Sensors - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 20

errors can occur, unless the extra heat is dispersed. This forces us to choose a small-
sized resistance device with a quick response or a larger resistance device and better
heat release.
A second solution is to keep the measuring current low (usually between 1 and
5 mA).

2.6 Humidity Sensors

Humidity is the amount of water vapor in the given substance (usually a gas). It is
an important parameter in a variety of fields, including room air humidity in patient
monitoring and exhibit perseveration in museums, meteorological observations,
soil humidity in agriculture, and process control in industrial applications.
Humidity can be measured as the absolute humidity (ratio of water vapor to the
volume of substance), relative (compared to the saturated moisture level) or dew point
(temperature and pressure at which the observed gas starts to turn into liquid). Most
common humidity sensors are based on capacitive, resistive, and thermal conductivity
measurement techniques.

2.6.1 Capacitive RH Sensors

In a capacitive RH sensor, change in dielectric constant is almost directly propor-
tional to relative humidity in the environment. Relative humidity sensors have
three-layer capacitance construction and consist of thermoset polymer, platinum
electrodes, and a silicon chip with integrated voltage output signal conditioning.
These sensors have low temperature coefficient, and response times that range
from 30 to 60 s. They offer near-linear voltage outputs, wide RH ranges and con-
densation tolerance, and are stable over long-term use. However, the capacitive
effect of the cable connecting the sensor to the signal conditioning circuitry is large
compared to the small capacitance changes of the sensor. This limits the distance
from sensing element to signal conditioning circuitry.

2.6.2 Resistive Humidity Sensors

Resistive humidity sensors measure the resistance change in a medium such as a
conductive polymer or a salt. Resistance usually has an inverse exponential rela-
tionship to humidity. Response times of these sensors are 10-30 s.
Resistive humidity sensors are small size, low cost, and are usable from remote
locations.