Главная страница | Контакты | Случайная страница Автомобили Астрономия Биология География Дом и сад Другие языки Другое Информатика История Культура Литература Логика Математика Медицина Металлургия Механика Образование Охрана труда Педагогика Политика Право Психология Религия Риторика Социология Спорт Строительство Технология Туризм Физика Философия Финансы Химия Черчение Экология Экономика Электроника

Determination of the air humidity using the Assmann aspiration psychrometer

The significant disadvantage of August psychrometer is its dependence on the air velocity. The air velocity influences the evaporation intensity and the device’s wet thermometer cooling.

This disadvantage has been eliminated in Assmann psychrometer due to the usage of the ventilator (see fig. 6.2-b). The ventilator produces the constant air movement at the 4 m/sec speed near thermometers’ reservoirs. As a result data does not depend on the air velocity either inside or outside of the premises. Furthermore, thermometers;’ reservoirs of this psychrometer are protected with reflecting cylinders around psychrometer’s reservoirs from the radiant heat.

The cambric of Assmann aspiration psychrometer wet thermometer is dampened using the pipette, the spring of the aspiration devise is set or the psychrometer with electrical ventilator is plugged in. After these procedures the psychrometer is hung up onto the support at the determination point. The data of wet and dry thermometers are taken 8-10 minutes later.

The absolute air humidity is calculated using the Sprung formula:

,

where: А – absolute air humidity in Hg mm;

t – maximum pressure of water vapour at the wet thermometer temperature (see the table of saturated water vapours, table 3);

0.5 – constant psychometric coefficient;

t – temperature of the dry thermometer;

t₁ – temperature of the wet thermometer;

В – barometric pressure at the determination moment in Hg mm.

Relative humidity is determined using the following formula:

,

where: Р –the value of relative humidity to be found, %;

А – absolute humidity, Hg mm;

F – maximum humidity at the dry thermometer temperature, Hg mm (see table 3).

Relative humidity is determined using the psychrometric tables for aspiration psychrometers. The value of the relative humidity is found at the intersection point of the dry and wet thermometer data (see table 5).

Hair or membrane hygrometers are used for the determination of the relative humidity of the air. These devices measure the relative humidity directly. The hygrometer operation is based on the facts, that the degreased hair lengthens, and the membrane/diaphragm weakens when it’s damp, and vice-versa when they are dry (see fig. 6.2-c).

Table 5

Determination of the relative humidity based on the Assmann psychrometer data, %

Table 4

The relative humidity standards for residential, public and administrative premises (abstract from Building Norms and Rules 2.04.05-86)

Note:^* Allowable humidity is 75% for regions with the estimated outdoor air relative humidity more than 75%.

Standards are set for people who continuously stay in premises for more than 2 hours.

Humidity deficit (the difference between the maximum and absolute air humidity) is determined using the table of saturated water vapours. The absolute air humidity, calculated using Regnault or Sprung formulas is subtracted from the value of maximum air humidity according to the dry psychrometer’s thermometer.

Physiological humidity deficit (the difference between the maximum air humidity at 36,5^оС body temperature and absolute air humidity) is determined using the same table of saturated water vapours (see table 3).

Dew point (temperature when the absolute air humidity is maximum) is determined using the same table of saturated water vapours (see table 3) in reverse direction. The temperature when the absolute air humidity is equivalent to the maximum, is found using the value of absolute humidity.

Interdependency between different air humidity indices can be seen on the diagram (see fig. 6.3).

Fig. 6.3. Interdependency between different air humidity indices

The scheme shows, that the rise of temperature provokes the maximum humidity increase in geometric progression, the absolute humidity – in arithmetical progression. When the air temperature rises, the relative humidity is decreases. As a result the amount of water in the air (absolute humidity) is essentially lower in cold seasons than in summer, but is closely related to saturation (maximum humidity). That is why the relative humidity is high in cold seasons and low in summer usually.

The daily temperature, the air humidity and the atmospheric pressure variation are determined using the thermograph, hygrograph and barograph respectively (see fig. 6.4).

Fig. 6.4. Self-recording devices

(а – thermograph; b – hygrograph; c - barograph.)

Electrothermoanemometer is the combined measurement device and is presented on the figure 6.5.

Fig. 6.5. Electrothermoanemometer

(1 – galvanometer; 2 – power switch; 3 – clips for network connection; 4 – sensor plug; 5 – switch for the temperature or air velocity determination; 6 – “measurement-control” switch; 7 – voltage control lever; 8 – sensor (micro-thermistor); 9 – switch protective case.)

Atmospheric pressure is measured using the aneroid barometer. Its scale is graduated in Hg mm or (see fig. 6.6) or in kilopascals.

Fig. 6.6 Aneroid barometer