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CHAPTER 5. THE CALIBRATION AND USE OF NET RADIOMETERS

CHAPTER 5. THE CALIBRATION AND USE OF NET RADIOMETERS

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SHERWOOD B. IDS0



and for scheduling irrigations (Franzoy and Tankersley, 1970; Heermann

and Jensen, 1970; Jensen, 1969, 1972; Jensen and Heermann, 1970; Jensen et ai., 1970, 1971). Thus, the study of net radiation has become a

subject of considerable importance for many soil scientists and

agronomists.

Over the years, several attempts have been made to develop techniques

€or estimating net radiation from other more easily measured parameters

(Davies, 1967; Fritschen, 1967; Gay, 1971; Idso et al., 1969b; Linacre,

1968, 1969; Monteith and Szeicz, 1961; Pasquill, 1949; Penman, 1948

Richardson, 1931; Scholte-Ubing, 1961; Stanhill et al., 1966). Owing to

effects of clouds, however, such procedures have not achieved a degree

of accuracy that is satisfactory for all-weather utilization. Indeed, even when

these procedures are applied only to clear sky conditions, Idso (1968,

1971a) and Nkemdirim (1972, 1973) have shown that they may not be

adequately generalized. Direct measurement thus remains the only reliable

method for obtaining precise information on net radiation under all the

varied conditions existing in the natural environment. Consequently, this

paper presents a summary of some of the basic aspects of net radiometers

and techniques for using them.

It.



Instruments



The primary component of all net radiometers is the thermopile. This

device usually takes the form of a flat black plate of some insulating material with a series of thermocouple junctions located on each side. The temperature difference across the plate is proportional to the net radiation to

which it is exposed; and by means of a calibration constant, the electrical

output of the thermopile is converted to yield this flux.

The basic design of the more common thermopiles in use today was

described by Funk (1959, 1962a) and Fritschen (1963, 1965). Other

variants were reported by Baumgartner (1952), Campbell et al. (1964),

Frttschen and Van Wijk (1959), and Hofmann (1952). Gates (1964),

Monteith (1959), and Monteith and Szeicz (1962) outlined the manufacture of simple versions. In addition, there is the economical net radiometer

transducer which utilizes a pair of thermometers. It has been described

by Suomi and Kuhn (1958) and Tanner et al. (1960, 1969).

There are two major types of net radiometers-ventilated and shielded.

In the first type, a forced stream of air is directed over the thermal transducer, or thermopile, to equalize as much as possible the effects of convection from its upper and lower surfaces. Examples of such instruments are

those developed by Gier and Dunkle (1951) and Suomi et al. (1954).

In the second type, transparent wind shields are used to reduce unequal

convection effects. Some of the materials that have been used to construct



CALIBRATION AND USE OF NET RADIOMETERS



263



FIG. 1. The thermopile of a Fritschen net radiometer showing the epoxy bobbin

around which are wound the several thermocouple junctions. The bobbin is encased

in a flat disk of similar epoxy and is now ready to be painted black.



these wind shields are rock salt coated with selenium (Aleksandrov and

Kurtener, 1941), sylvite coated with selenium (Khvoles, 1952), thallium

bromoiodide (Houghton and Brewer, 1954; Kreitz, 1953; Stern and

Schwartzmann, 1954), filters of germanium and silicon (Mills and Crawford, 1955), sheets of mica (Fritschen and Van Wijk, 1959), Saran plastic

film (Fritschen, 1960), and polystyrene (Fritschen, 1963). The material

used by most manufacturers today is polyethylene (Monteith, 1972).

Since many good reviews dealing with the various commercially available

net radiometers and their operating characteristics already exist (Gates,

1962, 1965a; Kondrat’yev, 1965; Monteith, 1972; Robinson, 1962; Sellers,

1965; Tanner, 1963), this information is not repeated, nor is the detailed

theory underlying their operation. Instead, this review is devoted to practical discussions of calibration techniques, basic considerations of data acquisition, and methods of modifying net radiometers for a variety of different

applications.

