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PH:512PH:512

MEASUREMENT

TECHNIQUES

PH512: Measurement Techniques 2 0 2 6

Principles of measurement systems; low pressure generation and measurement; low

temperature generation and measurement;

Instruments: X-ray diffractometer, LASER, Spectrometers - FTIR, UV-Vis, near IR,

Raman, Photoluminescence;Raman, Photoluminescence;

Microscopes: Optical, SPM, SEM, TEM;

Magnetic measurement systems: VSM, SQUID,

Thermal measurement system: DSC,

Resonance Spectroscopy: ESR, NMR; Optical spectrum analyzer.

Scientific seminar on related topics

Texts / References:

1. A. D. Helfrick and W.D.Cooper, Modern Electronic Instrumentation

and Measurement Techniques, PHI (1996).

2. J. P. Bentley, Principles of measurement systems, Pearson Education

Ltd, England (2005).

3. A. S. Morris, R. Langari, Measurement and Instrumentation: Theory

and Application, Academic Press, London (2012).

4. G.C.M. Meijer, Smart Sensor Systems, John Wiley & Sons Ltd, UK

(2008).(2008).

5. A. Ghatak and K.Thyagarajan, Optical Electronics, C.U.P. (1991).

6. D. A. Skoog, F. J. Holler and T. A. Nieman, Principles of Instrumental

Analysis, Saunders Coll. Publ. (1998).

7. H. J. Tichy, Effective Writing for Engineers, Managers, Scientists,

John Wiley & Sons (1988).

8. M. Alley, The Craft of Scientific Presentations: Critical Steps to

Succeed and Critical Errors to Avoid, Springer-Verlag New York (2003).

Exam Quiz-1 Mid Sem Quiz-2 End Sem

Date 26/08/2015 23/09/2015 28/10/2015 28/11/2015

Exams & Evaluation

Date 26/08/2015 23/09/2015 28/10/2015 28/11/2015

Marks 10 30 10 50

20/Lab reports 20/Scientific Seminars

+ Attendance 10

50

For the course related information, look in to

https://intranet.iitg.ernet.in/moodle/https://intranet.iitg.ernet.in/moodle/

Use your IITG e-mail user ID and password

In 2014

1,000,000,000,000 = one trillion

INTRDUCTION TO VACUUM

The word "vacuum" comes from the Latin "vacua", which means "empty". However,

there does not exist a totally empty space in nature, there is no "ideal vacuum".

Vacuum is only a partially empty space, where some of the air and other gases have been

removed from a gas containing volume ("gas”comes from the Greek word "chaos" =

infinite, empty space).

In other words, vacuum means any volume containing less gas particles, atoms and

molecules (a lower particle density and gas pressure), than there are in the surroundingmolecules (a lower particle density and gas pressure), than there are in the surrounding

outside atmosphere. Accordingly, vacuum is the gaseous environment at pressures below

atmosphere.

The atmospheric air around us is said to contain nearly 2.5x1019 molecules for every cubic

centimetre space.

Any given space having molecular density less than this is said to be under vacuum

conditions. The technology dealing with the production of such reduced pressure

environments using different scientific concepts is known as vacuum technology.

Why is a Vacuum Needed?

Contamination

(usually water)Clean surface

Atmosphere (High)Vacuum

(usually water)

To provide a clean surface

• A vacuum provides a clean environment

– Devoid of possible contamination from other

gases that may be present from the

atmosphere

Why Might We Need Vacuum?

atmosphere

– Devoid of particles that may react with physical

processes that are intended to take place

– Devoid of pressure that may limit restrict a

desired physical process

A vacuum environment provides a “clean slate” for chemical processes. Once the

nitrogen, oxygen, and other gases from the air have been removed from a

vacuum chamber, known amounts of specific gases can be injected into the

chamber for chemical processing to occur.

In the presence of atmospheric gases, the particles of matter present can affect

certain manufacturing and imaging processes

Some physical processes will not occur at acceptable temperatures or will

proceed much more slowly in the presence of atmospheric pressureproceed much more slowly in the presence of atmospheric pressure

The nanotech industry uses various levels of vacuum in the processing,

visualization, and analysis stages of manufacture.

