ARM TR-021

> Pyranometer Handbook
>
> November 2004
>
> T. Stoffel
>
> Work supported by the U.S. Department of Energy,
>
> Office of Science, Office of Biological and Environmental Research

# Contents {#contents .Heading-Front}

[1. General Overview 1](#general-overview)

[2. Contacts 1](#contacts)

[3. Deployment Locations and History
1](#deployment-locations-and-history)

[4. Near-Real-Time Data Plots 1](#near-real-time-data-plots)

[5. Data Description and Examples 1](#data-description-and-examples)

[6. Data Quality 7](#data-quality)

[7. Instrument Details 7](#instrument-details)

**Tables**

#  {#section .unnumbered}

[1. 6](#__RefHeading___Toc87239232)

# General Overview

A pyranometer is a type of radiometer used to measure broadband solar
irradiance on a planar surface. Accurate measurements of solar
irradiance are needed to understand the primary source of energy input
to the earth-atmosphere-ocean system. The ARM Program uses pyranometers
for ground-based, in situ shortwave radiation measurements collected by
the ARCS/GNDRAD, ARCS/SKYRAD, SIROS, and SIRS instrument platforms.
(Note: for additional information on the pyranometer, see the [SIRS
page](http://www.arm.gov/instruments/static/sirs.stm).)

# Contacts

## Mentor

Tom Stoffel\
National Renewable Energy Laboratory\
1617 Cole Blvd.\
Golden, CO 80401-3393\
Voice: 303-384-6395\
Fax: 303-384-6391\
E-mail: <Tom_Stoffel@nrel.gov>

## Instrument Developer

The Eppley Laboratory, Inc.\
12 Sheffield Ave.\
Newport, RI 02840\
Phone: 401-847-1020\
[http://www.eppleylab.com](http://www.eppleylab.com/)

# Deployment Locations and History 

This section is not applicable to this instrument.

# Near-Real-Time Data Plots

This section is not applicable to this instrument.

# Data Description and Examples

## Data File Contents

### Primary Variables and Expected Uncertainty

Depending on orientation and mounting system hardware, a pyranometer can
be used to measure various shortwave, or solar, irradiances (Watts per
square meter):

1.  Downwelling total hemispheric: An unshaded pyranometer mounted on a
    stable horizontal platform will provide measurements of the combined
    direct \"beam\" and diffuse \"sky\" irradiance components.

2.  Downwelling diffuse: A pyranometer mounted on an automatic solar
    tracker, equipped with a properly designed shade mechanism to block
    the direct solar rays, will provide measurements of diffuse
    irradiance.

3.  Upwelling diffuse: A pyranometer installed in an inverted horizontal
    position, i.e., mounted with the detector pointed at the ground,
    will measure the amount of reflected shortwave irradiance.\\

#### Definition of Uncertainty

Model PSP pyranometers are capable of providing 1-minute averages of
down welling global (total hemispheric), diffuse (sky), and upwelling
(reflected) shortwave irradiance measurements within +/- 5% or +/- 10
Watts per square meter, whichever is larger, at the 95% confidence
level. This estimate assumes the instrument has been calibrated within
the last year to the World Radiometric Reference (WRR), properly
installed, and is adequately maintained. A detailed analysis of the
response characteristics contributing to the measurement uncertainty
estimate is provided in the \"Theory of Operation\" section.

There are a number of conditions under which pyranometer measurements
may be incorrect. The data user should examine the data quality flags
(described in \"Explanation of Flags Applied During Data Ingest) and
data quality reports (DQR) to determine whether significant malfunctions
have occurred due to problems with a pyranometer. The more frequent
sources of error are described briefly here.

Contaminated optical surfaces can reduce or enhance the apparent solar
flux at the detector. Accumulations of dew, frost, dust, snow or ice on
the outer dome are generally reduced or eliminated by the use of a
ventilator or when the pyranometer is mounted in the inverted position.
Reduction of the dome transmittance due to contamination will decrease
the signal from a pyranometer. Under some circumstances, the presence of
dew, frost, snow or ice can enhance the amount of solar radiation
reaching the pyranometer detector. In small quantities and in locations
on the dome that can reflect more photons onto the detector than
available in the direct beam, the signal from a pyranometer may increase
from the expected value.

