Broadband radiometers manufactured by Eppley Laboratories Inc. are commonly used to measure irradiance from both ground-based and aircraft platforms. Namely, the pyranometer (Model PSP) measures irradiance in the .3 to 3.0 micron spectral region while the pyrgeometer (Model PIR) senses energy in the 4 to 50 micron region. The two instruments have a similar thermopile construction but different filters to achieve the appropriate spectral selection. During the fall of 1986, the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) commenced with the first cirrus Intensive Field Observation (IFO) conducted in Central Wisconsin. Due to the nature of this field project, pyranometers and pyrgeometers manufactured by Eppley 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 over a temperature range from -20 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. It is meant to serve as a supplement to calibration procedures

Cox, Stephen K.

Smith, William L., Jr.

English

NASA Technical Reports Server

T E ~ E R A T U R E  S E ~ S I T I ~ T Y  OF EPPLEY BROA~BAND RADIOMETERS 
WILLIAM L. SMITH. JR., STEPREN K .  Cox 
Depart& of Atnwophrne Scirna. CdDmdo Slats Uniwrdy, Ibtt Cdlinr, CO 805e3 
VINCE GLOVER 
Rcremh Amation M i l y  of the Atmorpheric nchdogv Diririon, Noriond Ceder for Atnwbpheric Reread, Boulder, CO 80507 
Eppfey Lo(Mdo+y Ine. modd PIR wrpamvtcro and nwdd PSP pyronmnetcro haw b e d  in lrmpcdure 
compenrdion circuitry derigned to limit nl0ti.c trmro in the wuorurenuni of mdidion to f t?% for a temptmfurt 
mnpr of -PO C to +do C. A pmacdurr developed to e:* thio op&ah'on and to Mcrminc the dative renriliuly 
to lcmprdurer M o r  -PO C u dercdod Rea& of thi~ di%&*on and application lo data c o n d i o n  are dro  
pnrentcd 
1. INTRODUCTION 
Broadband radiometers manufactured by E p  
pley Laboratories Inc. are commonly used to 
measure irradiance from both ground-based and 
aircraft platforms. Namely, the 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 filters to 
achieve the appropriate spectral selection. 
The precision pyranometer has been de- 
scribed b Albrecht and Cox (1976) and Robin- 
son (19663. These instruments are typically sent 
to the manufacturer 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 instru- 
ment, see Albrecht ct  ai. (1974). Several other 
authors have made contributions in assessing 
pyrgeometers. For example, 
points out that the main problem with achiew- 
ing accurate irradiance measurements from the 
pyrgeometer is associated with errors caused by 
temperature gradients within the instrument it- 
self. He suggests minimizing these errors by uti- 
lizing a newly constructed pyrgeometer which 
demonstrates a markedly reduced sensitivity to 
temperature gradients. 
During the fall of 1986, the First IS- 
CCP (International Satellite Cloud Climatol- 
servation (IFQ) conducted in central 
Regional Experiment (FIRE com- 
the first cirrus Intensive Fie I d Ob- 
sin. Due to  the nature of this field project, 
pyranometers and pyrgeometers manufactured 
by Eppley were flown on NCAR's high altitude 
research aircraft, the Sabreliner. Inherent in the 
construction of these radiometers is tempera- 
ture compensation circuitry designed to make 
the instrument sensitivity nominally constant 
(within f 2%) over a temperature range from 
-20 C to +40 6. Because the Sabreliner flew 
at high altitudes where temperatures were as 
cold as -70 C, i t  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. I t  is 
meant to serve as a supplement to the calibra- 
tion procedures referenced above. 
Six Eppley Laboratory Inc. manufactured ra- 
diometers were flown on the NCAR Sabreliner 
during the FIRE first cirrus IFQ. Of these six, 
only three were available for this particular cal- 
ibration at NCAR's Research Aviation Facility 
(RAF) and included two pyranometers and one 
pyrgeometer. Because this calibration is con- 
cerned with radiometer thermopile characteris- 
tics only, the calibration procedure can be sim- 
plified by converting them all to visible radiome- 
ters. 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 lront of the radiometers 
