Humidity measurement

[Ref: DK5:p128-130]

Hygrometer - instruments measuring humidity

• Most instruments measure relative humidity

Classification

• Elements vary with humidity
  * Hair hygrometer
  * Humidity transducer
• Measure temperature
  * Wet and dry bulb
  * Dew point hygrometer
• Removing water
• Mass spectrometer

Hair hygrometer

Principle: Hair gets longer as humidity rises

Mechanism: increased humidity leads to increased breaking of hydrogen bonds which maintains the coiled structure of keratin.

Accuracy: Accurate for relative humidity 30%-90%

Advantage: simplicity

Wet and dry bulb hygrometer

Principle

Relative humidity affects rate of evaporation of water.

Temperature differences between the two bulbs are due to evaporation of water, which depends on the relative humidity.

Setup

2 mercury thermometer. Wet one reads temperature of the wick (drenched in water). Dry one reads ambient temperature.

The bigger the temperature drop, the faster the evaporation, and the lower the relative humidity.

NB:

• Depends on sufficient air movement in the vicinity of the web bulb to prevent a localized increase in humidity

Dewpoint hygrometer

aka Regnault's hygrometer

Principle

Decrease in temperature
--> decrease in saturated vapour pressure

When SVP drops to or below actual vapour pressure
--> condensation forms

NB: The temperature at which this dew first appears is the dewpoint.

Setup

• Diethyl ether in a silver tube.
• Air blown through ether and cooling it.
• Dewpoint found.
• Look up in table the SVP at dewpoint (which would be equal to the actual vapour pressure).
• Relative humidity
  = SVP at dewpoint/SVP at ambient temperature

Other methods

Absolute humidity can also be determined by transducers, based on changes in electrical resistance or in capacitance of a substance as moisture is absorbed.

Mass spectrometry can measure the water content in excess of the saturation.