February 1988.FIRE series no. 5.Includes bibliographical references.Eppley Laboratory Inc. model PIR pyrgeometers and model PSP pyranometers have built in temperature compensation circuitry designed to limit relative errors in the measurement of radiation to + /- 2% for a temperature range of -20 C to +40 C. A procedure developed to verify this specification and to determine the relative sensitivity to temperatures below -20 C is described . Results of this calibration and application to data correction are also presented.Sponsored by the National Science Foundation and National Aeronautics & Space Administration

Cox, Stephen K.

Glover, Vince

Smith, W. L., Jr.

English

February 1988.FIRE series no. 5.Eppley Laboratory Inc. model PIR pyrgeometers and model PSP pyranometers have built in temperature compensation circuitry designed to limit relative errors in the measurement of radiation to + /- 2% for a temperature range of -20 C to +40 C. A procedure developed to verify this specification and to determine the relative sensitivity to temperatures below -20 C is described. Results of this calibration and application to data correction are also presented.Sponsored by the National Science Foundation and National Aeronautics & Space Administration

Smith, W. L., Jr., author

Cox, Stephen K., author

Glover, Vince, author

Mountain Scholar (Digital Collections of Colorado and Wyoming)

/ (QC85~ I C&J l1D·L/c23 f}T5L LIBRARIES MAR 21 1988 COLORADO STATE U IVERSITY Temperature Sensitivity of Eppley Broadband Radiometers by W.L. Smith Jr., Stephen K. Cox, Vince Glover FIRE Series No. 5 Funding Agencies: National Science Foundation National Aeronautics & Space Administration TEMPERATURE SENSITIVITY OF EPPLEY BROADBAND RADIOMETERS FIRE Series No. 5 by W.L. Smith Jr.,1 Stephen K. Cox1 Vince Glover2 February, 1988 Atmospheric Science Paper No. 423 1 Affiliation - Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523 2 Affiliation - Research Aviation Facility of the Atmospheric Technology Division, National Center for Atmospheric Research, Boulder, CO 80307 TABLE OF CONTENTS ABSTRACT l. INTRODUCTION 2. LABORATORY SET UP 3. CALIBRATION PROCEDURE 4. RESULTS 5. APPLICATION TO DATA CORRECTION 6. SUMMARY 7. ACKNOWLEDGEMENTS TABLE 1 8. REFERENCES FIGURE 1 FIGURE 2 1 2 2 3 4 6 6 8 8 9 10 11 12 TEMPERATURE SENSITIVITY OF EPPLEY BROADBAND RADIOMETERS Abstract Eppley Laboratory Inc. model PIR pyrgeometers and model PSP pyranometers have built in temperature compensation circuitry designed to limit relative errors in the measurement of rac.iat ion to + /- 2% for a temperature range of -20 C to +40 C. A procedure developed to verify this speci-fication and to determine the relative sensitivity to temperatures below -20 C is described . Results of t his calibration and application to data correction are also presented. 1. INTRODUCTION Broadband radiometers manufactured by Eppley Laboratory Inc. are commonly used to measure irradiance from both ground-based and aircraft platforms. Namely, the precision pyranometer (Model PSP) measures irradiance in the .3-3 micron spectral region while the pyrgeometer (Model PIR) senses energy in the 4-50 micron region. The two instruments have a similar thermopile construction but different optical filters to achieve the appropriate spectral selection. The precision pyranometer has been described by Albrecht and Cox (1976) and Robin-son (1966). These instruments are typically sent to the man facturer for calibration relative to a group of reference standards. Calibration of the pyrgeometer is more complex and can be performed following the procedures of Albrecht and Cox (1977). For a description of this instrument, see Albrecht et al. (1974). Several other authors have made contributions in assessing the performance of pyrgeometers. For example, see Weiss (1981), Bradley and Gibson (1982) and Ryzner and Weber (1982) . Foot (1986) points out that the main problem with achieving accurate irradiance measurements from the pyrgeometer is associated with errors caused by temperature gradients within the instrument itself. He suggests mi nimizi ng these errors by utilizing a newly constructed pyrgeometer which demonstrates a markedly reduced sensitivity to temperature gradients. During