Measurement of Humidity

Basic Terminology

• Dry Bulb Temperature
• Wet Bulb Temperature
• Dew point
• Absolute Humidity
• Molal Humidity
• Relative/Percentage relative humidity/Relative saturation
• Saturation Humidity
• Percentage/ Percentage absolute humidity/ Percentage saturation

hygrometer

• A hygrometer  is an instrument used for measuring the moisture content in the atmosphere.
• Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed.
• Modern electronic devices use temperature of condensation (the dew point), or changes in electrical capacitance or resistance to measure humidity differences.

Modern hygrometers

• Metal-paper coil type
• Hair tension hygrometers
• Psychrometers (Wet and Dry Bulb thermometer)
• Chilled mirror dew point Hygrometers

Classical hygrometers

• Capacitive hygrometers
• Resistive hygrometers
• Thermal hygrometers

Metal-paper coil type

• The metal-paper coil hygrometer is very useful for giving a dial indication of humidity changes.
• It appears most often in very inexpensive devices, and its accuracy is limited, with variations of 10% or more.
• In these devices, water vapour is absorbed by a salt-impregnated paper strip attached to a metal coil, causing the coil to change shape.
• These changes (analogous to those in a bimetallic thermometer) cause an indication on a dial.

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Hair tension hygrometers

• These devices use a human or animal hair under tension. The hair is hygroscopic (tending toward retaining moisture);
• its length changes with humidity, and the length change may be magnified by a mechanism and indicated on a dial or scale

Psychrometers

Psychrometers (Wet and Dry Bulb thermometer)

• Relative humidity is computed from the ambient temperature as shown by the dry-bulb thermometer and the difference in temperatures as shown by the wet-bulb and dry-bulb thermometers.
• Relative humidity can also be determined by locating the intersection of the wet and dry-bulb temperatures on a psychrometric chart.
• The two thermometers coincide when the air is fully saturated, and the greater the difference the drier the air.

Sling psychrometer

The sling psychrometer, where the thermometers are attached to a handle or length of rope and spun around in the air for a few minutes, is sometimes used for field measurements, but is being replaced by more convenient electronic sensors.

Chilled mirror dew point Hygrometers

• Dewpoint is the temperature at which a sample of moist air (or any other water vapor) at constant pressure reaches water vapor saturation.
• At this saturation temperature, further cooling results in condensation of water.
• Chilled mirror dewpoint hygrometers are some of the most precise instruments commonly available.
• These use a chilled mirror and optoelectronic mechanism to detect condensation on the mirror surface.

Chilled mirror dew point Hygrometers

• The temperature of the mirror is controlled by electronic feedback to maintain a dynamic equilibrium between evaporation and condensation on the mirror, thus closely measuring the dew point temperature.
• An accuracy of 0.2 °C is attainable with these devices, which correlates at typical office environments to a relative humidity accuracy of about ±1.2%.

Chilled mirror dew point Hygrometers

• These devices need frequent cleaning, a skilled operator and periodic calibration to attain these levels of accuracy.
• Even so, they are prone to heavy drifting in environments where smoke or otherwise impure air may be present

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Capacitive Hygrometers

• In capacitive hygrometers, the effect of humidity on the dielectric constant of a polymer or metal oxide material is measured.
• With calibration, these sensors have an accuracy of ±2% RH in the range 5–95% RH. Without calibration, the accuracy is 2 to 3 times worse.
• Capacitive sensors are robust against effects such as condensation and temporary high temperatures.
• Capacitive sensors are subject to contamination, drift and aging effects, but are suitable for many applications.

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Resistive Hygrometers

• In resistive hygrometers, the change in electrical resistance of a material due to humidity is measured.
• Typical materials are salts and conductive polymers. Resistive sensors are less sensitive than capacitive sensors – the change in material properties is less, so they require more complex circuitry.

Resistive Hygrometers

• The material properties also tend to depend both on humidity and temperature, which means in practice that the sensor must be combined with a temperature sensor.
• The accuracy and robustness against condensation vary depending on the chosen resistive material.
• Healthy, condensation-resistant sensors exist with an accuracy of up to ±3% RH.

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thermal hygrometers

• In thermal hygrometers, the change in thermal conductivity of air due to humidity is measured. These sensors measure absolute humidity rather than relative humidity