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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.
