Campbell EPPLEY PSP User manual

EPPLEY PSP
Precision Spectral Pyranometer
Instruction Manual
6/92
COPYRIGHT (c) 1992 CAMPBELL SCIENTIFIC, INC.

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The EPPLEY PSP -- Precision Spectral Pyranometer is warranted by CAMPBELL SCIENTIFIC, INC. to
be free from defects in materials and workmanship under normal use and service for twelve (12) months
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1
EPPLEY PSP
PRECISION SPRECTRAL PYRANOMETER
1. GENERAL
The EPPLEY Precision Spectral Pyranometer
(PSP) is a high quality pyranometer. It is
primarily used where high accuracy is required
or where it is used as a standard to calibrate
other pyranometers.
This manual provides information for interfacing
a CR10, 21X, and CR7 datalogger to a PSP.
An instruction manual provided by EPPLEY
contains the sensor calibration constant and
serial number. Cross check this serial number
against the serial number on your PSP to
ensure that the given calibration constant
corresponds to your sensor.
2. MEASUREMENT INSTRUCTION
The PSP (refer to Figure 1) outputs a low level
voltage ranging from 0 to a maximum of about
12mV depending on sensor calibration and
radiation level. A differential voltage
measurement (Instruction 2) is recommended
because it has better noise rejection than a
single-ended measurement.
If a differential channel is not available, a single-
ended measurement (Instruction 1) is a
possibility. As a test, wire the PSP as shown in
Figure 2 and make single-ended and differential
measurements. Compare results to determine
the acceptability of a single ended
measurement.
Figure 1. PSP Schematic
NOTE FOR 21X USERS: Slight ground
potential differences are created along the
21X analog terminal strip when the
datalogger power supply is powering
external peripherals. If the peripherals draw
30mA or greater, the PSP must be
measured differentially.
INPUT RANGE
An example showing how to determine the
optimum input range for a given sensor
calibration and maximum irradiance follows.
This is an example only. Your values will be
different.
EXAMPLE
-Sensor Calibration: Assume the sensor
calibration is 8.61 microvolts W-1 m2. This
is equivalant to 8.61 millivolts kW-1 m2.
-Maximum Irradiance: A reasonable
estimate of maximum irradiance at the
earth`s surface is 1 kW m-2.
-Input Range Selection: An estimate of the
maximum input voltage is obtained by
multiplying the calibration by the maximum
expected irradiance. That product is
8.61mV for this example. Select the
smallest input range which is greater than
the maximum expected input voltage. In this
case the 15mV range for the 21X and CR7,
and the 25mV range for the CR10 are
selected.
Measurement integration time is specified in the
input range parameter code. A more noise free
reading is obtained with the slow or 60 Hz
rejection integration. A fast integration takes
less power and allows for faster throughput.
MULTIPLIER
The multiplier converts the millivolt reading
to engineering units. Commonly used units
and how to calculate the multiplier are
shown in Table 1.

EPPLEY PSP
2
Table 1. Multipliers and Output Processing
OUTPUT/
UNITS MULTIPLIERS PRCSS
kJ m-2 (1/C)*t (Total)
kW m-2 (1/C) (Averages)
cal cm-2 (1/C)*t*(0.0239) (Total)
cal cm-2 min-1 (1/C)*(1.434) (Averages)
C= (EPPLEY calibration)
t= datalogger program execution
interval in seconds
3. OUTPUT FORMAT CONSIDERATIONS
The largest number that the datalogger can
output is 6999 in low resolution and 99999 in
high resolution (Instruction 78, set resolution). If
the measurement value is totalized, there is
some danger of overranging the output limits,
as shown in the following example.
EXAMPLE
Assume that daily total flux is desired, and the
datalogger scan rate is 1 second. With a
multiplier that converts the readings to units of
kJ m-2 and an average irradiance of .5 kW m-2,
the maximum low resolution output limit will be
exceeded in less than four hours.
Solution #1 - Record average flux density and
later multiply the result by the number of
seconds in the output interval to arrive at total
flux.
Solution #2 - Record total flux using the high
resolution format. The drawback to high
resolution is that it requires 4 bytes of memory
per data point, consuming twice as much
memory as low resolution.
4. CONNECTIONS
Differential and single-ended connections to the
datalogger are shown in Figures 2 and 3,
respectively.
*AG in Figure 2 refers to Analog Ground in the
CR10 which is the same as ground for the 21X
and CR7.
Figure 2. Differential Measurement
Connection
On a differential measurement, the low side of
the signal is jumpered to AG to keep the signal
in common mode range, as shown in Figure 2.
Figure 3. Single-ended Measurement
Connection
*AG in Figure 3 refers to Analog Ground in the
CR10, which is the same as ground for the 21X
and CR7.
5. RESOLUTION
The Eppley PSP outputs approximately 12mV
at 1400 watts meter-2 (solar constant). The
CR10 measures this sensor on the 25mV range
while the 21X and CR7 measure it on the 15mV
range.
An example of the datalogger resolution of the
PSP measurement with a PSP calibration of
8.61 mVw-1m2is given below:
CR10 differential resolution:
= ±0.00333mV * (1w/m2)/(.00861mV)
= ±0.387w/m2

EPPLEY PSP
3
CR10 single=ended resolution:
= ±0.00666mV * (1w/m2)/(.00861mV)
= ±0.773w/m2
21X differential resolution:
= ±0.001mV * (1w/m2)/(.00861mV)
= ±0.116w/m2
21X single-ended resolution:
= ±0.002mV * (1w/m2)/(.00861mV)
= ±0.232w/m2
6. SYSTEM ACCURACY
CR10 accuracy:
0.2% of Full Scale Range
= ±(0.002 * 25mV) * (1 wm-2/0.00861mV)
= 5.81w
21X accuracy:
0.1% of Full Scale Range
= ±(0.001 * 15mV) * (1 wm-2/0.00861mV)
= 1.74w
Eppley temperature dependance:
±1% over range from -20 to +40oC
Eppley linearity:
±.5% over range from 0 - 2800wm-2
Eppley cosine response:
±1% over range from 0 - 70o
±3% over range from 70-80o
System accuracy calculation example:
When solar radiation = 1117wm-2 at 10ofrom
normal (=1100wm-2 at 0o) and the temperature
is 25oC.
System accuracy = square root of the sum of
the squares of the random individual
component accuracies.
=±[(5.8w/m2)2+ (11w/m-2)2+ (5.5w/m-2)2
+(11w/m-2)2].5
=±17.5w/m-2 = ±1.6% of reading