On time-scales of a few minutes, the presence of scattered clouds,
especially towering cumulus, can reflect additional solar radiation onto
the pyranometer to cause irradiance measurements in excess of the
corresponding extraterrestrial value (a physical limit).

Improper alignment of the solar tracking disk can leave all or part of
the pyranometer detector surface unshaded from the direct radiance,
resulting in high values for down welling diffuse. Interruptions of
electrical power to the solar tracker can cause such misalignments. The
Model PSP is a passive instrument, providing a signal even under this
condition.

The down welling hemispheric shortwave irradiance will be in error if
the pyranometer detector is not horizontal. Tilted toward the sun by as
little as one degree, the pyranometer output can be noticeably biased.

Local obstructions to sun such as trees, buildings, power lines, or
towers can shade or reflect sunlight naturally available to the
pyranometer detector. The location of a SIRS measurement station has
been selected to eliminate or greatly reduce the effects of such
obstructions to accurate irradiance measurements. Generally, deviations
of the local horizon within an elevation angle of 5? will not result in
measurable differences from a pyranometer.

### Secondary/Underlying Variables

This section is not applicable to this instrument.

### Diagnostic Variables

This section is not applicable to this instrument.

### Data Quality Flags

See [SIRS](http://science.arm.gov/tool/dod/showdod.php?Inst=sirs) Data
Object Design Changes for ARM netCDF file header descriptions.

### Dimension Variables

This section is not applicable to this instrument.

## Annotated Examples

This section is not applicable to this instrument.

## User Notes and Known Problems

This section is not applicable to this instrument.

## Frequently Asked Questions

**How do you pronounce \"pyranometer\"?**

Webster\'s dictionary gives the first pronunciation as
\"peer\`-o-nom-et-ur\" and \"pie\`-rah-nom-et-ur\" as an alternate.

**Why are nighttime data typically negative?**

Shortwave radiometers should produce a null signal in the absence of
solar radiation. However, all commercially available pyranometers based
on thermoelectric transducers, can exhibit a non-zero output signal in
the absence of solar radiation. These signals are a result of thermal
gradients within the pyranometer.

The Eppley Laboratory, Inc. Model PSP is a thermoelectric, single-black
detector design with well-established performance characteristics
\[Hulstrom, 1989\]. Two Schott Glass hemispheres protect the thermopile
detector from the weather. These inner and outer domes also reduce the
thermal exchange between the detector, or thermopile \"hot\" junction,
and the environment. The body of the instrument is a relatively massive
bronze casting that is used to control the \"cold\" or reference
junction of the thermopile. Recent studies of this and other models of
pyranometers suggest thermal offsets are producing as much as 30 Wm-2
during clear-sky, nighttime conditions. Similarly, daytime measurements
of down welling diffuse shortwave irradiance must be corrected for
thermal offsets. ARM will use a correction method based on correlations
with the net infrared and observed diffuse irradiances \[Dutton, et al,
2001\]. Beginning in 2001, all SIRS diffuse irradiance measurements will
be made with an Eppley Model 8-48, a so-called \"black and white\"
detector designed to almost eliminate the thermal offset.

**How often should the outer dome be cleaned?**

Research-quality irradiance measurements require frequent cleaning of
all optical surfaces. Ventilators reduce the need for maintenance by
producing a steady flow of air over the pyranometer dome preventing dew
and frost formation and, to a lesser extent, accumulations of dust, snow
and ice. The frequency and amounts of contamination are site dependent.
Some studies suggest the effect can be about 5% for daily total global
irradiation \[Marion, 19XX\]. An analysis of historical SIRS data to
determine the effects of cleaning will begin in 2001. Additionally, a
measurement experiment comparing duplicate measurement systems with
varying cleaning intervals is planned for later the same year.

