Electromagnetic Interference-Classification

Electromagnetic Interference can be classified into two broad categories
Narrow band and Broadband
Narrow band interference originates from devices and frequencies used for communications such as radio, television and cell phones.  The term narrowband implies that the interference is mostly in one particular frequency or a narrow set of frequencies.  This type of interference can be easily filtered out. 
Broadband Interference originates from sources such as electric power transmission lines.  This radiation is also called unintentional radiation as the radiation is not deliberate by incidental to the power transmission.  Broadband Electromagnetic interference occupies a large part of the electromagnetic spectrum.  It does not have a limited frequency range like narrowband interference.  This kind of EMI is mostly caused by electric phenomena such as corona, switching transients, power line radiation . 
Fluorescent lights, brush arcing in motors and contact chattering in relays can also cause broadband interference in electronic devices located close by.
Electromagnetic interference can also be classified into Conducted interference and Radiated interference.
In conducted interference, the interference is transmitted through physical contact (conduction) between the devices.  An example may be noise in a television when an electric appliance in the house is switched on.
Radiated interference occurs when the interference radiates out of the conductors into space and couples with other devices.  For instance, the radiation from a power line can get coupled to a telephone line running alongside.
Another method of classification of Electromagnetic interference is based on the duration. 
Continuous interference and Impulse Noise
Continuous interference comes from sources such as power devices or from natural sources.  This interference is of long duration.
Impulse Noise is of short duration.  This is caused usually by switching actions. 

Faraday Cages

A Faraday Cage is a a cage or an enclosure made of conductive material.   The Faraday cage is used to protect a device or a region from external electric fields.  The Faraday cage is named after Sir Michael Faraday who invented them in 1836.

Faraday proved that no electric field can exist inside a room made of conductive material. 

The Faraday cage can block static and non-static electric field.  The charge passes through the conductive surface of the cage.

When an airplane or a car is struck by lightning, it acts as a Faraday's cage protecting the passengers from the lightning. 

Faraday cage are used in laboratories to protect sensitive instruments. 

Applications

Electric conductors used in instrumentation are protected from external interference by the use of a Shield.  The shield serves to protect the conductors from electromagnetic couplings.

Electromagnetic Interference (EMI)

EMI or Electromagnetic interference is a term which is commonly used  to describe interference in electric circuits by an external source.

There are many different sources for Electromagnetic Interference.  EMI is generally classified based on its source into two broad categories.

• Man-Made EMI which originates from other man made sources such as power lines, electric appliances and circuits.  The switching of electric circuits can cause interference.  Microwave ovens can also cause EMI
• Naturally occurring EMI which refers to interference from natural sources such as Lightning Discharges, Solar phenomena, etc.

EMI causes disturbance to electronic systems such as radio and television and wireless communication systems.  Analog communication systems based on amplitude modulation (AM) are particularly susceptible to EMI interference as they have low selectivity.

Protecting Devices against EMI

1. Electric and Electronic Devices are usually protected against EMI by enclosing them in a metallic enclosure. 
2. If possible, move known sources of EMI such as microwave ovens, cell phones away from sensitive electronic circuits.
3. Ensure that the device is properly grounded. 
4. To protect a large area such as a room from Electromagnetic Interference, a Faraday cage may be constructed.  A Faraday Cage is an enclosure made of conductive material or a mesh.

High Temperature Pressure Sensors-An Overview

High Temperature Pressure Sensors are used in applications where pressure has to be measured at high temperatures.  Ordinary pressure sensors cannot operate at such high temperatures.  These sensors work on the piezo resistive principle. 

The usual construction is a diaphragm which is filled with oil.  The pressure of the media to be measured is thus conducted to the oil.   The oil is carried via a capillary tube to the measuring chamber.   The temperature of the oil drops in the capillary.  The piezo resistive element then measures the pressure of the oil which is the pressure of the media.

Thus the pressure sensor is able to measure pressure at elevated temperature. 

Pressure Transducers using the piezo resistive principle can measure pressure at temperatures up to 300 degrees Celsius. 

Pressure Sensors designed for operation at very high temperatures use interferometer based sensors.

Aneroid Barometer

Aneroid Barometer are a portable form of barometer.  The aneroid barometer does not use a barometric liquid like mercury.  Instead, of the liquid it has a corrugated metal sheet which deforms depending on the shape of the machine. 

The Aneroid Barometer gets its name from the shape of the Aneroid Capsule inside the instrument.  The Aneroid Capsule is an evacuated circular corrugated disc. 

The corrugated disc deforms under air pressure.  A mechanism of gears and linkages is used to convey the deflection to a pointer across a dial.

The Aneroid Barometer is an extremely handy instrument.  It can be used in any orientation. 

A modified form of the Aneroid Barometer is used as the Altimeter to indicate the Altitude ( See Article).

The Aneroid Barometer is sensitive to temperature.  The metallic components used in its construction expand and contract with the temperature.  Hence, to compensate for the variation due to the temperature, a bimetallic strip is used.

The disadvantages of the Aneroid barometer are

• It is less precise than a mercury barometer
• It requires frequent calibration

Barometers -An Overview

A barometer is an instrument which measures atmospheric pressure.  The barometer was invented by the Italian scientist, Evangelista Torricelli. 

The barometer in its basic form consists of a bowl of mercury and a tube which is sealed at one end.  The tube is filled with mercury.  It is then inverted over the bowl.  The tube is made to stand vertically over the bowl with the help of a stand.

The level of mercury in the tube now falls in the tube.  This is the indication of the atmospheric pressure.  The column of mercury in the tube is supported by the atmospheric pressure.  The level of mercury in the tube is dependant on the atmospheric pressure.

The standard atmospheric pressure is 76 cms of mercury.

one bar of pressure = 75.0061561303 cms of mercury

Technically, the barometer can also be made using water.  However, a very long tube may be required which is not practical.

The standard atmospheric pressure can support a column of water 10 metre high.

The mercury barometer is not a portable instrument.  Hence, it has been replaced by the Aneroid Barometer.

See Article on Aneroid Barometer

What is the unit of Vacuum ?

The unit of Vacuum is Torr.  Torr is named after Evangelista Toricelli, the inventor of the barometer. 

One Torr is equal to 1/760 of a standard atmosphere. 

One Torr is also defined as the pressure necessary to support a column of mercury which is 1 mm high at 0�C.  One Torr is equal to 1333.2 microbars.