SMITHSONIAN MISCELLANEOUS COLLECTIONSVOLUME 110, NUMBER 11
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THE SMITHSONIAN STANDARDPYRHELIOMETRY
P.VC G. A P.HOTResearch Associate, Smithsonian Institution
(Publication 3945)
CITY OF WASHINGTONPUBLISHED BY THE SMITHSONIAN INSTITUTIONAUGUST 5, 1948
Z^t Boxi (gafttmor* ^vteeBALTIMORE, HO., O. S. A.
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Till-: SMITJISONIAN STANDARDI'^'kIIELIOMETRYP.Y C. G. AP.P.OTRcsciirilt .Issociatc, Smithsonian InslitnlionSince 1910 nearly a hundred copies of the silver-disk pyrhehometerhave been prepared at the Smithsonian Institution, ihey are in usein many CDuntries. Observers, even those using other types (jf ])yr-hehometer, often express their results in terms of "the Smithsonianstandard scale" which is carried to them by these silver-disk instru-ments, standardized against the water-llow pyrhehometer. Aidrichand Abbot, in 1947, made a painstaking comparison at Mount Wilsonbetween two silver-disk instruments and the water-flow pyrhehometer.They obtained within one part in a thousand the same result as in1934 and earlier.^ Various observers have investigated old silver-diskinstruments and find no evidence that there has been a change of theirsensitiveness since 1910.So the question of the standard scale depends on the adequacy oftlie water-flow pyrhehometer as a standard. (Originally this instrumentcomjirised a single deep test-tube-like blackened chamber of metalwith hollow walls. In these walls, in the extreme rear wall, and inthe walls of a hollow cone not quite at the rear, on which all the sun'srays fell directly, a current of water constantly flowed to carry offthe solar heat as fast as absorbed. An electrical thermometer, meticu-lously calibrated by means of an extremely delicate standard mercurythermometer, registered the rise of temperature between the entranceand the exit of the stream of water. A carefully gaged diaphragmadmitted the solar rays to the chamber. Other diaphragms of slightlylarger diameter, along the vestibule and within the chamber, servedthe double purpose of opposing air currents, and of obstructing theentrance or the escape of stray light. The rate of flow of the waterwas determined by frequent weighings.As all of the entering beam of sunlight fell upon the hollow black-ened cone near the extreme rear of the chamber, over 95 percent of
• Aldricli, L. B., and .\bbot, C. G., Smithsonian pyrliclionictry and thestandard scale of solar radiation. Smithsonian Misc. Coll., vol. 1 10. No. 5, 1948.SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 110, NO. 11
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the rays would be immediately absorbed on that cone and would giveup their heat there into the flowing water. The remaining 5 percent orless would be scattered over an entire hemisphere, of which nearlythe whole solid angle was included in the blackened walls of thechamber. Over 95 percent of the trifling amount of radiation scatteredfrom the cone, impinging upon these walls, would be absorbed onthem and this heat also would be communicated to the flowing water.Only' the measured aperture, through which solar rays entered,was open to free escape of the scattered rays As this aperturesubtended but 0.012 hemisphere as viewed from the hollow cone le sthan 0.012 of 5 percent of the introduced solar radiation could reelyescape So, theoretically, the chamber was fully 99-94 percent blackLest some unforeseen error should lurk in the device, two coilsof insulated wire were wound upon the cone. One coil w^s woundin shellac directly upon the rear wall of the cone, being behind thewater stream within the cone, but in front of the water stream inthe extreme back wall of the chamber. This coil was more favorab ysituated than solar heating to convey electrically produced heat tothe flowing water. The other insulated coil was of several millimetersthickness, was doughnut-shaped, and was stuck on with shellac to tl^front rim of the hollow cone, outside the area covered by the beamof sunUght. This coil was very unfavorably situated to give upelectrically produced heat to the flowing water, since it must hrstgive its heat to the air, and then to the walls of the chamber^I have been describing Standard PyrheHometer No. 3. On pages61 and 6^ of Annals of the Smithsonian Astrophysical Observatory,volume 3, 1913, there are given 24 tests, half with each of the twoheating coils, where electrically introduced heat was measui-ed byabsorption in the flowing water. The results of 12 tests at Wash-ington, April 18, 22, and 23, 1910, showed no certain difference asbetween the two coils, and gave a mean result of 99-85 percent heatfound The results of 12 tests at Mount Wilson, October 10 and n,191 1 also equally divided between the two coils, gave 100^6 percentheat found. These results come to well within their probable errorat exactly 100 percent heat found. They therefore indicate that heatintroduced in the chamber, no matter whether more or less favorablyfor measurement than solar heat, is completely absorbed and accu-rately measured by the instrument. This, as we shall see later, isa critically important result. , 1 1 ^f ^,.rNot content with this method of fixing the standard scale of pyr-heliometry we constructed another instrument of the hollow-chambertype It was called the water-stir pyrheliometer, because, mstead
