| |
KF5JRV > TECH 14.08.16 15:27l 67 Lines 3720 Bytes #999 (0) @ WW
BID : 248_KF5JRV
Read: GUEST
Subj: Hygrometers
Path: IW8PGT<CX2SA<N0KFQ<KF5JRV
Sent: 160814/1315Z 248@KF5JRV.#NWAR.AR.USA.NA BPQK1.4.65
Measuring Air Humidity
Early hygrometers used hair, whalebone or catgut - materials that stretch when
subjected to humidity - to gauge the dampness of air. The rate of water
evaporation from a wet surface is another way of determining the quantity of
water vapour in the surrounding atmosphere. Humidity plays a large role in how
we 'feel' temperature.
Studied since the fifteenth century, dew-point is the temperature at which
moisture in the air condenses as liquid. An investigation into this phenomena
by the Grand Duke of Tuscany, Ferdinand II (1610-1670), led to the
construction of a condensation hygrometer.
He noticed that during the hot summer months water formed on the outside of an
iced glass depending on the temperature and wind. Curious about the nature of
the atmosphere and the conditions that changed water vapour into liquid,
Ferdinand II invented his instrument. Consisting of a cone-shaped vessel
filled with ice, Ferdinand's crude hygrometer enabled him to conduct
experiments on how temperature and air circulation affected water vapour in
our atmosphere and the conditions that produced dew.
In 1751, Charles le Roy (1723-1789) attempted to chart dew-point by pouring
cold water into a glass container, observing the formation of condensation and
then measuring the water's temperature. The water was then decanted into a
second glass and the process repeated.
Wet and dry bulb hygrometers were more reliable than condensation hygrometers
for measuring air humidity. First developed in 1755 by William Cullen
(1710-1790) and Joseph Black (1728-1799), wet and dry bulb hygrometers
measured humidity based on the evaporation of water.
Cullen and Black noticed that the level of a wet thermometer would drop
slightly as it dried. With further studies, Cullen and Black observed how
evaporation produced a coldness that could be measured. If one thermometer was
kept in a wet muslin sleeve, indicating the temperature at which water
evaporated, and a second thermometer measured normal air temperature, the
difference provided a measurement of humidity. The rate of evaporation and the
amount of cooling was determined by the amount of water vapour already in the
air.
The Swiss savant Horace-Bénédict de Saussure (1740-1799) experimented with a
variety of materials to test air-humidity and found that degreased human hair
was a ready and reliable medium. Damp weather lengthened human hair
whilst dry weather shortened it. The tension produced from the changing
hair-length moved a pointer that indicated humidity along a marked scale.
While hair hygrometers were used for meteorological purposes, they were also
produced for domestic settings. Lambrecht's domestic hygrometer from 1896
functioned as a medical aid, indicating the quality of air inside the home.
In nineteenth-century miasma theory of disease, humidity and air-quality were
closely associated with notions of health. An inscription on the reverse of
the hygrometer suggests further reading to help educate home dwellers on the
dangers of unhealthy airs. Recommended books included Dr Fleischer's Healthy
Air and Dr Wurster's Temperature of the Human Skin and Its Relation to Cold
and Catarrh. To calibrate the hair hygrometer a wet pigeon's feather was used
to moisten the hair and the instrument adjusted to read 95% on the dial.
Evaporimeters
Evaporimeters determined the rate of air evaporation from a wet surface to the
atmosphere, which is another way of measuring air humidity. Not only were
evaporimeters used to study conditions of the atmosphere, but the instrument
became useful for early forms of 'climate control'.
73, Scott kf5jrv
KF5JRV @ KF5JRV.#NWAR.AR.USA.NA
Read previous mail | Read next mail
| |