but on the outside of the chamber. In addi- 
1 83 
https://ntrs.nasa.gov/search.jsp?R=19910001171 2020-03-19T20:14:25+00:00Z
TEMPERATURE SENSITIVITY O F  EPPLEY BROADBAND RADIOMETERS 
tion, an extra pyranometer was mounted on the 
outside of the chamber to monitor any changes 
in the light source output. Signal wires con- 
nected to  the radiometers were passed through 
a datalogger to a computer where thermopile 
output, chamber temperature, room tempera- 
ture and the sink temperature of the pyrgeome- 
ter were recorded and monitored in real-time. 
For a more visual depiction of the laboratory 
set up, see figure 1. 
3. CALIBRATION PROCEDURE 
The temperature of the insulated environ- 
ment chamber was controlled so that data 
could be sampled every 5 C from +26 C to - 
63 C. Again, because we are concerned with 
thermopile characteristics only, the calibration 
procedure can also be simplified by ensuring 
that the radiometers are in thermal equilib- 
rium when data is recorded. The sink temper- 
ature 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 C chamber temperature increment), the 
sink temperature of the pyrgeometer was moni- 
tored with respect to the chamber temperature 
to determine when the pyrgeometer was in ther- 
mal 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 ra- 
diometers 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 cover- 
ing the plated glass window, thus shielding the 
radiometers from all shortwave radiation. This 
provided information concerning the dark cur- 
rent 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 in 5 degree intervals. At the begin- 
ning of each day, a room temperature calibra- 
tion data point was recorded. This data relates 
the output voltage of each radiometer being cal- 
ibrated to the output voltage of the pyranome- 
ter monitoring the source while all radiometers 
are at the same room temperature. So, for each 
instrument being calibrated, 
where T, = room temperature 
V, = voltage output of source monitor at T, 
V, = voltage output of radiometer being 
Vd = dark voltage of radiometer 
p = function relating V, to  Vm at Tr. 
Then, for each subsequent calibration point, 
calibrated at Tr 
WT?) = dT)[Vc(T) - Vd(T)I 
where T = temperature of chamber and of the 
thermal mass of the radiometers being cali- 
brated 
q = function relating V, at T to Vm at T, 
and the relative sensitivity is 
K ( T ,  Tr) = p(Tr)/q(T)- 
Thus, the relative sensitivity K(T,T,)  is nor- 
malized to express the temperature dependence 
of each radiometers thermopile given a constant 
source of energy and with respect to some ref- 
erence temperature, taken here to  be the room 
temperature (T,) of 26 C. 
4. RESULTS 
Results of this calibration for the available ra- 
diometers are shown in figure 2. Within the 
electronically compensated temperature range' 
from +40 C to -20 C, our results depict nom- 
inal instrument sensitivity within about 2.5%, 
close to Eppley Laboratory Inc. f 2% speci- 
fication. Outside of this range (i.e* below -20 
C), errors could be as large as 7% at  -63 6. A 
third order polynomial proved to provide the 
best fit to these data. These curves are also 
shown in figure 2. The corresponding coeffi- 
cients for these curves have been used to correct 
the data collected on board the Sabreliner dur- 
ing FIRE. Since two of the pyranometers flown 
on the Sabreliner were unavailable for this post 
mission calibration, an average was taken of the 
two curves that were developed and this third 
curve used to correct the data collected by the 
uncalibrated instruments. Similarly, one pyrge- 
ometer was also unavailable, so the curve devel- 
oped from the calibrated pyrgeometer was also 
applied to data collected by the uncalibrated 
pyr geometer. 
TIQN 
5. APPLICATION TO DATA CQRREC- 
The application of this calibration to data 
correction is straight-forward because the orig- 
inal calibrations were done a t  room temper- 
atures near the 26 C reference temperature 
used in this calibration. Eppley Laboratory 
Inc. states in their calibration documentation 
that the adopted calibration temperature for 