the fall of 1986, the First ISCCP (International Satellite Cloud Climatology 2 Project) Regional Experiment (FIRE) commenced with the first cirrus Intensive Field Ob-servation (IFO) conducted in central Wisconsin. Due to the nature of this field project, pyranometers and pyrgeometers manufactured by Eppley Laboratory Inc. were flown on NCAR's high altitude research aircraft, the Sabreliner. Inherent in the construction of these radiometers is temperature compensation circuitry designed to make the instrument sensitivity nominally constant (within +/- 2%) over a temperature range from -20 C to +40 C. Because the Sabreliner flew at high altitudes where temperatures were as cold as -70 C, it was necessary ·to determine the radiometers relative sensitivity to temperatures below -20 C and apply appropriate corrections to the FIRE radiation data set. A procedure to perform this calibration is outlined below. It is meant to serve as a supplement o the calibration procedures referenced above. 2. LABORATORY SET UP Six Eppley Laboratory Inc. manufactured radiometers were flown on the NCAR Sabre-liner during the FIRE first cirrus IFO. Of these six, only three were available for this particular calibration at NCAR's Research Aviation Facility (RAF) and included two pyra-nometers and one pyrgeometer. Because this calibration is concerned with radiometer ther-mopile characteristics only, the calibration procedure can be simplified by converting them all to visible radiometers. This conversion only requires that the domes be visibly clear. The three radiometers were mounted inside an environment chamber at NCAR's RAF so that they faced , and were encompassed in the field of view of, a plated glass window in the door. A light source was mounted directly in front of the radiometers but on the outside of the chamber . In addition, an extra pyranometer was mounted on the outside of the chamber to monitor any changes in the light source output. Signal wires connected to the radiometers were passed through a datalogger to a computer where thermopile output, chamber tem-perature, room temperature and the sink temperature of the pyrgeometer were recorded and monitored in real-time. For a more visual depiction of the laboratory set up, see figure 1. 3 3, CALIBRATION PROCEDURE The temperature of the insulated environment chamber was controlled so that data could be sampled every 5 degrees celsius from +26 C to -63 C. Again, because we are concerned with thermopile characteristics only, the calibration procedure can also be sim-plified by ensuring that the radiometers are in thermal equilibrium when data is recorded . The sink temperature of the pyrgeometer is the best indication of the temperature of the thermal mass of the radiometer. So, at each calibration point (i.e. each 5 degree C cham-ber temperature increment), the sink temperature of the pyrgeometer was monitored with respect to the chamber temperature to determine when the pyrgeometer was in thermal equilibrium. While the temperature of the chamber's environment could be changed rather rapidly, it was found to take nearly an hour for the thermal mass of the pyrgeometer to achieve thermal stability. Since the pyranometers and pyrgeometer are nearly identical in construction and thermal mass, it was assumed that all radiometers in the environmental chamber reached thermal equilibrium at the same time. The instrument output was sampled at 1 Hz and recorded in 10 second averages. The source was chopped at each calibration point by covering the plated glass window, thus shielding the radiometers from all shortwave radiation. This provided information concern-ing the dark current of the instruments. Because of the time it took for thermal equilibrium to be reached at each calibration point, it took several days to cover the entire temperature range from +26 C to -63 C using 5 degree intervals . At the beginning of each day, a room temperature calibration data point was recorded. This data relates the output voltage of each radiometer being calibrated to the output voltage of the pyranometer monitoring the source while all radiometers are at the same room temperature. So, for each instrument being calibrated, 4 where Tr= room temperature V m = voltage output of source