The Abbot silver-disk pyrheliometer. Subjects: Pyrheliometer. Note: At head of title: Hodgkins Fund. Publication Physical Description: 11 p. illus. 24 cm. Get this from a library! The Abbot silver-disk pyrheliometer (with one plate). [Loyal Blaine Aldrich; Smithsonian Institution. Hodgkins Fund.]. Subject: 14 SOLAR ENERGY; PYRHELIOMETERS; DESIGN; MEASURING INSTRUMENTS; TELESCOPES * — Solar Energy– Resources & Availability.

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Standard and secondary standard solar radiation instruments are islver.

This instrument uses a cylindrical blackbody cavity dik absorb radiation that is admitted through a collimating tube. The design was modified by Abbot in to include the use of two thermally identical chambers, dividing the cooling water between them and beating one chamber electrically while the other is heated by solar radiation; when the instrument is adjusted so as to make the heat produced in the two chambers identical, the electrical power input is a measure of the solar energy absorbed.

Robinson and Coulson provide detailed discussion and bibliography on this topic. Two of these secondary standard instruments are of importance.

The side exposed to radiation is blackened, and the bulb of a precision mercury thermometer is inserted in a hole in the side of the disc and is in good thermal contact with the disc. The silver disc is suspended on wires at the end of a collimating tube, which in later models has dimensions such that 0. Thus any point on the detector sees an aperture angle of 5.

A shutter alternately admits radiation and shades the detector at regular intervals; the corresponding changes in disc temperature are measured and provide the means to calculate the absorbed radiation. The other secondary standard of particular importance is the AngslrSm compensation pyrheliometer, first constructed by K. Bach strip can be electrically heated, and each is fitted with a thermocouple. With one strip shaded and one strip exposed to radiation, a current is passed through the shaded strip to heat It to pyrheliometed same temperature as the exposed strip.

When there is no difference in temperature, the electrical energy to the shaded strip must siler the solar radiation absorbed by the exposed strip. Solar radiation is determined by equating the electrical energy to the product of incident solar radiation, strip area, and absorptance. After a determination is pyrneliometer, the position of the shutter is reversed to.

The Angstrom instrument serves, in principle, as an absolute or primary standard. The Abbot and AngstrSm instruments arc used as secondary standards for calibration of other instruments, and there Is a pyrheliometrlc scale associated with each of them.

The first scale, based on measurements with the Angstrom instrument, was established m the Angstrom scale ofor AS Reviews of the accuracy of these instruments and intercomparisons of hem led to the conclusions that measurements made on SSI3 were 3. SS13 is very close to WRR. Operational or field instruments arc calibrated against secondary standards and are the source of most of the data on which solar process engineering designs must be based.

A cross section of a recent model of the Eppley is shown in Figure 2.

The Abbot silver-disk pyrheliometer

The instrument mounted on a tracking mechanism is shown in Figure 2. The detector is at the end of the collimating tube, which contains several diaphragms and is blackened on the inside. The aperture angle of the instrument is 5.


In this instrument the assembly of copper diaphragms and case has very large thermal capacity, orders of magnitude more than the hot junctions. Other pyrheliometers were designed by Molt-Gorczynski, Yaaish- evskiy, and Michclson. The dimensions of the collimating systems are such that the detectors are exposed to radiation from the sun and from a portion of the sky around the sun.

It is difficult to generalize from the few data available, but it appears that thin clouds or haze can affect the angular distribution of rotation within the field of view of standard pyrheliomeiers. The WMO recommends that calibration of pyrheliometers only be undertaken on days in which atmospheric clarity meets or exceeds a minimum value.

Instalments for measuring total beam plus diffuse radiation are referred to as pyranom- eters, and it is from these instruments that most of the available data on solar radiation are obtained. The detectors for these instruments must have a response independent of wavelength of radiation over the solar energy spectrum.

These factors are discussed in more detail by Coulson The detector assembly was placed in a nearly spherical glass bulb, which has a transmittance greater than 0. The response of this Eppley was dependent on ambient temperature, with sensitivity decreasing by fl. It is possible to add temperature compensation to the external circuit and remove this source of error.