Ill.



Calibration Methods



The most common method for calibrating net radiometers is the occulting technique (Collins and Rabich, 1971) or, more simply, the so-called

“shading technique.” Calibration is accomplished by positioning a net

radiometer alongside a pyranometer or solarimeter and alternately shading



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and unshading both instruments simultaneously from the direct rays of the

sun (or some other light source) with identical small, opaque shields. In

the laboratory, a high-pressure xenon arc lamp is generally used as the

light source (Collins, 1970), although tungsten filament lamps have also

been used (Drummond, 1956; Fritschen, 1963; Hill et al., 1966).

If the net radiometer is positioned high enough above the ground so

that the shadow from the shield used to shade it from the direct beam of

the light source is insignificant in altering the radiation it receives from

below, the electromotive force (emf) change of the net radiometer’s output



FIG. 2. Calibrating a net radiometer by the common shading technique. The

lower left picture shows obscuration of the sun by the shield over the net

radiometer; and the lower right picture depicts the shading of the Eppley pyranometer

used as the standard.



CALIBRATION AND USE OF NET RADIOMETERS



265



between shaded and unshaded conditions can be equated to the known

energy flux change recorded by the standard pyranometer to yield the Calibration factor of the net radiometer. Important points to remember when

using this technique include the following: (a) the net radiometer must

not be positioned too close to the ground when calibrating; (b) the accuracy of the resulting calibration factor will be no better than that of the

solarimeter used to obtain it; and (c) this technique gives the calibration

factor for response to shortwave radiation (0 5 A _< 3 p m ) , which may or

may not be the same as the net radiometer’s response to long-wave radiation (A > 3 pm).

Chamber techniques are generally used to obtain long-wave radiation

calibration factors. Two major variations are presently in vogue, one due

primarily to Fritschen (1963) and the other to Funk (1959, 1962a,b).



FIG. 3. Back and front views of the calibration chamber constructed by L. J.

Fritschen for the calibration of net radiometers at the US. Water Conservation

Laboratory. It can be used for both short- and long-wave calibrations. In the

former instance, an artifical light source is used to alternately illuminate a net radiometer and a pyranometer located at the central chamber; in the latter instance,

the central chamber divider is removed, and the net radiometer is positioned there

and calibrated by calculation of the net long-wave radiation exchange between the

upper and lower chamber halves, which are kept at different temperatures by circulating water.



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SHERWOOD B. IDS0



Fritschen’s chamber, developed from a prototype of Johnson (U.S.

Weather Bureau, unpublished manuscript, 1956), is composed of two cubical halves with their common side removed. The inner copper walls are

painted black and have copper tubing attached to their backsides. Two

water streams of different temperature flow through the tubes attached to

the upper and lower sets of five walls to create a net flux of thermal radiation. The net radiometer is positioned in the center of the chamber; and

its output is equated with the calculated net long-wave radiative flux; this

flux is derived from thermocouple-obtained wall temperatures and a knowledge of the wall emittance, which may be measured by a variety of techniques (Buettner and Kern, 1965; Conaway and Van Bavel, 1966; Fuchs

and Tanner, 1966, 1968; Idso and Jackson, 1969). The formula given by

Fritschen for calculating the net radiation in his chamber is not exact, however, and the correct expression derived by Idso (1970a) should be used

when employing this technique.

The long-wave calibration technique of Funk was developed from an

earlier version of MacDowall ( 1955 ) which makes use of a more complex

arrangement of two chambers. The net radiometer is positioned in the

center of one of the insulated chambers and allowed to view the second

(a blackbody cavity of different temperature) through a small aperture

€or only a short period of time. The blackbody cavity most used today

is an electrically heated cylindrical cavity of cast aluminum described by

Collins (1968), the emissivity of which is known to be better than 0.999.

