Vaisala INTERCAP HMDW80 SERIES Specifikace stáhnout pdf (Strana 9)

If saturation vapor pressure is

reached in air or in a gas mixture,

the introduction of additional water

vapor requires that an equal amount

condenses out of the gas as a liquid

or a solid. A psychrometric chart

shows graphically the relation

between saturation vapor pressure

and temperature. In addition, vapor

pressure tables can be used to see

the saturation vapor pressure at

any temperature, and there are

also a number of computer-based

calculation programs available.

Effect of Pressure

on Humidity

Dalton's law states that a change in

the total pressure of a gas must have

an effect on the partial pressures of

all of the component gases, including

water vapor. If, for example, the

total pressure is doubled, the partial

pressures of all component gases are

doubled as well. In air compressors,

a pressure increase "squeezes" water

out of the air as it is compressed.

This happens because the partial

pressure of water vapor (P

) is

increased, but the saturation vapor

pressure is still only a function of

temperature. As pressure builds in

a receiver tank and P

reaches P

water condenses into liquid and must

ultimately be drained from the tank.

Some humidity measurement instruments allow an individual humidity parameter

to be chosen which can then be followed on a graphical display. This picture shows

different views of the Vaisala HUMICAP Hand-Held Humidity and Temperature Meter

HM70 display.

Relative Humidity

When thinking conceptually of water

vapor as a gas, it's easy to define

relative humidity. Relative humidity

(RH) can be defined as the ratio of

the partial water vapor pressure

) to the water vapor saturation

pressure (P

) at a particular

temperature:

%RH = 100% × P

/ P

Relative humidity is strongly

temperature dependent as the

denominator in the definition (P

)

is a function of temperature. For

example, in a room with an RH of 50%

and a temperature of 20°C, increasing

the temperature of the room to 25°C

will decrease the RH to about 37%,

even though the partial pressure of

the water vapor remains the same.

Pressure will also change relative

humidity. For example, if a process

is kept at a constant temperature,

relative humidity will increase by a

factor of two if the process pressure

is doubled.

Dew Point Temperature

If a gas is cooled and gaseous water

vapor begins to condense in the

liquid phase, the temperature at

which condensation occurs is defined

as the dew point temperature (T

At 100%RH the ambient temperature

equals the dew point temperature.

The further negative the dew point

is from the ambient temperature, the

smaller the risk for condensation and

the drier the air.

Humidity calculators are also available

for mobile phones.

Dew point directly correlates with

saturation vapor pressure (P

The partial pressure of water vapor

associated with any dew point can

be easily calculated. Unlike RH, dew

point is not temperature dependent

but it is affected by pressure.

Typical applications for dew point

measurement include various drying

processes, dry air applications, and

compressed air drying.

Frost Point Temperature

If the dew point temperature is below

freezing – which is the case in dry gas

applications – the term frost point (T

)

is sometimes used to explicitly state

that the condensing phase is ice. The

frost point is always slightly higher

than the dew point below 0°C as the

water vapor saturation pressure of

ice is different to water. People also

often refer to dew point for subzero

values, even though they mean frost

point. Ask for clarification if you are

not certain.

Parts Per Million

Unit parts per million (ppm) is

sometimes used for low levels of

humidity. It is the ratio of water vapor

to dry gas or total (moist) gas, and is

expressed either by volume/volume

(ppm

vol

) or mass/weight (ppm

Parts per million (ppm

vol

) can be

quantitatively expressed as follows:

ppm

vol

= [P

/(P - P

)] × 10

The ppm parameter is typically used

when defining the water vapor content

of pressurized and dry pure gases.

Humidity

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