substances due to moisture absorption, cooling of wet surfaces by evaporation,
absorption of radiation by water or when measuring the temperature of a surface
when dew forms on it. The principles of measuring atmospheric humidity, for the
application of the turbulent covariance method, based on the use of infrared
analyzers and detectors for analyzing the absorption of radiation by water vapor,
were developed in Chap. 3.
Hair hygrometers are based on the principle that moisture causes changes in the
length of human hair strands. Human hair varies by about 2.5% in length, when the
relative humidity ranges from 0% to 100%. Changes in the dimensions of human
hair and other materials are magnified mechanically and transmitted in digital or
analog terms. Air length also varies with temperature and fatigue, so calibration is
critical with this type of equipment. The time constant for this type of hygrometer is
high, in the order of tens of minutes (Campbell 1997).
Methods based on psychrometric relationships are very common. These
methods, consisting of the use of psychrometers are fundamentally based on the
evaporative cooling of a water surface and measurement of the consequent lowering
of temperature, from which the humidity is calculated. In this context, the aspiration
ventilated psychrometer is based on the temperature measurements of the dry and
wet bulb thermometers. The temperature of the humid bulb thermometer, in
conditions of non-saturated air, will always be lower than that of the dry
thermometer and by assuring adequate conditions of ventilation and radiation
shielding, the calculation of partial pressure of water vapor and other moisture
properties, according to the principles mentioned in Annex A2.
For high accuracy demanding operations, such as comparison experiments and
calibrations, dew point and frost point hygrometers are commonly used. Dew point
hygrometers are expensive devices based on the water condensation on a mirrored
surface for direct measurement of the dew point temperature.
The condensation hygrometers use a thermoelectric cooling system to cool the
surface until condensation occurs. A radiative beam, reflected from the mirrored
surface to a photocell, is indicative of the beginning of the condensation process.
The photocell re-emits the signal to the controller of the surface cooling process, so
that the mirrored surface remains at the dew point temperature (Campbell 1997). In
meteorological networks, capacitive sensors are widely used. Hair hygrometers are
still used for temperatures below 0° to which other psychrometers do not have
enough precision (Foken 2017).
Another very common category of humidity sensors is electrical sensors. These
instruments are based on absorption principles, through a volume of material or
surface adsorption, which causes variations in electrical properties such as electrical
resistance or capacity (Fig. A1.10). The resistance units can be coated with
materials such as lithium chloride or aluminum oxide. The capacitive units have a
polymer coating with a mesh, which measures the change in electrical capacity,
with the adsorption or desorption of water, and are also radiation-shielded. The
measurement of variations in resistance or capacity of these sensors, in general,
328
Annex A1: Instrumentation in Environmental Physics
absorption of radiation by water or when measuring the temperature of a surface
when dew forms on it. The principles of measuring atmospheric humidity, for the
application of the turbulent covariance method, based on the use of infrared
analyzers and detectors for analyzing the absorption of radiation by water vapor,
were developed in Chap. 3.
Hair hygrometers are based on the principle that moisture causes changes in the
length of human hair strands. Human hair varies by about 2.5% in length, when the
relative humidity ranges from 0% to 100%. Changes in the dimensions of human
hair and other materials are magnified mechanically and transmitted in digital or
analog terms. Air length also varies with temperature and fatigue, so calibration is
critical with this type of equipment. The time constant for this type of hygrometer is
high, in the order of tens of minutes (Campbell 1997).
Methods based on psychrometric relationships are very common. These
methods, consisting of the use of psychrometers are fundamentally based on the
evaporative cooling of a water surface and measurement of the consequent lowering
of temperature, from which the humidity is calculated. In this context, the aspiration
ventilated psychrometer is based on the temperature measurements of the dry and
wet bulb thermometers. The temperature of the humid bulb thermometer, in
conditions of non-saturated air, will always be lower than that of the dry
thermometer and by assuring adequate conditions of ventilation and radiation
shielding, the calculation of partial pressure of water vapor and other moisture
properties, according to the principles mentioned in Annex A2.
For high accuracy demanding operations, such as comparison experiments and
calibrations, dew point and frost point hygrometers are commonly used. Dew point
hygrometers are expensive devices based on the water condensation on a mirrored
surface for direct measurement of the dew point temperature.
The condensation hygrometers use a thermoelectric cooling system to cool the
surface until condensation occurs. A radiative beam, reflected from the mirrored
surface to a photocell, is indicative of the beginning of the condensation process.
The photocell re-emits the signal to the controller of the surface cooling process, so
that the mirrored surface remains at the dew point temperature (Campbell 1997). In
meteorological networks, capacitive sensors are widely used. Hair hygrometers are
still used for temperatures below 0° to which other psychrometers do not have
enough precision (Foken 2017).
Another very common category of humidity sensors is electrical sensors. These
instruments are based on absorption principles, through a volume of material or
surface adsorption, which causes variations in electrical properties such as electrical
resistance or capacity (Fig. A1.10). The resistance units can be coated with
materials such as lithium chloride or aluminum oxide. The capacitive units have a
polymer coating with a mesh, which measures the change in electrical capacity,
with the adsorption or desorption of water, and are also radiation-shielded. The
measurement of variations in resistance or capacity of these sensors, in general,
328
Annex A1: Instrumentation in Environmental Physics