monitor at Tr Ve = voltage output of radiometer being calibrated at Tr Vd = dark voltage of radiometer p = function relating Ve to V m at Tr. Then, for each subsequent calibration point, where T = temperature of chamber and consequently of the thermal mass of the radiometers being calibrated q = function relating Ve at T to V m at Tr and the relative sensitivity is Thus, the relative sensitivity K(T ,Tr) is normalized to express the temperature depen-dence of each radiometers thermopile given a constant source of energy and with respect to some reference temperature, taken here to be the room temperature (Tr) of 26 degrees C. 5 4. RESULTS Results of this calibration for the available radiometers are shown in figure 2. Within the electronically compensated temperature range from +40 C to -20 C, our results depict nominal instrument sensitivity within about 2.5%, close to the Eppley Laboratory Inc. +/-2% specification. Outside of this range {i.e. below -20 C), errors could be as large as 7% at -63 C. A third order polynomial proved to provide the best fit to these data. These curves are also shown in figure 2. The corresponding coefficients for these curves are listed in table 1 and have been used to correct the data collected on board the Sabreliner during FIRE. Since two of the pyranometers flown on the Sabreliner were unavailable for this post mis-sion calibration, an average was taken of the two pyranometer curves that were developed and this third curve used to correct the data collected by the uncalibrated instruments. Similarly, one pyrgeometer was also unavailable, so the curve developed from the calibrated pyrgeometer was also applied to data collected by the uncalibrated pyrgeometer. 5. APPLICATION TO DATA CORRECTION The application of this calibration to data correction is straight- forward because the original manufacturer 's calibrations were done at room temperatures near the 26 C reference temperature used in this calibration. Eppley Laboratory Inc. states in their calibration doc-umentation that the adopted calibration temperature for the pyranometer is 25 C. For the pyrgeometers, the calibration procedure following the methods of Albrecht and Cox {1977) dictates that the temperature of the radiometer thermopile is near the room temperature of the laboratory. Albrecht et al. (1974) report that the sink temperature is representative of the th~rmopile sensor temperature. This is because the temperature difference between the hot and cold junctions of the thermopile is, at most, 0.5 K for typical irradiance mea-surements in the atmosphere. The relative sensitivity {K), determined in section 3, can be applied directly to the pyranometer measurements to get the corrected value. For example, 6 SWCORR = 1/K(T.,) * C * V where V = pyranometer thermopile output voltage C = calibration constant supplied by Eppley Laboratory Inc. to convert thermopile output voltage to units of irradiance (i.e. W /M**2) K(T.,) = the relative sensitivity at a temperature T., which is the sink temperature of an adjacent pyrgeometer SWCORR = the corrected irradiance. For the pyrgeometer, an equation derived by Albrecht et al. {1974) and Albrecht and Cox ( 1977) of the form H where E + u, r: - f3 u (TJ - r:) ,, f - emissivity of the thermopile ( usually taken to be unity) <T = Stefan Boltzmann constant Td, T., = dome temperature and sink temperature, respectively r,, /3 = calibration constants E = thermopile voltage H = incoming irradiance can be further corrected for temperature sensitivity by applying K(T.,) to the first term such that HCORR 1 -- * K(T.,) E + EuTf - f3u(TJ - Tf) ,, where H CORR is H but corrected for temperat~e sensitivity. 7 However, because the infrared irradiance {from the pyrgeometer) is a function of the summation of several terms, the effect of the temperature sensitivity correction will be somewhat less for the pyrgeometer data than for the pyranometer data. Several FIRE data sets were analyzed to determine the magnitude of error between corrected and uncorrected pyranometer and pyrgeometer measurements. Irradiance errors for the pyrgeometer were found to be, at most, 2-3 W /m**2 which is within the noise of the instrument. Because the temperature sensitivity correction is applied directly to the pyranometer measurements, er-rors could be as large as 7% of the uncorrected measurement at -63 C. In addition, errors as large as 10% were found in cloud fractional absorptance {CFA) calculations (See Ackerman and Cox {1981) for a description of CFA). 