22
2.3
Properties of the pyrgeometer
The pyrgeometer consists of a thermopile sensor and a silicon window integrated in the CNR 4 body. The
thermopile is coated with a black absorbent coating. The paint absorbs the radiation and converts it to heat. The
resulting heat flow is converted to a voltage by the thermopile. Most electrical specifications are determined by the
thermopile and the resistor.
Spectral specifications are determined by the absorber paint and the window. The window serves both as
environmental protection and as a filter. It only transmits the relevant Far Infrared radiation, while obstructing the
Solar radiation. The upper thermopile has a dome shaped window so that its field of view is 180 degrees, and that
its angular characteristics fulfil the so-called cosine response as much as possible, in this field of view. It causes
water droplets to run of more easily.
The field of view of the lower pyrgeometer is 150 degrees. It is limited due to the use of a flat window. This does
not produce a large error because the missing part of the field of view does not contribute significantly to the total,
and is compensated for during calibration.
There is no international standard that classifies pyrgeometers. Pyrgeometers have two specific properties that
deserve special attention. The first is the so-called window-heating offset; the second is the influence of water
deposition on the window.
2.3.1
Window heating offset
The window heating offset is a measurement error that is introduced by the heating of the pyrgeometer window by
the sun. It only occurs during the day. During a sunny day, the upper pyrgeometer will suffer from this. This error
can be reduced by shading or ventilating. On a sunny windless day with a solar irradiance of 1000W/m
2
, an error of
6 Watts per square meter can be expected. The window will absorb part of the solar radiation and will heat up. As a
result of this heating, heat will irradiate towards the thermopile. This results in an error source, in the Infrared
range. This error is neglected, however, in the net radiation calculation this is justified because the solar radiation is
always dominant when this error occurs. Due to its construction the window heating offset in the CNR 4 is
extremely small compared with other instruments.
2.3.2
Water deposition on the pyrgeometer window
The second specific error source of a pyrgeometer is the substantial measurement error introduced as the result of
water deposition on the window. Water will completely obstruct the transmission of Far Infrared radiation. Water
deposition will occur when it rains, snows, or when dew is deposited.
In the case of rain or snow, the resulting error is not very significant, mainly due to the fact that under these cloudy
conditions, the pyrgeometer signal will be close to zero anyway. The cloud base temperature is generally close to
ambient temperature. The conditions under which dew can form are much more likely to produce significant errors.
A typical situation occurs at night, with a cloudless sky, low wind speeds, and high humidity (so-called clear,
windless nights). Under these conditions, the upward-facing pyrgeometer signal is large, typically -100 Watts per
square metre. When dew occurs, this reading can drop to zero, resulting in a 100 Watts per metre square error.
Generally speaking this kind of error is too large, and if possible it should be avoided. Ventilation and heating can
prevent dew deposition with the CNR 4's optional CNF 4. Heating will keep the instrument window above the dew
point and ventilation will keep the domes clean from rain and snow.
2.3.3
Specifications of the Pyrgeometer
The output of the pyrgeometer is a small voltage, in the mV range. It is proportional to the temperature difference
between the pyrgeometer and the object that it faces. This implies that for calculation of the absolute quantity of
Far Infrared radiation, that is emitted by the sky or the ground you also need to take the pyrgeometer temperature
into account. This temperature is measured by a Pt-100 that is incorporated in the body of CNR 4. The calculation of
the Far Infrared irradiance is described in chapter 1.
Pyrgeometer Specification
Unit
Value
Spectral range
µm
4.5 to 42 (50% points)