Both Funk‘s and Collin’s analyses of the net radiation to which the net

radiometer is exposed, however, are slightly in error. Idso (1970a) provides a more exact evaluation of this flux.

Chamber techniques of the Funk and Fritschen type require no other

standard radiometer or solarimeter for comparison; and they can yield very

accurate long-wave calibration factors when properly utilized. The chambers are somewhat complex, however, and hardly worth the effort to construct for the calibration of just a few radiometers. Thus, they generally

are used only in major laboratories, such as the CSIRO Division of Atmospheric Physics, which calibrates some 400 instruments annually (Collins

and Rabich, 1971), and the U.S. Water Conservation Laboratory, which

at its peak production of Fritschen-type net radiometers calibrated about

200 annually.

A simpler technique for obtaining long-wave calibration factors of net

radiometers was developed by Idso ( 1970a,b). In this method a net radiometer is positioned anywhere from 5 to 20 cm above the floor of a constant-temperature room. Then, a blackened metallic plate about 40 cm

on a side (which was previously cooled about 10°C below the temperature

of the room) is brought into the room and set beneath the radiometer.



CALIBRATION AND U S E OF N E T RADIOMETERS



267



FIG.4. The setup of the flat-plate technique for obtaining long-wave calibration

factors of net radiometers.



The output of the radiometer is subsequently monitored, along with the

temperature of the flat plate (by means of 4 or 5 embedded thermocouples), as the cool plate gradually warms to ambient room temperature.

Since all radiative fluxes in the room are constant, except for the flux

emitted by the warming plate, the millivolt output of the net radiometer

can be plotted against the radiation received from the plate and its calibration factor determined as the slope of this line. The radiation from the plate

is specified by the plate’s emittance, temperature, and view factor with respect to the radiometer, which may be determined from a variety of standard heat transfer texts (Eckert and Drake, 1959; Jakob, 1957; Sparrow

and Cess, 1966). This technique has also been shown to be suitable for

calibrating soil heat flux plates (Idso, 1972b).

Both Fritschen and Funk concluded from their studies of short- and longwave calibration factors of polyethylene-shielded net radiometers that these

instruments were about 5% more sensitive to short-wave radiation than

to longwave radiation. A similar conclusion was also reached by Collins

and Kyle ( 1966). Thus, for years, practically all polyethylene-shielded net

radiometers have had a small strip of white paint applied to their thermopiles to reflect about 5 % of the incident solar radiation and thereby equate

their short- and long-wave calibration factors. Idso ( 1970a), however,

showed that the inaccuracies in the original derivations of the equations

representing the net long-wave radiation fluxes in the chambers of Fritschen

and Funk were such as to completely, and rather exactly, negate the need

for this paint. Experimentation with the flat plate technique also confirmed

this analysis. Thus, for the most exacting requirements of net radiation

measurement, polyethylene-shielded instruments without any white paint

on their transducers should be employed.



SHERWOOD B. IDS0



268



IV.



Utilizing the Basic N e t Radiometer



Most of the primary considerations involved in net radiometry are discussed by Fritschen (1963, 1965) and Funk (1959, 1962a). In addition,

there is an instruction manual by Fritschen and Mullins (1965) that is

whollly devoted to this subject. The methodology covers the physical installation of net radiometers, leveling of net radiometers, preventing of internal

and external condensation of water on radiometer domes, cleaning of

domes, and replacement of damaged domes. Only a few additional comments are required.