**What is the maximum signal cable length?**

Daytime pyranometer signals typically range from 10 microvolts to 15
millivolts, depending on the amount and distribution of clouds. The
measurement accuracies of all-weather data acquisition systems is
typically ?3 microvolts. Experience has shown a \"good\" signal cable
made of 18 AWG to 22 AWG copper, twisted pair, shielded, and properly
grounded can be used at lengths approaching 50 m before effecting the
pyranometer signals.

**When should the PSP desiccant be changed?**

When the indicating silica gel turns pink, the desiccant needs to be
recharged or replaced. White or clear silica granules indicate complete
saturation and the failure of a seal within the radiometer.

**Why is the direct irradiance sometimes greater than the unshaded
pyranometer?**

Direct normal (beam) irradiance varies with solar zenith angle and the
amounts of absorbing and scattering elements in the atmospheric path to
the receiver. Global (total horizontal) irradiance is the sum of the
direct normal and diffuse (sky) components: Global = Direct Normal x
Cos(Z) + Diffuse. Depending on the site location (latitude) and time of
year (solar declination), the direct normal can exceed the global
irradiance or vice versa.

**What is the maximum irradiance expected from a pyranometer
measurement?**

\>Unshaded = Global = 1400 Wm^-2^ (greater than exoatmospheric) in the
presence of towering cumulous or when the solar disk is near the edge of
a cloud.\
\>Shaded = Diffuse = 600 Wm^-2^ for mid-latitude, continental locations.

**Where can I send a pyranometer for recalibration?**

All radiometer manufacturers provide recalibration services, typically
using artificial sources. The ARM Program has developed the Radiometer
Calibration Facility at the SGP to provide annual recalibration of all
SIRS shortwave radiometers (pyranometers and pyrheliometers) using
natural sunlight.

**How often should a pyranometer be recalibrated?**

Depending upon exposure, a properly maintained pyranometer can maintain
a responsivity (calibration factor is proportional to the inverse of
responsivity) within ?0.5% for up to one year. Annual recalibration of
field radiometers is an accepted practice for maintaining
research-quality measurements.

**Is it necessary to remove a pyranometer from service for
calibration?**

No. Traveling standards can be used to compare with field instruments,
but the resulting calibration uncertainties are greater than using
reference standards and the ?summation? technique. Field calibrations
require stable, clear-sky conditions during the comparisons. ARM is
evaluating the Alternate Calibration Method in which the unshaded
(global) and shaded (diffuse) instruments are periodically (seasonally)
exchanged at the site. Comparisons of the irradiances at similar solar
zenith angles and the coincident direct normal irradiance can produce
calibration factors for the two pyranometers.

**What is \"cosine response\"?**

The angular response of a perfect pyranometer detector would vary
exactly with the cosine of the incoming direct irradiance. Manufacturers
typically specify the departure from perfect cosine response at
incidence angles less than 85? on the order of ?5%.

**What are the effects of ventilators on pyranometer performance?**

The forced air over the pyranometer dome helps reduce the thermal
gradient between the hot and cold thermopile junctions, thereby reducing
the pyranometer zero-offsets.

**Does it matter *how* I install a pyranometer?**

The pyranometer must be level (parallel to horizon) within 0.5?

An air gap between the pyranometer base and mounting surface should be
at least 5 mm to insulate the instrument from unwanted solar heating and
reduce the chances for water to penetrate the gasketed inspection plate
in the base of the instrument.

Mounting material should reduce the heat transfer to the pyranometer
(gratings work well).

**Does it matter *where* I install a pyranometer?**

Provide a free horizon for full solar access all year. Obstructions
should be no higher than 5? above the local horizon, far from any highly
reflective surfaces (window glass or metal walls). Avoid possibilities
of shading by trees, buildings, power poles, roof vents, etc.

**Does the amount of circumsolar (forward scattering around the solar
disk) effect pyranometer measurements?**

Unshaded pyranometers are not affected by the redistribution of direct
or diffuse irradiance. Shaded pyranometers will detect the amount of
circumsolar irradiance ?leaking? past the shade device, typically
occulting a 5? field of view.

