Humidity

1. Humidity and Moisture
measurement
Sejal Patel
Lecturer IC

2. Define Humidity and Moisture with units
❑Humidity
• Humidity is the presence of water in air.
• Humidity is the quantity of water vapor retained by a gas
Units:
Common measurement units are dew point (°F or °C), grams of water
per cubic meter of air (g/m³)
❑Moisture
• Moisture is often defined as the amount of water absorbed or
adsorbed by a solid or a liquid.

3. Define terminologies for humidity
❑Relative Humidity
• It is defined as the ratio of percentage of how much moisture is in the air
versus how much it can hold at any temperature. It depend upon
temperature and water holding capacity
❑Specific Humidity
• It is the ratio of mass of total water vapor present in 1kg of dry air. unit
g/kg
❑Absolute Humidity
• Absolute humidity, in comparison, measures the mass of water vapor
present in a unit volume of air at a given temperature and pressure. unit
gram/meter cube
Full Air
RH=0%
25%water
RH=25%

4. ❑Dew Point
• The temperature by which unsaturated air can be converted to
saturated air called dew point .
❑Hygrometer
• Hygrometer is the instrument that is used for measurement of the
moisture content of gases

5. Importance of moisture and humidity measurement in process
industries
• Moisture is an unwanted contaminant that exists in industrial gases
and the atmosphere, and is able to penetrate virtually any surface
including such metals as copper, bronze and carbon steel.
• Therefore, it is important to first accurately measure the moisture
content in order to subsequently control or remove the unwanted
moisture.
• By understanding when and how to measure and manage moisture,
we are able to improve product quality, minimize equipment damage,
save energy, reduce costs, and meet contractual obligations.
• The downside of humidity includes condensation, corrosion and
contamination.

6. • Moisture can:
✓Penetrate virtually any surface
✓Render test results useless
✓Result in poor product quality
✓ Cause corrosion in tubing
✓ Lead to ice formation at low temperature
✓ Cause premature wear and equipment failure
✓ React with other chemicals and gases
✓Clearly moisture has the capacity to cause expensive problems
and potentially catastrophic failures
so to avoid all above problem moisture measurement in industry

7. Wet and dry bulb type hygrometer
• 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.

8. • 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
equations can be stored in a microprocessor, thus making
this a direct sensing method for RH and dew point.

10. • Pv=Psat-kPt(td-tw)
• Where, Pv partial vapour pressure
• Psat saturation partial pressure
• PT total pressure (All pressures in heights of Hg)
• td, tw dry & wet bulb temperatures respectively (°F)
• k a constant

11. • Application
❑This type of psychrometer is used in air conditioning system for
maintaining humidity at a specific value.
• Advantage
❑More accurate than other type of hygrometer.
• Disadvantages
❑ slow response when compared with modern types.
❑Need intermediate calculations.

12. HAIR HYGROMETER
Many materials like wood, animal hair, animal membrane,
paper etc are sensitive to humidity.
As humidity changes, their dimension also change. A hair
hygrometer is shown in figure.
It was invented by De Saussure in 1783 and is still being used
in different forms.

14. Principle
• Hygrometers are humidity sensors, The hair hydrometer is a type of
absorption hydrometer and uses the technique of mechanical
moisture detection.
• Principle of Hair hygrometer
• Due to humidity, several materials undergo a change in physical,
chemical and electrical properties.
• This property is used in a transducer designed and calibrated to
directly read the relative humidity.
• Certain hygroscopic materials, such as human hair, animal
membranes, wood, paper, etc., undergo changes in the linear
dimensions when they absorb moisture from the surrounding air. This
change in the linear dimension is used as the measurement of the
humidity present in the air.

15. Construction
• Human hair is used as a humidity sensor.
• The hair is arranged on a parallel beam and separated from each
other to expose them to the surrounding air / atmosphere.
• Number of hairs are placed in parallel to increase the mechanical
strength.
• This hair arrangement is placed under a small tension by the use of a
tension spring to ensure proper functioning.
• The hair arrangement is connected to an arm and a link arrangement
and the link is attached to a pointer rotated at one end.
• The pointer sweeps over a calibrated scale of humidity.

16. Working
• When air humidity is to be measured, this air is made to
surround the hair arrangement and the hair arrangement
absorbs moisture from the surrounding air and expands or
contracts in the linear direction.
• This expansion or contraction of the hair arrangement moves
the arm and the link and, therefore, the pointer to a suitable
position on the calibrated scale and, therefore, indicates the
humidity present in the air / atmosphere.