In order to understand how a vacuum system works, it is necessary that the

learner understand the basics of gases and gaseous environments.

Once the basics of gases are understood, an understanding of the operation of

the components that make up a vacuum system such as pumps, gauges, and

materials will follow.

Why do we want to learn about a vacuum?

Vacuum technology is widely used in a variety of industries. Here are some applications you would

have certainly heard about:

i) An early application of vacuum technology came around 1900 when the first major industrial use

was for light bulbs and TV tube production (later on). It has been shown that filaments emit

electrons under vacuum which is the major property used in television technology.

ii) The second major application is in the electronic industry. Many processes that occur in a

semiconductor fabrication facility require vacuums of different levels, including the deposition

of thin films of material on computer chips.

iii) Another major application is in space technology. The main issue in space technology is how to

design the space station or shuttle in order to maintain a pressurized cabin. Also, it is

important to design safe space-suits to protect astronauts during their missions in open space.

These are examples of how vacuum technology helps us. Now we will proceed to learn about

vaccum and how it is created and measured.

Classification of vacuum ranges

Low vacuum (viscous flow) region: The number of gas molecules in the enclosed volume is much

larger compared to the dimensions of a given chamber. While 760 – 0.5 Torr is known as low

vacuum. The mean free path of the gas molecules is very small at atmospheric pressure so that the

flow of the gas is limited by its viscosity.

0.5 to 10-2 Torr is referred to as medium vacuum.

In this range, the mean free path of the molecules is similar to the dimensions of the vacuum

enclosure, the flow of the gas is governed by viscosity as well as by molecular phenomena; this is

the intermediate flow.the intermediate flow.

High vacuum: The gas molecules in the system are located principally on the surfaces as the mean

free path is larger than the characteristic dimensions of the chamber. Particles would freely travel

in the vacuum enclosures colliding more with the walls of the chamber than with other molecules.

The flow of gas under such conditions is molecular. The range extends from 10-3 to 10-7 Torr

Ultra high vacuum: This range covers the pressures from 10-7 to 10-16 Torr. Where the time to

form a monolayer is longer than the usual time for laboratory measurements. Thus clear surface

can be prepared and their properties can be studied. In this range the associated flow is purely

molecular.

Viscous and Molecular Flow

Viscous Flow

(momentum transfer

between molecules)

Molecular Flow

(molecules move

independently)

FLOW REGIMESViscous Flow: Distance between molecules is small; collisions between molecules

dominate; flow through momentum transfer; generally P greater than 0.1 Torr

Intermediate Flow: Region between viscous and molecular flow

Molecular Flow: Distance between molecules is large; collisions between molecules and

wall dominate; flow through random motion; generally P smaller than 10 Torr

Mean Free PathMean Free Path

Characteristic DimensionViscous Flow: is less than 0.01

Mean Free Path

Characteristic DimensionMolecular Flow: is greater than 1

Mean Free Path

Characteristic DimensionIntermediate Flow: is between 0.01 and 1

Two dimensionless parameters, namely Reynolds number and Kundsen number,

are usually used to define the various flow regimes.

For fluid flow through cylindrical pipe in viscous regime, the Reynolds number is

defined as Re=ρVD/μ, where ρ is the density, V is the velocity, D is the diameter

of the pipe and μ is the viscosity of the gas.

For Re>2100 (high pressure) the flow is entirely turbulent

For Re<1100, the flow is entirely laminare

Similarly the Kundsen number (Kn=λ/D) defined as the ratio of mean free path of

a molecule to the diameter of the pipe can be used to distinguish different flow

regimes as:

1/Kn= D/λ>110 corresponds to viscous / turbulent flow

1< D/λ<110 corresponds to intermediate (laminar flow) flow

D/λ<1 corresponds to molecular flow.