**What effects do rapidly changing cloud scenes have on pyranometer
measurements?**

The time constant of the Model PSP is about 1 second for 1/e. The Model
8-48 is 5 seconds for 1/e. Changes in irradiance are detectable if the
radiometer time response is fast enough. Radiometers with detectors
based on the photoelectric effect are very fast (\~10 microseconds), but
have restricted spectral response. Thermoelectric (thermopile) detectors
have broadband spectral response, but are slower (2 to 5 seconds). When
comparing solar irradiance data from different sites or solar irradiance
components from the same site, it is very important to know the
measurement characteristics of the radiometers, especially time
response. SIRS data, based on PSP pyranometers, a NIP pyrheliometer, and
PIR pyrgeometers, include 1-minute averages, standard deviations,
minima, and maxima for the 2-second irradiance samples. These data
attempt to capture the changing radiation fields due to clouds, solar
position, and other factors.

**What is the difference between measurements made with a silicon
detector vs. thermopile detector pyranometer?**

At several locations, ARM has collocated radiometers based on these two
detector types. Their response characteristics are generally quite
different:

[]{#__RefHeading___Toc87239232 .anchor}Table 1.

  --------------------- ------------------------ ------------------------
  **Response            **Thermopile**           **Photodiode**
  Characteristic**                               

  Spectral              280 nm ? 2,950 nm        400 nm ? 1100 nm

  Time Constant         1 sec (1/e)              \~10 microseconds
                                                 (10-90%)

  Angular               ?1% up to 70?, ?3%       3% up to 80? incidence
                        70?-80?                  angle

  Temperature           ?1% from ?20? to +40?C   ?0.15% per degree C
                                                 (max)

  Linearity             ?0.5% up to 2800 Wm^-2^  1% up to 3000 Wm^-2^

  Stability             \< 1% per year           \< 2% per year
  --------------------- ------------------------ ------------------------

# Data Quality

## Data Quality Health and Status

The following links go to current data quality health and status
results.

-   [DQ HandS](http://dq.arm.gov/) (Data Quality Health and Status)

-   [NCVweb](http://dq.arm.gov/ncvweb/ncvweb.cgi) for interactive data
    plotting using.

The tables and graphs shown contain the techniques used by ARM\'s data
quality analysts, instrument mentors, and site scientists to monitor and
diagnose data quality.

## Data Reviews by Instrument Mentor

This section is not applicable to this instrument.

## Data Assessments by Site Scientist/Data Quality Office

All DQ Office and most Site Scientist techniques for checking have been
incorporated within [DQ HandS](http://dq.arm.gov/) and can be viewed
there.

## Value-Added Procedures and Quality Measurement Experiments

Many of the scientific needs of the ARM Program are met through the
analysis and processing of existing data products into \"value-added\"
products or VAPs. Despite extensive instrumentation deployed at the ARM
CART sites, there will always be quantities of interest that are either
impractical or impossible to measure directly or routinely. Physical
models using ARM instrument data as inputs are implemented as VAPs and
can help fill some of the unmet measurement needs of the program.
Conversely, ARM produces some VAPs not in order to fill unmet
measurement needs, but instead to improve the quality of existing
measurements. In addition, when more than one measurement is available,
ARM also produces \"best estimate\" VAPs. A special class of VAP called
a Quality Measurement Experiment (QME) does not output geophysical
parameters of scientific interest. Rather, a QME adds value to the input
datastreams by providing for continuous assessment of the quality of the
input data based on internal consistency checks, comparisons between
independent similar measurements, or comparisons between measurement
with modeled results, and so forth. For more information, see the [VAPs
and QMEs web page](http://www.arm.gov/data/vaps.stm).

# Instrument Details

## Detailed Description

### List of Components

A pyranometer is the lowest component-level device used by an ARM
instrument platform. Please refer to ARCS/GNDRAD, ARCS/SKYRAD, SIROS, or
SIRS for a description of how pyranometers are used in these systems.

