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KELEMBABAN UDARA

PENGERTIAN

• Kelembaban udara menggambarkan kandungan uap air di udara

yang dapat dinyatakan sebagai kelembaban mutlak, kelembaban

nisbi (relatif) maupun defisit tekanan uap air.

• Kelembaban nisbi membandingkan antara kandungan/tekanan

uap air aktual dengan keadaan jenuhnya atau pada kapasitas

udara untuk menampung uap air.

• Kapasitas udara untuk menampung uap air (pada

keadaan jenuh) tergantung pada suhu udara

• Defisit tekanan uap air adalah selisih antara tekanan uap

air jenuh dengan tekanan uap aktual.

• Pengembunan akan terjadi bila kelembaban nisbi

mencapai 100 %.

KERAPATAN UAP AIR

Adalah massa uap air per satuan volume udara yang mengandung

uap air tersebut.(kelembaban mutlak)

Ρv = kerapatan uap air (kg m-3)

Mv= massa uap air (kg) pada volume udara sebesar V

V = volume udara (m3)

ρv = mv /V

• Pada daerah lembab seperti di daerah tropis, ρv akan

lebih tinggi daripada daerah dengan temperatur yang

relatif kering terutama pada musim dingin (winter).

• Pada musim dingin kapasitas udara untuk menampung

uap air menjadi kecil

TEKANAN UAP AIR

• Hukum Gas Ideal :

ea = Tekanan uap air (mb)

R = Tetapan gas umum (8.3143 J K-1 mol -1)

T = suhu mutlak (K)

V = volume udara (m3)

• Jumlah mol adalah n = m/Mv dan Mv = 18.016 untuk uap (H2O),

serta ρv = mv /V, maka

Berdasarkan persamaan di atas, maka tekanan uap ditentukan oleh

kerapatan uap air (ρv ) serta suhu udara (T)

ea = mv RT/(18.016 V)

= 0.056 ρv RT

KELEMBABAN SPESIFIK

• Specific humidity (q)

• Perbandingan antara massa uap air (mv), dengan massa udara

lembab, yaitu massa udara kering (md) bersama-sama uap air

tersebut (mv)

q = m/(md + mv)

Nisbah campuran (r) (mixing ratio), massa uap air dibandingkan

dengan massa udara kering

r = mv/md

KELEMBABAN RELATIF

• Relative humidity (RH)

• Perbandingan antara kelembaban aktual dengan kapasitas udara

untuk menampung uap air.

RH = 100 ea / es

ea = kelembaban aktual/tekanan uap air aktual

es = kapasitas udara untuk menampung uap air/tekanan uap jenuh

KELEMBABAN RELATIF (RH)

• Bila RH 100% maka, ea = es

• Es tergantung pada suhu udara (T)

• Makin tinggi T, kapasitas untuk menampung uap air/ es

meningkat.

• Pada ea yang tetap, RH akan lebih kecil bila suhu udara

meningkat, sebaliknya RH makin tinggi bila suhu udara

rendah.

Temperature Degrees

Celsius

Vapor (g) per Kilogram of

Dry Air

50 88.12

40 49.81

30 27.69

20 14.85

10 7.76

0 3.84

Saturation mixing ratio (at 1000 mb).

TEKANAN UAP JENUH

es = 6.1078 e (17.239 T/(T + 273)

• Es = tekanan ua[ jenuh (mb)

• T = suhu Udara (o C)

DEFISIT TEKANAN UAP AIR (vpd)

• Selisih antara tekanan uap air jenuh dengan tekanan uap

aktualnya

• Semakin tinggi defisit uap air, maka udara semakin kering

vpd = es - ea

SUHU TITIK EMBUN

• Suhu titik embun (dew point, Td)

• Pada tekanan uap air (ea) tetap maka pendinginan udara

(suhu udara turun) akan meningkatkan RH sampai 100%.

• Suhu pada waktu tercapai ea = es disebut suhu titik

embun.

SEBARAN KELEMBABAN

• ea yang tetap antara siang dan malam, menyebabkan RH akan lebih rendah

pada siang hari ytetapi lebih tinggi pada malam hari

• RH lebih tinggi pada malam hari dam mencapai maksimum pada pagi hari

sebelum matahari terbit.