NO. II STANDARD PYRHErjOMIiTRY—ABBOT 3
of carrying off absorbed lieat in a flowing stream of water, thechamber was immersed in a water bath whose rate of rise of tempera-ture, and coohng corrections, were observed after the methods ofexact calorimetry. In this instrument only one insulated coil of wirewas introduced, but it was wound in part within the wall of the sidesof the chamber. Thus it had almost identically the same facility t(jgive up its heat to the water as did the solar rays. Tests of electricalheating with this instrument were made on October 24 and 26, 1912,and recorded on page (^"j of Annals, volume 3. Six tests gave 100.05percent of heat found, and the results are even more consistent thanthe excellent ones with the water-flow pyrheliometer. Silver-diskpyrheliometer APO 8|,is, which we have ever since used as secondarystandard, was compared on a number of occasions from 1910 to 1912,some at Washington, others at Mount Wilson, and with both thewater-flow and the water-stir standards. The results are given atthe bottom of page 70, Annals, volume 3. They give the followingindependent determinations of the constant for APO Sbis : 0.3798,0-3791. 0.3809, 0.3786, 0.3792, 0.3770, 0.3772.Many years later the silver-disk pyrheliometers were altered tohave longer vestibules so as to reduce the angular area of sky nearthe sun to which they were exposed. The water-flow standard pyr-heliometer was also changed. A Russian, V. M. Shulgin, made thevaluable suggestion that by using two chambers rather than one in thewater-flow pyrheliometer, with the water stream divided just at theentrance of their walls, inequalities in rate of water flow would bethe same in both. Plence if the solar heating in one chamber wascontinually being balanced by electrical heating in the other, theinequalities of flow of water would cease to produce fluctuations inthe readings. In 1932 we introduced Shulgin's method, and, dependingon the results oj iqio to 1912, to the effect that solar heating and elec-trical heating are equally efficiently absorbed, all subsequent standard-izations of pyrheliometers by Smithsonian observers are based on theuse of the standard water-flow pyrheliometer as an electrical compen-sation instrument. That is, we no longer measure the water-flow rate,or the rise of temperature of the water, but we balance solar heat inone chamber against electrical heat in the other, and reverse chambersas respects heating again and again. I repeat, ice noxv absolutelydepend on the experiments I have quoted, of the years iQio to 10T2,which prove that in our pyrheliometer electrical heat and solar heatare both fully absorbed in the water stream.Prior to the adoption of W M. Shulgin's suggestion of using twochambers in the water-flow pyrheliometer, we found great difficulty
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in producing a constant water stream. Air bubbles were carriedalong, and local fluctuations in temperature occurred owing to aircurrents affecting the short rubber tubes which had to be introducedto allow free movement. These irregularities, both of mechanicaland heat natures, caused accidental differences of successive measure-ments so appreciable that great numbers of comparisons with silver-disk pyrheliometers had to be made to obtain accurate results. Whatwith this source of error, and the effect of sky radiation from nearthe sun, which was minimized by using the longer vestibules of thesilver-disk pyrheliometers after the year 1925, we found that theearlier determinations of the constants of silver-disk pyrheliometerswere too high by 2.3 percent. This correction we published in theyear 1934.' Nevertheless, so as not to upset the world's system ofpyrheliometry, and the comparability over a long term of years ofSmithsonian solar-constant results contained in volumes 2 to 6 ofAnnals of the Smithsonian Astrophysical Observatory, while we admitthat the 191 3 scale of pyrheliometry is 2.3 percent too high, we andthose who follow us still use the Smithsonian scale of 1913.The variability of the brightness of the sky may still slightly affectsilver-disk pyrheliometry. However, as stated at pages 53 to 55,Annals, volume 6, we now eliminate variations of sky brightness as asource of error in solar-constant measurements.
- Abbot, C. G., and Aldrich, L. B., The standard scale of solar radiation.Smithsonian Misc. Coll., vol. 92, No. 13, 1934.