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Humidity Measurement Principle
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by Editorial Staff
Humidity measurement is an important tool for predicting the climate Select Category
outdoors as well as controlling the climate indoors. Humidity control is
especially important in living, storage, and manufacturing sites.
Definitions
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Scientifically stated, humidity is the measure of water vapor present in a
gas. Vapor is a term that refers to the gaseous form of a substance that Karthikeya on PLC
normally exists as a solid or liquid. When liquid exists as a gas, it exerts Program for Automatic
pressure on its surroundings. This pressure determines the amount of vapor Bottle Rejection System
in the air at a given temperature. This value, known as vapor pressure, Jordan on
varies with temperature and pressure, as well as from substance to Instrumentation and
substance. Water, for example, has a high vapor pressure at temperatures Electrical Teams
near boiling (at the boiling point the vapor pressure = the atmospheric Interactions (Detail
pressure). A low vapor pressure at temperatures below freezing (all solids Design Engineering)
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generally have low vapor pressures; if they did not, they would evaporate). Mehar on PLC Practice
The vapor pressure rises as pressure decreases, which explains why water Questions
boils at a lower temperature in higher altitudes (low pressure). Absolute
Florin Aurel Cheche on
humidity is a measure of the mass of the water vapor present in a specified
Pneumatic Cylinder
volume. Because the mass of water vapor is difficult to measure, a more
Movement Control with
common measurement called relative humidity is used. Relative humidity
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(RH) is the percentage of the amount of water that the air can hold at a
Ben on GET Command –
given temperature. The following equation calculates the percent relative
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humidity.
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Control… to Calibrate Load Cells in
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Programming for
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Where:
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Pa = actual pressure 1. Chlorine dioxide Analyzer
Principle
Ps = saturated pressure
2. Tunable Diode Laser
Relative Humidity is temperature dependent. At 100% RH, the actual Analyzer Working Principle
pressure of the water vapor is equal to the saturation pressure. The
3. Automated Calibration
temperature where this exists is called the dew point. Any cooling below the
dew point causes water to condense. If the atmosphere stabilizes at a 4. Single-beam Non-dispersive
constant humidity during the day, the falling temperatures at night may go Analyzer
below the dew point causing the vapor to condense. The condensing vapor 5. Non-Dispersive Analyzers
creates the phenomenon commonly known as dew. A crucial fact to note is
6. Dual-beam Non-dispersive
that it is only necessary for the sensor to obtain one measurement, absolute
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humidity, RH, or dew point, because the other two can be calculated using
the ambient temperature, graphs or equations. 7. TOC Analyzer Working
Principle
Sensing Methods
8. Gas Analyzers Interview
In general, obtaining a measure of humidity is not an easy task. Many of the Questions and Answers
instruments have poor accuracy, narrow bandwidth, contamination 9. What is Gas Metering
problems, and hysteresis. Some of the instruments are large, awkward, and System?
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expensive pieces of equipment. To compound the problem there are few 10. Chromatograph Sample
highly accurate devices for humidity generation and measurement, needed Valves
for use in sensor calibration. Discussed below, are the different methods of
humidity sensing.
Psychrometric Method
The oldest method for measuring relative humidity is the psychrometric
method. Psychrometry is commonly known as the “wet” and “dry” bulb
method. A psychrometric sensor does not directly sense humidity, but rather
it senses temperature to indirectly find relative humidity. The sensing
elements can be thermometers, RTD’s, or thermistors. The first sensing
element, the dry bulb, measures ambient temperature. The second sensing
element, the wet bulb, is enclosed in a wick saturated with distilled water.
Air forced across the wet bulb creates evaporation, which cools it below
ambient temperature. The amount of evaporation (cooling) is dependent on
the vapor pressure of the air. Using the wet and dry bulb temperatures, the
relative humidity can be looked up on a psychrometric chart. Looking up the
%RH on a chart for every measurement is time-consuming and
cumbersome. With today’s technology, psychrometric charts and dew point
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equations can be stored in a microprocessor, thus making this a direct
sensing method for RH and dew point.
The sling psychrometer dates back to the late 19th century. It used mercury
thermometers for temperature measurement, and swinging the bulb around
created air movement across the wet bulb. Nowadays, the units have built in
fans to ventilate the wet bulb. A psychrometric sensor has good precision
with %RH resolutions of 0.1%, humidity ranges from 10 – 100% at
temperatures from 32oF to 140oF, and accuracy’s of + 2%. The drawbacks of
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a psychrometric sensor are a slow response time and they are substantially
more expensive.
Dew Point Method
Another method for measuring humidity is using a dew point sensor. There
are two common types of dew point sensors, the cooled condensation
surface type, or a solution of saturated lithium chloride. The solution of
saturated lithium chloride does not directly sense relative humidity.