• Hal tersebut menyebabkan proses pengembunan bila udara bersentuhan

dengan bidang/permukaan yang suhunya lebih rendah dari suhu titik embun.

• Embun terbentuk pada tempat-tempat yang terbuka atau tidak ternaungi

seperti bagian terluar dari tajuk pohon dan di rumput (tidak terlindungi benda

lain.

• Tempat tersebut memiliki suhu terendah karena paling banyak kehilangan

energi melalui pancaran radiasi gelombang panjang.

PENGUKURAN

• Terdapat beberapa alat pengukuran kelembaban.

• Alat ukur kelembaban disebut psychrometer atau hygrometer.

• Kelembaban juga diukur secara global menggunakan remote yang

diletakkan di staelit.

• Satelit tersebut dapat mendeteksi konsentrasi air di troposfer pada ketinggian

antara 4 hingga 12 kilometer.

• Satelit pengukur uap air memiliki sensor yang sangat sensitif terhadap

radiasi sinar infra merah.

• Uap air secara khusus diserap dan diradiasikan kembali dalam band

spektrum ini.

• Satelit ini sangat penting dalam pengamatan kondisi iklim (seperti formasi

petir) dan dalam peramalan cuaca.

ALAT UKUR

METAL/PULP COIL TYPE

• The familiar metal/paper coil hygrometer is useful for giving a dial

indication of humidity changes, but it appears most often in very

inexpensive devices and their accuracy is very limited.

• A search through many identical units in a display might show

differences in indicated humidity of 10% or more. In these

devices, humidity is absorbed by a salt-impregnated paper strip

attached to a metal coil, causing it to change shape. These

changes in length (analogous to those in a bimetallic

thermometer) cause an indication on a dial.

ALAT UKUR

HAIR TENSION HYGROMETERS

• These devices use a human or animal hair under tension. The

length of the hair changes with humidity and the length change

may be magnified by a mechanism and/or indicated on a dial or

scale. The traditional folk art device known as a "weather house"

works on this principle.

E ELECTRONIC 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 one of the most

precise instruments commonly available. These use a chilled

mirror and optoelectronic mechanism to detect condensation on

the mirror surface.

ALAT UKUR

ALAT UKUR

• 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 ±0.5%. These devices need frequent

cleaning, a skilled operator and periodic calibration to attain these levels of

accuracy.

• For applications where cost, space, or fragility are relevant, other types of

electronic sensors are used, at the price of a lower accuracy. Incapacitive

humidity sensors, the effect of humidity on the dielectric constant of

a polymer or metal oxide material is measured.

ALAT UKUR

• 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.[1] Capacitive sensors are subject to contamination, drift and

aging effects, but are suitable for many applications.

• In resistive humidity sensors, the change in electrical resistance of a

material due to humidity is measured.[1] 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.

ALAT UKUR

• 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. Robust, condensation-resistant sensors exist

with an accuracy of up to ±3% RH.

• In thermal conductivity humidity sensors, the change in

thermal conductivity of air due to humidity is measured. These

sensors measure absolute humidity rather than relative humidity.[

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PSYCHROMETER

• A psychrometer consists of two thermometers, one which is dry and one

which is kept moist with distilled water on a sock or wick. The two

thermometers are thus called the dry-bulb and the wet-bulb. At temperatures

above the freezing point of water, evaporation of water from the wick lowers

the temperature, so that the wet-bulb thermometer usually shows a lower

temperature than that of the dry-bulb thermometer. When the air temperature

is below freezing, however, the wet-bulb is covered with a thin coating of ice

and may be warmer than the dry bulb.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.

ALAT UKUR

• Relative humidity can also be determined by locating the

intersection of the wet- and dry-bulb temperatures on

a psychrometric chart. Psychrometers are commonly used

in meteorology, and in the HVAC industry for proper

refrigerant charging of residential and commercial air

conditioning systems. 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. Alternatively a whirling

psychrometer uses the same principle, however the two

thermometers are fitted into a device that resembles

a Ratchet or football rattle.