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Wednesday, December 24, 2014

What is outgassing in vacuum science?

The generation of gas resulting from the desorption is known as outgassing. The outgassing constant is defined as the rate at which gas appears to emanate from unit area of surface, and is usually measured in units of Torr.Liter.Sec-1.Cm-2.     

If the temperature of the material is raised (baking), the outgasssing rate rises to a peak value.

Together with the acceleration of desorption, heating may also have the effect of causing activated chemisorption of physically adsorbed gas (in particular water vapour), which can then be desorbed only by prolonged heating at much high temperatures.


Chemi-adsorbed water vapor continues to be evolved at temperatures in excess of 300 degC. It should therefore appear that a degassing programme should begin with pumping at room temperature to remove physically adsorbed water vapor, before baking is commenced.

What is desorption?

When a material is placed in Vacuum, the gas which was previously adsorbed begins to desorb i.e. to leave the material.

The desorption is influenced by

1.       Pressure
2.       Temperature
3.       Shape of material
4.       Kind of its surface

The pressure has a basic influence on the desorption phenomenon since according to its tendency of increasing over or decreasing below the equilibrium , the phenomenon of sorption or that of desorption appears.

The temperature has a clear influence on desorption phenomena. Desorption is endothermic, thus it is accelerated by increase of temperature.

The shape of the material influences desorption either if the gas is adsorbed or absorbed.


If the gas is adsorbed, then only the amount of surface is the influencing factor, but if the gas has to diffuse from the interior of the material to the surface, then the third dimension “thickness” is also influencing the rate of desorption. 

ABOUT ELECTROLYTIC CAPACITORS

They provide more capacitance for their size than any other type. Electrolytics all have one thing in common, instead of usual plates separated by a dielectric, the electrolytic capacitor has a metallic anode coated with an oxide film. This outer covering is the dielectric, and a liquid electrolyte acts as a cathode. A second metallic conductor serves primarily as the connection to the liquid cathode, providing an external termination.
In actual practice, porous paper is wrapped around the anode and saturated with the electrolyte to eliminate the spillage problem.

There are two common types of electrolytic capacitors: a)  Aluminum  b) Tantalum

Both employ the same basic principle. The aluminum or Tantalum anode is covered with an oxide film. A suitable liquid or solid electrolyte is the cathode. The Aluminum type is by far the most popular because of its lower cost. The aluminum –oxide film has a very high resistance to current in one direction and has very low resistance to current in opposite direction. In other words, film acts as a dielectric in first instance and as a plate in second case. Because of this electrolytic are polarized. If the designated polarity is not observed, the oxide film on anode will breakdown and migrate to cathode connection, resulting in prompt failure of capacitor.

Electrolytics are described as belonging to one of the three basic families;

Polarized type
This type has one anode, the liquid cathode and a cathode connection. Polarity must be observed.

Semi polarized type
In many energy storage applications a certain amount of current reversal is often encountered. In such cases this type is recommended. In this type, the primary anode has a relatively thick oxide surface. The cathode connection now becomes the secondary anode with a thin oxide surface. Also liquid cathode has a slightly different chemical composition.

Non-polarized

This type is used in audio cross over networks and ac motor starting applications. Here there is a complete reversal of polarity; therefore, two anodes are required. The cathode connection now becomes a second, and equal, anode. Obviously size will be affected. Infact, the third type called nonpolarized is just twice as large as a polarized type of equivalent capacitance and voltage rating.

Monday, December 22, 2014

MICA CAPACITORS

Mica is popularly known as Isin glass. Important characteristics of Mica:

i) Its ability to operate at very high temperature (upto 500 oC).
ii) The material is almost totally inert and will not change with age, either chemically or physically.
iii) Mica is usable as a dielectric in its naturally state.
iv) It can be readily split into very thin sheets.

Mica capacitors are made by a method of depositing a thin layer of silver on each side of sheet of Mica was developed. This is done by a type of silver screen process. The silver is then fired in a furnace.

The principal advantage of Mica capacitor is its excellent degree of stability over a wide range of operating temperatures.

Mica capacitors are also among the best types to use where radio frequencies are involved.

Disadvantages of Mica capacitors:

i) They are relatively bulky when compared to other capacitors on a pure capacitance vs volume basis.


ii) The flat plates and method of construction lead to resonant frequency problems in some circuits.

Tuesday, August 19, 2014

What is a relay?

A relay is an electromagnetic switch. An actuating current passed in a coil operates one or more galvanically separated contacts. In fact it is a remote controlled switch capable of switching multiple circuits, either individually, simultaneously or in sequence.    

The most widely used type of relay is electromechanical relay. In electromechanical relays the switching element is a mecahncial contact, actuated by an electromagnet. This is the msot widely used type of relay design. The principal internal functions of the electromechanical relay are:

  1. Conversion of electrical current into  magnetic field
  2. Conversion of magnetic field into a mechanical force
  3. This Mechanical force operates the contacts
  4. Contacts switch and conduct electrical current
Applications of Relay

Typical applications of relays are as follows:
  • Laboratory instruments
  • Telecommunication systems
  • Domestic appliances
  • Traffic control
  • Control of motors & solenoids
  • Air conditioning & heating

Natural Radioactivity in Soil (Bq.Kg-1)


Country
Type of Radioactive Isotope

K-40
U-238
Ra-226
Th-232
India
400
29
29
64
China
440
33
32
41
Japan
310
29
33
28
USA
370
35
40
35
Russia
520
19
27
30
Spain
470
NA
32
33

Mean values are given from wide range of values
Courtesy: UNSCEAR, 2000

Natural Background Radiation in Various Cities of India


City
µGy.yr-1
Cosmic
Terrestial
Total
Mumbai
280
204
484
Kolkata
280
530
810
Delhi
310
390
700
Chennai
280
510
790
Bangalore
440
385
825


As one shifts from Mumbai to Delhi, he is going to get 216 µGy additional natural radiation dose which is 10 times more than that from a nuclear power plant.