6. SUMMARY A calibration procedure to correct Eppley Laboratory Inc. broadband radiometers for thermopile temperature sensitivity has been presented. This calibration should be per-formed prior to field work where radiometers of this type, particularly the pyranometer, may be exposed to temperatures beyond the range of the temperature compensation circuit. 7. ACKNOWLEDGEMENTS The authors wish to express their appreciation to Mr. Christopher Johnson-Pasqua at Colorado State University and Mr. David McFarland at NCAR's RAF for their electronics expertise and assistance with the laboratory set up. Dr. Ron Schwiesow provided scientific guidance regarding the design of the laboratory experiment and Mssrs . Herminio Avila and Paul LeHardy devised and assembled the calibration fixtures . Special thanks are extended to Melissa Tucker for preparation of this manuscript. This research has been supported by the National Science Foundation under Grant ATM-8521214 and by the National Aeronau-tics and Space Administration under Grant NAG 1-554. 8 Table 1. Coefficients for the third order polynomial representation of the relation sen-sivity (K) for -63 T, +26 where K(T,) a + b * T, + c * T, * *2 + d * T, * *3 Radiometer Serial No. a b C d 14963F3 1.0212 -3 .8166E-4 -2.2971E-5 l.1316E-7 12149F3 1.0201 -5.8376E-4 -l.6416E-5 l.8673E-7 12148F3* 1.0207 -4.8271E-4 -l.9694E-5 l.4994E-7 14962F3* 1.0207 -4.8271E-4 -l.9694E-5 l.4994E-7 12150F3 1.0123 -4.5256E-4 -7.7408E-6 l.9354E-7 12151F3* 1.0123 -4.5256E-4 -7.7408E-6 l.9354E-7 *indicates that radiometer was unavailable for this calibration T, is sink temperature of nearest pyrogeometer 9 8. REFERENCES Albrecht, B., and S.K. Cox, 1977: Procedures for improving pyrgeometer performance. J. Appl. Meteor., 16, 188-197. Ackerman, S.A., and S.K. Cox, 1981: Aircraft Observations of the Shortwave Fractional Absorptance of Non-Homogeneous Clouds. J. Appl. Meteor., 20, 0128-0133. Ackerman, S.A., M. Poellot and S.K. Cox, 1974: Pyrgeometer measurements from aircraft . Rev. Sci. Instrum., 45, 33-38. Bradley, S.G ., and R.G. Gibson, 1982: On the use of pyrgeometers in cloud. J. Appl. Meteor., 21, 1155-1159. Foot , J .S., 1985: A new pyrgeometer. J. Atmos. Oceanic Technol., 3, 363-370. Robinson, N., 1966: Solar Radiation, Elsevier, 347 pp. Ryzner, E., and M.R. Weber, 1982: Comments "On the performance of pyrgeometers with silicon domes." J. Appl. Meteor., 21, 1208-1210. Weiss, A., 1981: On the performance of pyrgeometers with silicon domes . J. Appl. Meteor ., 20, 962-965. 10 RE 1 SIDE VIEW FRONT VIEW NCAR (RAF) ENVIRONMENT CHAMBER -+25.00 0 DOOR WITH 5 LAYER PLATED GLASS WINDOW TRANSMISSION ,v 15% LIGHT SOURCE 650 WATT QUARTZ HALOGEN BULB RADIOMETERS WITH CLEAR DOMES MOUNTED INSIDE CHAMBER SOURCE MONITOR INSTRUMENT OUTPUT DATALOGGER COMPUTER i Cl) ffi Cl) ! 0:: i Cl) i5 Cl) ! 0:: v~u-n"'1V'\JW J vl\UDIV"\l s;:J FALL 1987 1. 100 RADIOMETER ID 1 080 0-014963F3 ' A-A 12149F3 1.060 1.040 1.020+ 1.000 0.980+ 0.960 0.940+ ,e,8=1!1<~:e:a:~D iS· vfl .. o n-~o ~- 'I 0/uO. A O" 0/ o.e20 I 0.900 I I I I I I -80 -60 -40 -20 0 20 40 TEMPERATURE (DEGREES · CELSIUS) PYRANOMETER I 14963F3 1.100~-------------------, 1.080 1.060 1.040 1.020 1,000 0.980 0.960 0,940 0.920 • BEST FIT • (J 0-S..S 0.900--------------------- 80 -00 -~ -w O W 40 TEMPERATURE (DEGREES CELSIUS) i ffi ! 0:: i Cl) i5 Cl) ! 0:: PYRANOMETER l12149F3 1.100.-------------------------,, 1_.080 1.060 1.040 1.020 1.000 0.980 0.960 0.940 0.920 BEST F1T 0.900 +---+---+----+----+---- -f I -80 -60 -40 -20 0 20 40 TEMPERATURE (DEGREES CELSIUS) PYRGEOMETER I 12150F3 1.100---------------------, 1.080 1.060 1.040 1.020 1.000 0.980 0.960 0.940 BEST FIT Qlii3 D....D 0.920 0.900-------t----t---+---t------f ftj N -80 -00 -40 -w O W 40 TCMPERATURE (DEGREES CELSIUS) 

Temperature sensitivity of Eppley broadband radiometers

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Temperature sensitivity of Eppley broadband radiometers

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