Production of vacuum

Classification of the pumps: Because of natural inherent limitations, no single vacuum pump

exists which cover the pressure range from 760 to 10-13 Torr. Hence, depending on the

ultimate pressure needed in the vacuum system, combinations of different types of vacuum

pumps have to be used for different applications. These pumps are classified according to

their nature of operation as described below.

i) Positive displacement vacuum pumps: In these pumps the gas is displaced by varying

the volume of the vacuum chamber at a regular interval and is exhausted directly tothe volume of the vacuum chamber at a regular interval and is exhausted directly to

atmospheric pressure. Rotary oil sealed pumps and roots blower pumps fall under this

category.

ii) Ejectors: Here the pumped gas is entrained in a jet of the working medium. The diffusion

pump falls under this category.

iii) Molecular vacuum pumps: Here pumped gas molecules are imparted an additional

speed in a specified direction. Turbo-molecular pumps with a rotation speed of about

40,000 revolutions per minute fall under this category.

iv) Sorption vacuum pumps: here the gas is pumped by either physio-sorption

or chemi-sorption on to the surface of a sorbing material such as molecular

sieves, activated charcoal, titanium surface and other getter materials.

v) Ion pumps: these pumps work on the principle of ionization of the gas and

further directing the ions towards the neutral surfaces where ions are

adsorbed or trapped. Ion pumps are of different types such as evaporation-

ion pumps and sputter-ion pumps depending on the mode of generating

the adsorbing agent.

vi) Cryogenic vacuum pumps: in these pumps the gases and vapours are

frozen in low vapor pressure species in refrigerated cryo- panels. Cryo-

condensation and cryo-sorption pumps fall in this category.

Rotary Vane Pump

A rotary vane vacuum pump is an oil-sealed rotary displacement pump. The

pumping system consists of a housing (1), an eccentrically installed rotor (2), vanes

that move radially under spring force (3) and the inlet and outlet (4).

The outlet valve is oil-sealed. The inlet valve is designed as a vacuum safety valve

that is always open during operation.

The working chamber (5) is located inside the housing. Rotor and vanes divide theThe working chamber (5) is located inside the housing. Rotor and vanes divide the

working chamber into two separate spaces having variable volumes.

As the rotor turns, gas flows into the enlarging suction chamber until it is sealed off

by the second vane.

The enclosed gas is compressed until the outlet valve opens against atmospheric

pressure. In the case of gas ballast operation, a hole to the outside is opened, which

empties into the sealed suction chamber on the front side.

Fig. 1

Operating fluid, oil

Pump oil, which is also called as operating fluid, has multiple tasks to perform in a

rotary vane pump. It lubricates all moving parts, fills both the harmful space under

the outlet valve as well as the narrow gap between inlet and outlet. It compresses

the gap between the vanes and the working chamber and additionally ensures an

optimal temperature balance through heat transfer.

Operation of the transfer pump

Multi-stage pumps

Rotary vane vacuum pumps are built in single- and two-stage versions. Two-stage pumps

achieve lower ultimate pressures than single-stage pumps. Moreover, the effects of the gas

ballast on the ultimate pressure are lower, as the ballast gas is only admitted in the second

stage.

Vacuum safety valve

Depending upon the type of pump in question, rotary vane vacuum pumps can be equipped

with a vacuum safety valve. The vacuum safety valve disconnects the pump from the

vacuum recipient in the event of intentional or unintentional standstill, and uses the

displaced gas to vent the pumping system in order to prevent oil from rising into the

recipient.

After switching on the pump, it opens after a delay once the pressure in the pump has

reached the approximate pressure in the recipient.

These pumps are effective in the range from atmospheric pressure down to 10-3 Torr.

Mechanical pumps are often used as roughing pumps for other types of pumps, like

diffusion pumps.

Functions of Oil

• Seals

– Oil surface tension seals the duo-seal

– Fills gaps between the vanes, rotors & stators

• LubricatesLubricates

– Bearing areas and blade contact surfaces

• Cools

– Moves heat from rotors & stators to the oil box

• Protects parts from rust and corrosion

–Coats surfaces to protect from aggressive gas

Diffusion Pumps

Diffusion pumps are vapour jet pumps or vapour ejector pumps designed for pumping

rarefied gases in the high vacuum range (<10-2 Torr) of pressures. These are called the

diffusion pumps because of the fact that the molecules of the pumped gas penetrate the

vapour jet in a manner resembling the diffusion of one gas into another.

