• Version
• 109 Downloads
• 1.07 MB File Size
• 1 File Count
• December 1, 2016 Create Date
• December 1, 2016 Last Updated

National Advisory Committee for Aeronautics, Technical Notes - A Photoelectric Hygrometer

The main requisites of a satisfactory hygrometer'are__ _
reliability, portability, and instantaneous readings. It'is
well known that none of the humidity—measuring methods nowwm
in use satifies all_the foregoing requirements simultaneously.’
One or two of these desiderata usually are sacrificed to se-
cure the third.

With this in mind, a little—exploited method of measur—
ing the humidity has been studied: the change in the inten-
sity of the water vapor infrared absorption bands as the
humidity of.the atmosphere changes. A hygrometer based on
this nrinciple is called a snectral hygrometer. Such a hy—
grometer consists of a source of‘light which sends a beam of
radiation over an air path to a dispersing system which forms
a spectrum of the light source. Ehe radiation then falls up—
on a detecting system. The air path contains the air the
humidity of which is to be measured. The detector is placed
so that one of—the water vapor infrared bands will fall upon
it. In addition, a reference detector is placed in a region
of the spectrum wherein no water vapor bands are located. is
the humiditw.over the air path varies, the detector in the
hand is affected; whereas'the reference detector is not.
Since the absorption is a function of the number of—abeorbing
atoms in the ontioal path, such an instrument measures the
absolute humidity. '

A recent spectral h grometer was constructed by Hand.
(See references 1 and 2. A.sing1e detector, a thermocouple,
without any reference_detectorswas used. The source of light
was"the sun. and the optical path.wae the whole atmosphere;
The 94¢0 angstrom units band was used. Hettner (reference 3)
has'shownfthat the 9440 angstrcm units band is weak compared
to the 13,700 angstrom units band of water vapor. The weak
band could be used because of the tremendous path length.

In order to make the above instrument more practicable
for field‘use, Foskett and Foster (reference 4) used two
thermocouples following the original_development of Fowle
(reference 53gwho used bolometers; The thermocouple outputs
are-sent in opposite directions through a sensitive galvanom-
eterfiuha'changes of readings of which indicate the absolute
humidity over the air path. The air oaths varied from 1 to
50 meters. The 13,700 angstrom units_band was used.