First, it has been my experience that radiometer calibration factors are

not adversely affected unless the domes are extremely dust laden. Thus,

cleaning the domes should be a rather conservative practice, since they

are so easily scratched. Second, although a standard placement height of

1 meter is often suggested for net radiometers, this topic is a subject in

itself. For instance, routine experimentation has been conducted with net

radiometers at heights of 0.1 m (Van Bavel and Fritschen, 1965), 0.3

m (Van Bavel, 1967), 0.5 m (Rosenberg, 1966; Stanhill et al., 1966),

0.75 m (Idso et al., 1969b), 1.0 m (Davies and Buttimor, 1969; Fritschen,

1967; Stanhill and Fuchs, 1968; Thompson and Boyce, 1967), 1.5 m

(Ekern, 1965; Fitzpatrick and Stern, 1966; Monteith and Szeicz, 1961),

2.0 m (Begg et al., 1964; Denmead, 1969; Morgan et al., 1970; Rosenberg,

1969), and 2.5 m (Stanhill et al., 1966). Of this group of investigators,

only the ones using the lowest (0.1 m) and highest (2.5 m) heights gave

any reasons for their choices-and these were only qualitative at that.

Idso and Cooley (1971, 1972) conducted a comprehensive study of net

radiometer height placement. They showed that if net radiation measurements are required over a surface such as a dry, bare soil that becomes

significantly warmer than ambient air during clear days, net radiometers

should be located at a height of only 20 or 25 cm. The reason for choosing

this level is that the intervening air between the net radiometer and the

ground interacts with the radiation from the soil and modifies it significantly

over even smaller distances than this. Thus, true net radiation is theoretically measured only at the surface, if surface and air temperatures are

different. However, as the surface is approached, the shadow of the net

radiometer begins to alter significantly the radiation emitted and reflected

by the surface. The level of compromise between these two effects was

found to be between 20 and 25 cm for radiometers of the Funk and Fritschen type (about 6 cm in diameter). If for some reason a different level

is selected, Kondo (1972) has developed procedures for estimating the

required corrections to surface conditions.



CALIBRATION AND USE OF NET RADIOMETERS



269



When surface and air temperatures do not differ greatly, such as over

an irrigated field or crop, considerations of homogeneity of field of view

tend to predominate in selecting the height of net radiometer placement.

View factors must then be calculated for various heights to determine the

best level for representative sampling of the total system or a specific portion of it, i.e., the bare soil between crop rows. Standard heat transfer

texts may be consulted for this exercise or the two more agriculturally

oriented papers of Reifsnyder and Lull (1965) and Reifsnyder (1967),

which contain some useful graphs that often obviate the need for an investigator to make the actual calculations. Other examples of view factor utilization in net radiometry have been published by Gates (1962, 1965a) and

Waggoner and Reifsnyder (1961).



V.



Modifications for Different Applications



The basic Fritschen and Funk net radiometers are extremely versatile

instruments. Solar Radiation Instruments' (SRI) of Australia markets a

Funk type that is claimed to be suitable for use in nine different forms.

These are: (a) a regular net radiometer, (b) a short-wave balance meter,

(c) a solarimeter, (d) an albedometer, (e) a total hemisphrical radiometer,

( f ) a total albedo and long-wave emission radiometer, (g) an underwater

short-wave balance meter, ( h ) an underwater solimeter, and (i) an underwater albedometer.

Most of the procedures required to transform the basic net radiometer

into these other forms are rather simple and straightforward. By merely

removing the polyethylene domes and replacing them with glass, for instance, the net radiometer is transformed into a short-wave balance meter.

The SRI instrument may be purchased with interchangeable glass domes

specifically manufactured for this purpose. Working with the Fritschen net

radiometer, I have also made this transformation with the glass domes that

are normally used for the Kipp solarimeter, as they are of the proper size

to be held in place by the net radiometer's clamping rings.

By painting either the inner or outer surface of one of the glass domes

black to restrict light entry to only one side of the thermal transducer, the

shortwave balance meter may be transformed into either a solarimeter or an

albedometer, depending upon whether it is operated in an upright or an

inverted position. In both of those applications, however, additional steps

must be taken to ensure that the painted dome does not absorb an excessive

*Trade names and company names are included for the benefit of the reader

and imply no endorsement or preferential treatment of the product listed by the

U.S.Department of Agriculture.



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