High vacuum systems based on diffusion pumps include at least one diffusion pump and

one rotary pump connected in series. The rotary pump remove about 99.99% of the air

from the chamber. The remaining air and water vapor, down to any pressure from 10-3 to

10-9 Torr is removed by the diffusion pump discharging into the rotary pump.

Diffusion pumps are used when constant high speeds for all gases are desired for long

periods of time without much attention.

Diffusion pumps can not discharge directly into the atmosphere. A rotary pump is required

to reduce the pressure in the vacuum system to the correct operating range. The rotary

pump is then used to maintain proper discharge pressure conditions for the diffusion

pump.

Fig.2 illustrates the working mechanism of a diffusion pump. The arrows represents the pumping fluid

molecules and the small circles represent gas molecules being pumped. A pumping fluid of low vapor

pressure is boiled in the boiler.

The oil vapor flows up through the jet chimneys, reverses its direction at jet caps and emerges out

(downwards) from the nozzles at supersonic velocity.

The oil molecules condense on the pump walls which are water cooled and flow in the form of an oil

film, back down to the boiler where the oil is reboiled and evaporated.

Gas molecules present in the chamber above the jet assembly diffuse into the vapor stream where they

are given a downward momentum due to collisions with heavier oil molecules. Thus the gas moleculesare given a downward momentum due to collisions with heavier oil molecules. Thus the gas molecules

are forced by the jet into a region of higher pressure in the region of lower section of the diffusion pump.

The pressure is high enough for the backing rotary pump to have a finite pumping speed so that

accumulated molecules are drawn off through the for vacuum line.

Diffusion pumps operate only at quiet low pressures. The backing pressure usually required is 10-1 to 10-2

Torr. The ultimate vacuum attainable depends upon the vapor pressure of the pump liquid at

temperature of the condensing surfaces.

However, by providing a cold trap between the diffusion pump and the region being evacuated,

pressures far below the vapor pressure of the liquid may be achieved.

Fig.2

Previously mercury was used as the working fluid.

The advantages of mercury are: it does not decompose and it does not contribute to the

non condensable gas load to the system. Species of unknown molecular weight are not

introduced into the system and hence mass spectrometer analysis is not complicated.

Further, mercury does not react with most other gases or with hot filament.

Some disadvantages: It has smaller molecular weight, less effective as a pump fluid, its

toxic and reacts with metals.

At present silicone oils DC 704 are used due to its high resistance to oxidation atAt present silicone oils DC 704 are used due to its high resistance to oxidation at

elevated temperatures.

Back streaming: The transfer of diffusion pump fluid beyond pump flange mainly

depends on the type of oil and design of the pump. This back streaming can be

minimized by properly designed trap but at the expense of pumping speed. By placing

the LN2 traps above the diffusion pump can improve the performance of the pump in

two ways: This trap acts as a barrier for the low condensible vapors from pump to the

system and also acts for condensible vapors emanating from the system.

The pumping speed of a diffusion pump varies with the pressure for different gases. The

pumping speed for a diffusion pump is constant in its working range and limited by the

pump mouth diameter.

since the mouth diameter determines the number of molecules per second entering the

pump and getting entrained in the supersonic jets.

Diffusion pumps are commercially available with speeds ranging from 100 litrs/ sec to

45,000 litrs/ sec.

The speed of a vapor diffusion pump is higher for light gases such as H and He.The speed of a vapor diffusion pump is higher for light gases such as H and He.

The speed varies inversely with square root of M, where M is the molecular weight of the

gas being pumped.

A simple working formula based on the kinetic theory of gases to get the pumping speed

of a diffusion pump at 20oC for air is given by

S=11.6AH litre/sec where A is the area of the intake annulus (in cm2) around the top jet

cap and H is the Ho-factor whose value is in the range of 0.3 to 0.5.