17. • Absorption of moisture by hair increases its length.
• The amount of moisture it can absorb is dependent on the
temperature and partial vapour pressure of the atmosphere and
hence the extension of the hair is a function of the relative humidity.
• A bunch of hair is used to increase the mechanical strength of the
instrument.
• The element is maintained at slight tension by a spring. Excess of
stress on the hair element may lead to a permanent set in it.
• Compared to hair animal membrane has a longer elongation for the
same relative humidity but the calibration drift is also large.
• Apple wood has also a good extension property particularly if it is cut
at right angles to grain orientation.

18. • Advantages
❑ It can be very cheap.
❑ Used for controlling humidity at specific values.
❑ Range of 25-95% of relative humidity.
❑ 0-70ºC of operating temperature range.
• Disadvantages
❑ Accuracy over a large range is quite poor.
❑ Calibration is needed before use.
❑ Slow response.
❑Aging affects calibration and produces considerable drift.

19. Thin film capacitance type hygrometer method

20. Thin film capacitance type hygrometer method.
• The capacitance (measured in Farads) of a parallel plate capacitor
having two conductor plates can be calculated by:
• where A is the area of the two plates (in meters)
• r is the dielectric constant of the material between the plates, eo is
the permittivity of free space (8.85×10−12 F/m) and
• d is the separation between the plates (in meters).
• For humidity sensing, the area of cross-section and the gap between
the electrodes are fixed and the effective dielectric constant changes
due to change in humidity level

21. Electrolytic hygrometer method
• The principle of measurement utilized involves the
electrolysis of water into oxygen and hydrogen.
• Since two electrons are required for electrolysis of each
water molecule, the electrolysis current is a measure of the
water present in the sample.
• If the volumetric flow rate of the sample gas into the
electrolysis cell is controlled at a fixed value, then the
electrolysis current is a function of water concentration in
the sample.

22. Electrolytic hygrometer method

23. Electrolytic hygrometer cell.

24. • The commercial electrolysis hygrometer cell consists of a small
chamber containing two noble metal electrodes that support a thin
layer of desiccant.
• Moisture in the sample is absorbed by the desiccant and electrolyzed
by means of a voltage-regulated power supply connected to the
electrodes.
• Units are available for use in nonhazardous areas with the sample
flow control, electrolysis cell, and electronics packaged as a single
unit.
• When used in hazardous areas, the cell and flow controller are
housed in an explosion-proof conduit, and the electronic circuitry is
remote-mounted.

25. • Measured air flow (gas) is passed around the platinum
electrodes with phosphorus pentoxide, which absorbs water.
• When voltage is applied to the electrodes of water
decomposes into hydrogen and oxygen.
• Electrolysis current is proportional to the amount of
absorbed water, ie absolute humidity.

26. Infrared absorption hygrometer method
• Water absorbs electromagnetic radiation in the infrared (IR) region
of the spectrum.
• Specifically, infrared radiation of 1.4 and 1.93 µm wavelength is
absorbed strongly by water.
• By measuring the attenuation (decrease of light intensity) of a beam
of this wavelength as it passes through a sample, the moisture
content of the sample can be determined.
• However other factors such as reflection and dispersion of the radiant
energy will contribute to the attenuation.

27. • Therefore, it is necessary to either calibrate these factors out
of the measurement or to use a reference wavelength that is
not absorbed by moisture but is affected by all other factors
to the same extent as the measuring wavelength.
• The difference in the attenuation of the measurement and
reference wavelength is then a function of moisture content
only

29. • The sensing element consists of three groups of
components: an IR radiation source, sample cell, and
radiation detector.
• The radiation source consists of a lamp, filters to pass the
measuring and references wavelengths, and optics to direct
the beam through the glass sample cell.
• The radiation pickup consists of optics to collect the
transmitted radiation and a photocell to convert the
electromagnetic energy to an electric current.

30. • The measuring and reference wavelengths are allowed to
impinge alternately on the photocell so that two sets of
current pulses are produced.
• These pulses are converted into two direct current (DC)
signal levels whose ratio represents the moisture content of
the sample

31. Limitation
• There are several general limitations to the field of
application of this instrument.
• The sample fluid must not be corrosive to the glass sample
cell.
• It must have some minimum transparency for the
measurement and reference wavelengths.
• Additionally, there cannot be any other components present
in the sample that will selectively absorb either the
measuring or reference wavelength.