2 Vacuum Gas Low

Vacuum Techniques Gas lows ‫یاد آوری‬

Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws • Standard Pressure: 1 atm = 760 torr • Absolute Pressure: pressure relative to a perfect vacuum – e.g. psia (pounds per square inch absolute) – 0 psia = perfect vacuum – used for most vacuum applications – MUST be used for most of the gas law formulas • Gauge Pressure: pressure relative to atmosphere – e.g. psig (pounds per square inch gauge) – 0 psig = 14.7 psia = 1 atm – used for exhaust measurements and compressed gasses Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws • Standard Temperature: 0 oC = 32 oF = 273 K • Absolute Temperature (Kelvins): temperature relative to absolute zero – 0 Kelvins = - 273 oC = - 459 oF (absolute zero) – MUST be used for almost all formulas that involve temperature • Gauge Temperature (Fahrenheit and Celsius): temperature relative to an arbitrary zero point – 0 oC (freezing point of water, standard temperature)

Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws • What are the four measurable qualities of a gas? • Pressure, Temperature, Volume, Mass • Avogadro’s Law: – If two containers contain two DIFFERENT gasses at the same pressure, temperature, and volume, then they also contain the same number of molecules.

• Specifically, if: – Volume of the container = 22.4 liters – Temperature = 0 oC (standard temperature) – Pressure = 760 torr = 1 atm (standard pressure)

• Then: there will be 6.023 x 1023 molecules of the gas (one mole), REGARDLESS of what gas it is. – This number is known as Avogadro’s number (also called a “mole”) Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws • Avogadro’s Law (cont.): • How many molecules of gas will there be in ONE liter at STP? • Will the masses of the two containers be the same ? • No, the mass is proportional to the atomic weight of the gas. – One mole of a gas equals the atomic weight in grams. – What is the mass and volume of one mole of water vapor ? – What is the mass and volume of one mole of liquid water ?

Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws • Combine Boyle’s and Charles’ Laws to get the General Gas Law:

P1V1 P2V2 = T1 T2 – can use ANY units that are ABSOLUTE

Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws • Boyle’s Law: P1V1 = P2V2 ==> as volume decreases, pressure increases – assumptions: temperature is constant, closed non-rigid container – pressure can be in ANY units that are ABSOLUTE (no PSIG) – many vacuum pumps and gauges operate using this principle

Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws

• Charles’ Law:

V1/T1 = V2/T2 ==> as temperature increases, volume increases – assumptions: pressure is held constant due to changing volume – temperature must be in ABSOLUTE units (Kelvins) – this is useful for describing the behavior of gasses in a flexible container

Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws • The final step is the Ideal Gas Law: PV = nRT ==> relates pressure, volume, and temperature in a closed container – – – – –

P = Pressure (torr) V = Volume (liter) n = number of moles of the gas R = a constant (62.4 torr liter/mole K) T = temperature (Kelvins)

• Watch Units!! (must be in units listed above) Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws • Assumptions behind the Ideal Gas Law: – Gas molecules are separated by distances that are very large in comparison to their dimensions. – The molecules exert no force on each other except when they collide with themselves or the walls of the container.

• The Ideal Gas Law works best when there is: – low pressure – high temperature – non-polar gas molecules (not so good for water vapor) Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws • Dalton’s Law states that in a mixture of gasses, each gas exhibits a pressure relative to its percent of the total gas volume. – This is called the “partial pressure” of the gas. • P=PA+PB+PC •

n=nA+nB+nC

• The gas behaves the same as if it were the only gas in the container. – The sum of the partial pressures is equal to the total pressure of the system. • Partial pressure can be used in place of total pressure in all of the previously-discussed gas laws. Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws

• Dalton’s Law:

– What is the partial pressure of carbon dioxide in the atmosphere? Gas – Composition of Dry Air

% of Volume

Nitrogen

78.08

Oxygen

20.95

Argon

0.934

Carbon Dioxide

0.033

Neon

0.0018

Helium

0.000524

Methane

0.0002

Krypton

0.00011

Hydrogen

0.00005

Nitrous Oxide

0.00005

Xenon

0.0000086

Dr. G. Mirjalili, Physics Dept. Yazd University

Gas Laws • • • • • •

Gases and vapors If the material is above it critical temperature TC it is described as gas, below it is vapor. Vapor can be liquefied by compression but vapor can not. in Gas (at temperature above Tc,) compression the gas can not produce condensation . Gas obey the Boyle`s law (P=nkT) Vapor obey the Boyle`s law before saturation conditions. F

F

F Saturated vapor

P
P0 Dr. G. Mirjalili, Physics Dept. Yazd University

Liquid

Gas Laws • .

Vapor

Gas TC

Tc water=647K,

Tc N2= 126K

Dr. G. Mirjalili, Physics Dept. Yazd University

Maxwell distribution V = 8KT

V = 145 T

300K N u b e r

mπ

= 8 RT

600K

Vrms=√8kT/πm V km/s

V=√2kT/m

Dr. G. Mirjalili, Physics Dept. Yazd University

M

Mπ

Dr. G. Mirjalili, Physics Dept. Yazd University

• Impingement rate/flux, J per unit area:

Dr. G. Mirjalili, Physics Dept. Yazd University

Gases in vacuum system • Vacuum system pipe

P, n

pump Chamber

Dr. G. Mirjalili, Physics Dept. Yazd University

Relative Atomic & molecular masses •

Relative atomic masses H2 2

He

C

N

O

Ne

A

4

12

14

16

20

40

Relative molecular masses H2 2

H2O 18

N2 28

O2

CO

32

mH2=2/6.02X1023=3.3X10-27 kg mH2O=18/ 6.02X1023=3.0X10-26 kg

28

CO2 44

Mass of one H2 molecules Mass of one H2O melecules

Dr. G. Mirjalili, Physics Dept. Yazd University