Humidity and Moisture Sensors
Panca Mudji Rahardjo, ST.MT.
Electrical Engineering - UB
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.
• 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 near50%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.
• Humidity can be measured by instruments
called hygrometers. The first hygrometer was
invented by Sir John Leslie (1766–1832) .
• 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.
• 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 dryweight basis.
• The moisture in liquids with low water miscibility
is usually expressed as parts per million by weight
(PPMw).
• The term moisture generally refers to the
water content of any material, but for
practical reasons, it is applied only to liquids
and solids,
• whereas the term humidity is reserved for the
water vapor content in gases.
• The following are some useful definitions:
• Moisture: the amount of water contained in a liquid or
solid by absorption or adsorption which can be
removed without altering its chemical properties.
• Mixing ratio (humidity ratio) r: the mass of water
vapor per unit mass of dry gas.
• Absolute humidity (mass concentration or density of
water vapor): the mass m of water vapor per unit
volume v of wet gas: dw =m/v . In other words,
absolute humidity is the density of the water vapor
component. It can be measured, for example, by
passing a measured quantity of air through a moistureabsorbing substance (such as silica gel) which is
weighed before and after the absorption.
• Relative humidity: the ratio of the actual vapor
pressure of the air at any temperature to the maximum
of saturation vapor pressure at the same temperature.
Relative humidity in percent is defined as
where Pw is the partial pressure of water vapor and Ps
is the pressure of saturate water vapor at a given
temperature.
• The value of H expresses the vapor content as a
percentage of the concentration required to cause the
vapor saturation, [i.e., the formation of water droplets
(dew) at that temperature].
• An alternative way to present RH is as a ratio of the
mole fraction of water vapor in a space to the mole
fraction of water vapor in the space at saturation.
• Dew-point temperature: the temperature at
which the partial pressure of the water vapor
present would be at its maximum, or
saturated vapor condition, with respect to
equilibrium with a plain surface of ice.
The dew point is the temperature at which
relative humidity is 100%. In other words, the
dew point is the temperature that the air must
reach for the air to hold the maximum amount
of moisture it can. When the temperature
cools to the dew point, the air becomes
saturated and fog, dew, or frost can occur.
Sensor Types and Technologies
• Capacitive RH Sensors
• Capacitive RH sensors are used widely in industrial, commercial,
and weather telemetry applications. They dominate both
atmospheric and process measurements and are the only types of
full-range RH measuring devices capable of operating accurately
down to 0% RH.
• Because of their low temperature effect, they are often used over
wide temperature ranges without active temperature
compensation.
• In a capacitive RH sensor, change in dielectric constant is almost
directly proportional to relative humidity in the environment.
Typical change in capacitance is 0.2–0.5 pF for 1% RH change. Bulk
capacitance is between 100 and 500 pF at 50% RH at 25°C.
• These sensors have low temperature coefficient and can function at
high temperatures up to 200°C. They are able to fully recover from
condensation and resist chemical vapors. Response time ranges
from 30 to 60 seconds for a 63% RH step change.
• Applications for capacitive RH sensors are wide
ranging, including
– Automotive onboard devices such as windshield
defoggers
– Computer printers
– Medical devices such as ventilators and incubators
– Appliances such as microwave ovens, refrigerators,
and clothes dryers
– HVAC
– Recorders and data loggers
– Leak detection
– Weather stations
– Industrial and food processing equipment
– Environmental test chambers
• Advantages (capacitive RH sensor)
– Near-linear voltage output
– Wide RH range and condensation tolerance
– Interchangeable, if laser trimmed
– Stable over long-term use
• Disadvantages
– Distance from sensing element to signal
conditioning circuitry limited
• Resistive Humidity Sensors
• Resistive humidity sensors measure the
impedance change, which usually has an
inverse exponential relationship to humidity.
• Typically, the impedance change of a medium
such as a conductive polymer, salt, or treated
substrate is measured.
• Selecting Resistive Humidity Sensors
• Resistive sensors are small, low-cost humidity
sensors that provide long-term stability and
are highly interchangeable. They are suitable
for many industrial, commercial, and
residential applications, especially control and
display products.
• Advantages (resistive RH sensors)
– No calibration standards, so highly
interchangeable and field replaceable
– Long-term stability
– Usable from remote locations
– Small size
– Low cost
• Disadvantages
– Exposure to chemical vapors and contaminants
may cause premature failure
– Values may shift when water-soluble coatings are
used
• Thermal Conductivity Humidity Sensors
• Thermal conductivity humidity sensors (also
known as absolute humidity sensors) measure
absolute humidity by calculating the difference
between the thermal conductivity of dry air and
air containing water vapor.
• These sensors are constructed using two negative
temperature coefficient (NTC) thermistor
elements in a DC bridge circuit. One of the
elements is sealed in dry nitrogen, while the
other is exposed to the environment.
• The difference in the resistance between the two
thermistors is directly proportional to absolute
humidity.
• Selecting Thermal Conductivity Humidity
Sensors
Thermal conductivity humidity sensors are
commonly used in appliances, including
clothes dryers and microwave ovens. They are
used in many industrial applications including
wood-drying kilns, drying machinery,
pharmaceutical production, cooking, and food
dehydration.
• Advantages
– Very durable
– Work well in corrosive and high-temperature
environments up to 575°F
– Better resolution than capacitive and resistive
sensors
• Disadvantages
– Responds to any gas with thermal properties
different than dry nitrogen, which may affect
measurement.
Reference
• JACOB FRADEN. “HANDBOOK OF MODERN
SENSORS PHYSICS, DESIGNS, and
APPLICATIONS”. T h i r d E d i t i o n. Springer.
2003.
• Jon S. Wilson. “Sensor Technology Handbook”
Newnes. 2005.