Saturating a wick with resistive electrodes in the solution and an excitation
current through the wick creates joule heating. The heating causes some of
the solution to evaporate, which reduces the resistance and slows heating.
Eventually equilibrium is reached and the temperature of the solution can
then be related to the dew point.
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The cooled condensation surface type detects very accurately the
temperature at which condensation begins. The most common device used
is a mirror to detect for condensation. The system is set up so an LED (light
emitting diode) reflects off the mirror at an angle of about 45 degrees. A
photo-transistor detects the reflected light. Then, the temperature of the
mirror is electronically controlled. The system works by cooling the mirror’s
surface below the ambient temperature until condensation forms. The
condensation on the surface of the mirror causes the LED’s light to scatter.
The scattered light creates a sudden drop in the output of the photo-
transistor. At this point, the surface temperature of the mirror is read using
a temperature sensor such as an RTD or thermistor. This temperature is the
dew point. With a feedback loop, the cooling or heating of the mirror
continuously tracks the dew point. There are a few different designs of
condensation surfaces that are also used. The chilled mirror method is the
most stable and accurate method to determine relative humidity. It is crucial
to keep the mirror clean and ensure the temperature sensor and mirror is of
high quality. This method has the best humidity range (0-100%RH) and can
be used for numerous gases at many pressures. These instruments are
bulky and very expensive.
Hygrometric Method
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The hygrometric method of relative humidity sensing is the most common.
The instruments are generally compact, reliable, and inexpensive.
Hygrometric humidity sensors provide an output that is directly indicative of
humidity. The first humidity sensing elements were mechanical in nature.
Physical dimensions of various materials will change with the adsorption1 of
water. Some examples of these are hair, animal membrane, and some
plastics. To build a sensor from these materials the element is kept in
tension with a spring. A strain gage monitors the displacement caused by a
change in the moisture content of the air. The output of the strain gage is
directly proportional to the relative humidity. A second method of
hygrometry is coating an oscillating crystal (quartz) with a hygroscopic
coating. When the coating adsorbs water the mass changes which then
changes the crystal’s oscillating frequency. A more obscure method is an
electrolytic hygrometer. This method is complicated and not used frequently
enough to warrant explanation.
Recent strides in thin film and micromachining technology make it possible
to produce high quality resistive and capacitive hygrometric sensors. These
sensors have become more accurate, compact, and stable in recent years
making them popular in industry. The materials used to produce these
sensing elements have the ability to change their electrical characteristics
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with the adsorption of water. The materials have changed over the years
from electrolytic salts, to ceramics, to the recently popular polymers. The
new designs in sensing material have overcome many problems. The first
major problem with the sensors was narrow bandwidths. Each individual
sensor was only reliable over a span of 10 to 20% relative humidity.
Numerous manufactured sensors with specific spans cover the entire %RH
span. Then there was a problem when adsorbed water left impurities behind
on the surface. These impurities would change the electrical characteristics
of the sensing materials. The polymers used now have overcome these early
problems.
A capacitive sensor is built like a parallel-plate capacitor. The sensing
element serves as the dielectric. As the moisture in the air changes the
water vapor, the sensing polymer changes with adsorption, resulting in a
dielectric constant change. The dielectric constant is directly proportional to
the capacitance, which is inversely proportional to the %RH. New techniques
in producing thin films have made these, accurate, stable, and easy to
manufacture large quantities.
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The resistive types of hygrometers are the sensors ACI uses in their
products. The sensors consist of placing a thin film of sensing polymer over
a set of comb electrodes. Figure 1 is a representation of the physical parts of
the humidity sensor. The sensor adsorbs water into the humidity sensing
material, which changes the polymers resistance. Other materials adsorb the
water, which changes just the surface resistivity. Because the water is
absorbed the bulk resistance of the polymer changes making the sensor
resistant to surface contamination. The sensor requires an AC excitation
current. So there is no chance electrolysis or separation of the humidity-
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sensing polymer can occur. Thin film technology makes these sensors
accurate, stable, and easy to manufacture. The choice of material assures
they have fast response times with little hysteresis. Another attraction is
their small size and low cost.
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3 thoughts on “Humidity Measurement Principle”
behram baloch
May 16, 2016 at 3:18 pm
nice reading and learning material
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Reply
VJ
May 6, 2017 at 7:31 am
Congratulations Mr. Bharadwaj Reddy for all the wonderful information
that you keep sharing through this portal. I find it very useful for my
teaching profession to Instrumentations students.
Keep up the good work!
Reply
Sukhsevi D.
December 13, 2023 at 7:32 pm
For capacitive sensor : If there is different in comparing with salt
saturation method, how to calibrate :
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1)Possible to calibrate the sensor ?
2) Using correction factor ?
Reply
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