I84 
the pyranometer is 25 C. For the pyrgeometers, 
the calibration procedure following the meth- 
ods of Albrecht and Cox 1977) dictates that 
is near the room tern erature of the laboratory. 
Albrecht el al. (1974rreport that the sink tem- 
perature is representative of the thermopile sen- 
sor temperature. This is because the tempera- 
ture difference between the hot and cold junc- 
tions of the thermopile is, a t  most, 0.5 C for 
typical irradiance measurements in the atmo- 
sphere. 
The relative sensitivity (K) can be applied di- 
rectly to the pyranometer measurements to get 
the corrected value. For example, 
the temperature of the ra 6 iometer thermopile 
where V = thermopile output voltage of pyra- 
nometer 
C = calibration constant supplied by Eppley 
Laboratory Inc. to convert thermopile voltage 
output to units of irradiance (Le. W / M 2 )  
K(T,) = the relative sensitivity a t  a temper- 
ature T, which is the sink temperature of an 
adjacent pyrgeometer 
SWCORR = the corrected irradiance. 
For the pyrgeometer, an equation derived by 
Albrecht et ~ l .  (1974) and Albrecht and Cox 
(1977) of the form 
V 
'I 
E = - + COT," - /3~(Tj - TI") 
where 
6 - emissivity of the thermopile (usually taken 
to be unity) 
u = Stefan Boltzmann constant 
Td, T, = dome temperature and sink tempera- 
ture, respectively 
q,p = calibration constants 
V = thermopile voltage 
E = incoming irradiance 
can be further corrected for temperature sensi- 
tivity by applying K(Ts) to the first term such 
that 
where ECORR is E but corrected for temper- 
ature sensitivity. 
However, because the infrared irradiance 
(from the pyrgeometer) is a function of the sum- 
mation of several terms, the effect of the tem- 
less for the pyrgeometer data than for the pyra- 
nometer data. Several FIRE data sets were an- 
alyzed to determine the magnitude of error be- 
tween corrected and uncorrected pyranometer 
and pyrgeometer measurements. Irradiauce er- 
rors for the pyrgeometer were found to be, at 
most, 2-3 W/m2 which is within the noise ol 
the instrument. Because the temperature sensi- 
tivity correction is applied directly to the pyra- 
nometer measurements, errors could be as large 
as 7% of the uncorrected measurement a t  -63 C. 
In addition, errors as large as 10% were found in 
cloud fractional absorptance f CFA) calculations 
(See Ackerman and Cox (198i) f o r a  description 
of CFA). 
0. 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.  ACKNOWLEDGEMETJTS 
This research has been supported by the Na- 
tional Science Foundation under Grant ATM- 
8521214 and by the National Aeronautics and 
Space Administration under Grant NAG 1-554. 
REFERENCES 
Ackerman, S. A., and S. K. Cox, 1981: Aircraft 
Observations of the Shortwave Fractional 
Absorptance of Non-Homogeneous Clouds. 
Ackerman S. A., M. Poellot and S.K. Cox, 
1974: byrgeorneter measurements from air- 
craft. Rre. Sci. Inatrum., 45, 33-38. 
Albrecht, B., and S. K. Cox, 1976: Radiation 
data reduction procedures for Sabreliner, C- 
130 and DC-6 aircraft during the GARP At- 
lantic Tropical Experiment. Colorado State 
University AtmorpAcn'e Snence Paper NO. 44, Fort 
Collins, CO., 100 pp. 
Albrecht, B., and S. K. Cox, 1977: Procedures 
for improving pyrgeometer performance. J.  
Bradley, S.G., and R.G. Gibson, 1982: On the 
J.  Appl. Meteor., 20, 0128-0133. 
Appl. MC~COP., 16, 188-197. 
use of r eometers in cloud. J. Appl. Mefcor., 
21. ll&?lk59. 
Foot, 'J.S., 1985: A new pyrgeometer. J. A l m a .  
Robinson, N., 1966: Sdar Radiotion, Elsevier, 347 
Rvzner. E.. and M.R. Weber. 1982: Comments 
Ocanic Tshnol., 3, 363-370. 
PP- 
, .  - "On the performance of pyrgeometers with 
silicon domes." J.  ~ p p t .   etc cor., 21, 1208-1210. perature sensitivity correction will be somewhat 
185 
TEMPERATURE SENSITIVITY O F  EPPLEY BROADBAND RADIOMETERS 
Weiss, A., 1981: On the performance of pyr- 
eometers with silicon domes. J. A d .  Add8W.l 
80,962-965. 
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186 


Temperature sensitivity of Eppley broadband radiometers

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