handbook of modern sensors

13
Humidity and Moisture Sensors
13.1 Concept of Humidity
The water content in surrounding air is an important factor for the well-being of humans
and animals. The level of comfort is determined by a combination of two factors:
relative humidity and ambient temperature. You may be quite comfortable at −30 ◦ C
(−22 ◦ F) in Siberia, where the air is usually very dry in winter, and feel quite miserable
in Cleveland near lake Erie at 0 ◦ C(+32 ◦ F), where air may contain a substantial
amount of moisture. 1 Humidity is an important factor for operating certain equipment
(e.g., high-impedance electronic circuits, electrostatic-sensitive components,
high-voltage devices, fine mechanisms, etc.). A rule of thumb is to assure a relative
humidity near 50% at normal room temperature (20–25 ◦ C). This may vary from as low
as 38% for the Class-10 clean rooms to 60% in hospital operating rooms. Moisture is
the ingredient common to most manufactured goods and processed materials. It can be
said that a significant portion of the U.S. GNP(Gross National Product) is moisture [1].
Humidity can be measured by instruments called hygrometers. The first hygrometer
was invented by Sir John Leslie (1766–1832) [2]. To detect moisture contents, a
sensor in a hygrometer must be selective to water, and its internal properties should
be modulated by the water concentration. Generally, sensors for moisture, humidity,
and dew temperature can be capacitive, conductive, oscillating, or optical. The optical
sensors for gases detect dew-point temperature, whereas the optical hygrometers
for organic solvents employ absorptivity of near-infrared (NIR) light in the spectral
range from 1.9 to 2.7 µm [3] (see Fig. 14.18 of Chapter 14).
There are many ways to express moisture and humidity, often depending on the
industry or the particular application. The moisture of gases is expressed sometimes in
pounds of water vapor per million cubic feet of gas. The moisture in liquids and solids
is generally given as a percentage of water per total mass (wet-weight basis), but may
be given on a dry-weight basis. The moisture in liquids with low water miscibility is
usually expressed as parts per million by weight (PPM w ).
1 Naturally, here we disregard other comfort factors, such as economical, cultural, and political.