It is shown in Figure 2. The Eppley precision spectral pyranometer PSP utilizes a thermopile detector, two concentric hemispherical optically ground covers, and temperature compensation that.

See Stewart et ai. The thermopile assembly is covered with two concentric glass hemispherical domes to protect it from weather and is rectangular — in configuration with the thermocouples aligned in a row which results in some sensitivity to the azimuth angle of the radiation.

Pynmomelers are usually calibrated against standard pyrheliometers, A standard method has been set forth in the Annals of the International Geophysical Year IGY.

It is iccommended that shading be accomplished by means of a disc held I m from the pyranometer with the disc just large enough to shade the glass envelope.

Pyrheliometer, Silver Disk

Gate and precision are required in these calibrations. It is also possible, as described by Norristo calibrate pyranometers against a secondary standard pyranometer such as the Eppjey precision pyranometlr.

Direct comparison of the precision Eppley and field instruments can be made to determine the calibration constant of the field instruments. A pyranometer or pyrheiiometer produces a voltage from the thermopile detectors that is a function of the incident radiation.

It is necessary to use silfer potentiometer to detect and record this output. Radiation data usually must be integrated over some period of time, such as an hour or a day. Integration can be done by means of planimetry or by electronic integrators.

Much of the available radiation data prior to is probably not this good, largely because of infrequent calibration pyrehliometer in some instances because of inadequate integration procedures.

Another class of pynmometers, originally designed by Robtlzsch, utilizes detectors that are bimetallic elements heated by solar radiation: These instruments have the advantage of being entirely spring driven and thus require no electrical energy. Data from these instruments are generally not as accurate as that from thermopile-type pyranometers.

Another type of pyranometer is based on photovoltaic solar ceil detectors.

They are less precise instruments than the thermopile instruments and have some limitations on their use. They are also less expensive than thermopile instruments and are easy to use. Tire main disadvantage of photovoltaic detectors is their spectrally selective response. If the spectral distribution of incident radiation was fixed, a diso could be established that would remain constant; however, there are some variations in spectral distribution2 with clouds and atmospheric water vapor.


Photovoltaic detectors have additional characteristics of interest. Their response to changing radiation levels is essentially pyrhelimoeter and is linear with radiation. The Ll-COR instrument is fitted with an abot diffuser that substantially removes the dependence of response on the angle of incidence of the radiation.

The preceding discussion dealt entirely with measurements of total radiation on a horizontal surface. Two additional kinds of measurements are made with pyranometers: Measurements of diffuse radiation can be made with pyranometers by shading the instrument from beam radiation. This is usually done by means of a shading ring, as shown in Figure 2. The pyeheliometer is used to allow continuous recording of diffuse radiation without the necessity of continuous positioning of smaller shading devices; adjustments need to be made for changing declination only and can be made every few days.

The ring shades the pyranometer from part of the diffuse radiation, and a correction for this shading must be estimated and applied to the observed diffuse radiation Drummond,; Pyrheliometee. The corrections are based on assumptions of the distribution of diffuse radiation over the sky and typically are factors of 1.

An example of shade ring correction factors, to illustrate their trends and magnitudes, is shown in Figure 2. Measurements of solar radiation on inclined planes are important in determining the input to solar collectors. There is evidence that the calibration of pyranometers changes if the instrument is inclined to the horizontal. The reason for this appears to be changes in the convection patterns inside the gloss dome, which changes the manner in which heal is transferred from Hie hot junctions of the thermopiles to the cover and other parts of the instrument.

Courtesy of The Eppley Laboratory. Adapted from Couhon. Adapted from Stewart el at. The Bellani spherical distillation pyranometer is based on a different principle. U uses a spherical container of alcohol that absorbs solar radiation. The sphere is connected to a calibrated condenser receiver tube. The quantity of alcohol condensed is a measure Of integrated solar energy on the spherical receiver. Data on the total energy received by a body, as represented by the sphere, are of interest in some biological processes.

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The Abbot silver-disk pyrheliometer (with one plate)

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