### System Configuration and Measurement Methods

The ARM Program uses The Eppley Laboratory, Inc. Model PSP pyranometer
for shortwave hemispheric irradiance measurements. The PSP comprises a
circular multi-junction wire-wound Eppley thermopile which can withstand
severe mechanical vibration and shock. Its receiver is coated with
Parson\'s black lacquer providing non-wavelength selective absorption.
The instrument is supplied with a pair of removable precision ground and
polished hemispheres of Schott optical glass. Both hemispheres are made
of clear WG295 glass which is uniformly transparent to energy between
0.285 to 2.8?m. The instrument has a cast bronze body with a white
enameled guard disk (shield), a spirit level for accurately determining
the horizontal orientation, adjustable leveling screws and a desiccator
which can be readily inspected.

This thermoelectric device (see \"Theory of Operations\" for details)
produces electrical current proportional to the broadband shortwave
irradiance reaching the detector. A shielded, twisted pair, copper wire
signal cable connects the PSP to a data logger. The data logger is
generally programmable and provides 1-minute statistics of analog to
digital conversions made every few seconds (see other instrument
platforms for details, e.g., SIRS).

Pyranometers are located to provide maximum solar access (i.e., the
local horizon is unobstructed to the extent possible). The unshaded
pyranometer is mounted in a ventilator which is in turn secured to a
stable horizontal surface about 1.5 meters above the ground. The
pyranometer/ventilator system is carefully leveled to provide accurate
down welling shortwave irradiance. The ventilator provides a continuous
stream of air over the outer dome of the pyranometer to prevent dew,
frost, or dust accumulation that would effect the radiation reaching the
detector.

A solar tracker is used to position a shading disk or ball above a
pyranometer for down welling diffuse irradiance data. The shading device
blocks the direct (beam) radiation from reaching the pyranometer
detector. Continuously accurate tracker alignment with the sun\'s
position is required for reliable measurements of diffuse (sky)
radiation. In this configuration, the pyranometer is also mounted in a
ventilator for reduced maintenance.

The inverted pyranometer is mounted several meters above the ground,
typically on a meteorological tower mast, over a surface representative
of local ground cover. An inverted pyranometer is generally not mounted
in a ventilator as this orientation naturally protects the instrument\'s
optical surfaces from contamination.

### Specifications

This section is not applicable to this instrument.

## Theory of Operation

This section is not applicable to this instrument.

## Calibration

### Theory

**SKYRAD Shaded PSP**

On a clear day at Momote at noon it should measure about 100 watts per
square meter. The diffuse component will increase when clouds are
present.

**SKYRAD Unshaded PSP**

On a clear day at Momote at noon it should measure about 1100 watts per
square meter. The value will be reduced when clouds are present.

**GNDRAD PSP**

On a clear day at Momote at noon it should measure about 200 watts per
square meter reflected from the grass below.

### Procedures

This section is not applicable to this instrument.

### History

This section is not applicable to this instrument.

## Operation and Maintenance

### User Manual

This section is not applicable to this instrument.

### Routine and Corrective Maintenance Documentation

This section is not applicable to this instrument.

### Software Documentation

ARM netCDF file header descriptions may be found at
[SIRS](../%20http://science.arm.gov/tool/dod/showdod.php%3FInst=sirs)
Data Object Design Changes.

### Additional Documentation

This section is not applicable to this instrument.

## Glossary

Absolute Cavity Radiometer\
Broadband\
Diffuse Irradiance\
Direct Normal Irradiance\
Extraterrestrial Irradiance\
Global Irradiance\
Irradiance\
Radiance\
Shortwave Irradiance\
Solar Constant\
World Radiometric Reference

Also see the [ARM Glossary](http://www.arm.gov/about/glossary.stm).

## Acronyms

AHF: Automatic Hickey-Frieden. The model name for an absolute cavity
radiometer manufactured by The Eppley Laboratory, Inc.

ARCS: Atmospheric Radiation and Cloud Station

BORCAL: Broadband Outdoor Radiometer Calibration. The summation method
of pyranometer calibration as performed at NREL and the SGP/RCF.

