. (A)Aspirated thermocouple shield (aspirated TC), with constant air flow of 1.8 m�s–1. (B) Superaspiratedtemperature–relative humidity shield (superaspirated T-RH) with a constant air flow of 4.4 m�s–1.

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