During the measurement phase, the disk is heated by solar radiation and its temperature rises. The
intensity of this radiation is ascertained by measuring the temperature change of the disk between the
measurement phase and the shading phase with the mercury-in-glass thermometer.
The structure of a thermoelectric pyrheliometer is shown in Figure 7.8. This instrument uses a
thermopile at its sensor, and continuously delivers a thermoelectromotive force in proportion to the direct
solar irradiance. While Angstrom electrical compensation pyrheliometers and silver-disk pyrheliometers
Figure 7.7 Silver-disk pyrheliometer
g
e
f
a
d
c
b
a: Black-painted silver disk
b: Mercury-in-glass thermometer
c: Wooden container
d: Copper case
e: Cylinder
f: Diaphragm
g: Shutter 9
have a structure that allows the outer air to come into direct contact with the sensor portion, this type has
transparent optical glass in the aperture to make it suitable for use in all weather conditions. It is mounted
on a sun-tracking device to enable outdoor installation for automatic operation by JMA.
There are several types of thermoelectric pyrheliometer, but their structures are similar. Figure 7.8
shows the structure of the one used by JMA. Copper-plated constantan wire is used as the thermopile in
the sensor portion, which is attached to the bottom of the cylinder at right angles to the cylinder axis. The
cylinder is fitted with diaphragms to direct sunlight to the sensor portion. It is made of a metallic block
with high heat capacity and good thermal conductivity, and is enclosed in a polished intermediate cylinder
and a silver-plated outer brass cylinder with high reflectivity to prevent rapid ambient temperature changes
or outer wind from disturbing the heat flux in the radiation-sensing element. The cylinder is kept dry using
a desiccant to prevent condensation on the inside of the aperture window.