GNDRAD: Ground Radiation Instrument Systems. Radiometers positioned to
measure irradiance reflected by and emitted by the natural surface.

NREL: National Renewable Energy Laboratory. One of the eight national
laboratories operated for the US Department of Energy.

PSP: Precision Spectral Pyranometer. The model name for the instrument
manufactured by The Eppley Laboratory, Inc.

RCC: Radiometer Calibration and Characterization. The hardware and
software implementation developed by NREL for automating a BORCAL.

RCF: Radiometer Calibration Facility. Located at the SGP Central
Facility, the RCF is designed to perform calibrations of all broadband
longwave and shortwave radiometers used by the ARM Program. Calibrations
are traceable to international standards through the Metrology
Laboratory and the Solar Radiation Research Laboratory at NREL.

SGP: Southern Great Plains. One of three climate regions under study by
the ARM Program. SGP encompasses southern Kansas and northern Oklahoma.

SIROS: Solar and Infrared Radiation Observing System. Predecessor to the
SIRS radiometer platform for measuring broadband shortwave and longwave
surface irradiance budgets.

SIRS: Solar Infrared Radiation Station. Current radiometer platform used
in the SGP for measuring broadband shortwave and longwave surface
irradiance budgets.

SKYRAD: Sky Radiation Instrument Systems. Radiometers positioned to
measure shortwave (solar) and longwave (atmospheric or infrared) from
the sky dome.

WG295: A type of optical glass with uniformly high transmittance between
wavelengths of 0.285 and 2.8 microns.

WRR: World Radiometric Reference. The international calibration
reference for broadband shortwave irradiance radiometers maintained by
the World Radiation Center in Davos Switzerland.

Also see the [ARM Acronyms and
Abbreviations](http://www.arm.gov/about/acronyms.stm).

## Citable References

Coulson, K.L. 1975. *Solar and Terrestrial Radiation Methods and
Measurements*, Academic Press, New York, New York. ISBN 0-12-192950-7,
pp. 322.

Dutton, E.G., J.J. Michalsky, T. Stoffel, B.W. Forgan, J. Hickey, D.W.
Nelson, T.L. Alberta, and I. Reda. Measurement of Broadband Diffuse
Solar Irradiance Using Current Commercial Instrumentation with a
Correction for Thermal Offset Errors. *J. Atmospheric & Oceanic Tech*.
(Accepted for publication July 2000).

Forgan, B.W. 1996. A New Method for Calibrating Reference and Field
Pyranometers. *Journal of Atmospheric and Oceanic Technology*, Vol. 13,
p.638-645.

Hulstrom, R.L. 1989. *Solar Resources*, The MIT Press, Cambridge,
Massachusetts. ISBN 0-262-08184-9, pp. 408.

Iqbal, M. 1983. *An Introduction to Solar Radiation*, Academic Press,
New York, New York. ISBN 0-12-373750-8 (ISBN 0-12-373752-4 for
paperback), pp. 390.

Marion, \*\*\*\*ASES paper on MPA and cleaning study\*\*\*\*

Maxwell, E.L, and D.R. Myers, \"Daily Estimates of Aerosol Optical Depth
for Solar Radiation Models\" *Solar \'92, Proceedings of 1992 Annual
Conference of the American Solar Energy Society*, Cocoa Beach, Florida,
June 15- 18, 1992, p.323.

McCluney, W.R. 1994. *Introduction to Radiometry and Photometry*, Artech
House, Inc., 685 Canton Street, Norweed, Massachusetts. ISBN
0-89006-678-7, pp. 402.

Reda, I. 1999. *Improving the Shade/Unshade Method to Calculate the
Responsivities of Solar Pyranometers*. NREL/TR-26483.

World Meteorological Organization. 1971. *Guide to Meteorological
Instrument and Observing Practices*. 4^th^ edition, WMO-No. 8, TP. 3,
Geneva, Switzerland. (Note: Most WMO publications are available in the
U.S. from the American Meteorological Society, 45 Beacon St., Boston,
Massachusetts 02108-3693, Telephone: 617-227-2425.)