WE LIVE IN NATURALLY RADIOACTIVE WORLD

We are exposed to radiation from the sun and outer space, also from the naturally occurring radioactive materials present in the earth, the house we live in, the buildings where we work, the food and drink we consume.

Where we live in
The houses are made up of materials which contain radioactivity. Gamma radiation from walls, floor & ceiling, and Radon and Thoron progeny are major sources of radiation exposure. Especially in closed rooms, Radon is the significant dose contributing factor.

Natural Radioactivity (Bq.Kg-1) in Building Materials used in India
Material
Type of Radioactive isotope
K-40
Ra-226
Th-232
Cement
5-385
16-377
8-78
Brick
130-1390
21-48
26-126
Stone
48-1479
6-155
5-412
Sand
5-1074
1-5047
4-2971
Granite
76-1380
4-98
103-240
Clay
6-477
7-1621
4-311
Fly Ash
6-522
7-670
30-159
Lime stone
6-518
1-26
1-33
Gypsum
70-807
7-807
1-152

We are always in natural background radiation*
Cosmic-from the sun and outer space – 0.4mSv.Yr-1
Terrestrial – from the earth’s crust – 0.5mSv.yr-1
Radon – from decay of Uranium/radium – 1.2mSv.yr-1
Internal sources in the body(eg:  40K)  --- 0.3mSv.yr-1

Total Dose from Natural Sources: ----2.4 mSv.Yr-1

What we drink
Radioactivity in milk  is over 200 times more than in drinking water and 3 times that in beer. 

From a cup of milk , 180 beta particles of Ptassium-40 are emitted per minute

From a cup of tea, 91 beta particles of Potassium-40 are emittd per minute from tea leaves(excluding milk and water)

What we eat

Food stuff
K-40 (Bq.Kg-1)
Rice
40-90
Leafy vegetables
80-220
Brinjal
90-140
Carrot
60-120
Beetroot
90-120


Radiation Doses from Cosmic Rays

Distance above Ground
Dose Rate in µSv. h-1
~ 15 Km above ground
10
~ 10 Km above ground
5
~ 8 Km above ground
2
~ 2 Km above ground
0.1
Sea Level
0.03

Courtesy: Radiation Safety, IAEA (1996), NRPB, UK

Comparison Between Wastes From A Nuclear Reactor & Coal Based Thermal Power Plant [1000 Mwe Each]



Thermal Power Plant

Nuclear Power Plant

Ash:
320,000 tonnes

High Level:
27 tonnes spent fuel or 3 Cu.m after reprocessing
CO2:
6.5 Million tonnes

SO2:
44,000 tonnes

Intermediate level:
310 tonnes
NO2:
22,000 tonnes

Low Level:
460 tonnes

Tuesday, August 12, 2014

Units of Radioactivity

------------------------------------------------------------------------------------------------------------
        Physical Quantity                               SI Unit                                 Non-SI Unit
------------------------------------------------------------------------------------------------------------
             Activity                                       Becquerel                                Curie (Ci)

        Absorbed Dose                                 Gray                                        RAD

        Dose Equivalent                                Sievert                                     REM

        Exposure                                         Coulomb/Kg                              Roentgen
------------------------------------------------------------------------------------------------------------

BF3 Properties


  1. Molecular Weight  : 67.82
  2. Density of gas       : 3.06 gm / litre at 0 deg C, 760mm of Hg 
  3. Density of Liquid  : 1.68 gm/cc @ -128 deg C
  4. Density of solid     : 1.87 gm/cc
  5. Melting Point        : -128 deg C
  6. Boiling Point         : -100 deg C
  7. Triple Point          : - 123 deg C   

Sunday, July 20, 2014

The Cost of Oxygen!!


The cost of Oxygen. Very interesting!!!


In one day a human being breathes oxygen equivalent to three cylinders. 

Each oxygen cylinder on an average costs Rs 700, without subsidy. 

So in a day one uses Oxygen worth Rs 2100 and for a full year it is Rs 7,66,500. 

If we consider an average life span of 65 years; the costs of oxygen we use become a staggering sum of  Rs 500,00,000. Rs 50 million. 

All this oxygen is derived free of cost from the surrounding trees.. 

Very few people look at trees as a resource and there is rampart tree cutting going on everywhere which should stop.

Please pass this to your friends and care for trees.

Monday, April 7, 2014

Decay Level Scheme of Co-60

Cobalt-60 is a beta emitter that also releases gamma radiation. The energy released in the beta emission leaves the product element, nickel-60, in an excited state. When the nickel-60 descends to its ground state, it gives off photons in the gamma ray region of the radiant energy spectrum.