» Asmann Hydrometer (0905)

Immagini

DESCRIPTION

Exhibit 905 is an Asmann hydrometer for measuring the humidity of the air, using two thermometers, one with a humid bulb and the other with a dry bulb. The instrument consists of two mercury thermometers, t₁ and t₂, beside each other protected by a metallic cylinder (see design-1). The bulb of thermometer t₂ is maintained humid by a water-soaked gauze while t₁ is maintained dry. The scales of the two thermometers vary from -28° to +43° with 5 divisions per degree.
The temperature t₁ of the dry thermometer is the ambient temperature, the temperature t₂ of the wet thermometer is lower due to the evaporation of the water contained in the gauze. Evaporation occurs if the air is not already saturated of water vapor and it is more rapid and intense when the air is dry. The heat necessary for evaporation determines a lowering of the temperature of the wet thermometer down to an equilibrium minimum that then remains constant.

Evaporation is accelerated by a fan operated by a clock mechanism located in the upper part which is wound by turning the key and which turns the fan for two minutes with one charge. The drier the air, the greater the cooling due to evaporation.
One proceeds with reading the temperatures when they are stabilized. The difference (t₁ - t₂), called psicrometric difference, is related to the atmospheric pressure Pa in the following way:

Equazione: [P<sub>max</sub> -P_v= kP_a(t_1-t_2)\eqno{(1)}]

Where
P_max = maximum pressure of saturated vapor at the ambient temperature.
P_v = actual pressure of water vapor
P_a = actual atmospheric pressure
K = constant of the instrument.
From (1) one obtains the value of P_v

Equazione: [P_v = P<sub>max</sub> - K P_a(t_1 - t_2)]

The apparatus is conserved in a wood box, together with the key for winding the fan motor.

ISCRIZIONI

Nel retro dei termometri è scritto Centigrade R. Fuess, Berlin-Steglitz N° 5050, sul vetro di uno è scritto, preceduto da un logo: PTR 40321928

BIBLIOGRAPHY

[1] R. Fuess, Duentherstrasse 8, Berlin-Steglitz. - [S.l. : S.n.], [1932?]. - 24 p. : ill. ; 21 cm. + [4] p.
[2] Rolf Seidel, Hugo Noack, "Manuale dell'installatore frigorista", III edizione, Tecniche Nuove, Milano. Pagina 60.
[3] E.Perucca, "Fisica Generale e Sperimentale", Vol.I. Seconda edizione UTET, Torino, 1937. Pg.557-- 558.

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