14-05-2014, 12:40 PM
MICROWAVE OVEN
[attachment=63222]
BASIC DEFINITION
A microwave oven is a kitchen appliance that cooks or heats food
by dielectric heating. This is accomplished by using microwave
radiation to heat water and other polarized molecules within the
food. This excitation is fairly uniform, leading to food being more
evenly heated throughout (except in thick objects) than generally
occurs in other cooking techniques.
Principle
A microwave oven works by passing non-ionizing
microwave radiation, usually at a frequency of 2.45
gigahertz—a wavelength of 122 mm (4.80 in)—through the
food.
Microwave radiation is between common radio and
infrared frequencies.
Water, fat, and other substances in the food absorb
energy from the microwaves in a process called dielectric
heating.
Many molecules (such as those of water) are electric
dipoles, meaning that they have a positive charge at one end
and a negative charge at the other, and therefore rotate as
they try to align themselves with the alternating electric field
of the microwaves.
This molecular movement represents heat which is
then dispersed as the rotating molecules hit other molecules
and put them into motion.
Microwave heating is more efficient on liquid water
than on fats and sugars (which have a smaller molecular
dipole moment), and also more efficient than on frozen water
(where the molecules are not free to rotate).
Microwave heating is sometimes explained as a
resonance of water molecules, but this is incorrect: such
6resonance only occurs in water vapor at much higher
frequencies, at about 20 GHz.
Moreover, large industrial/commercial microwave
ovens operating at the common large industrial-oven
microwave heating frequency of 915 MHz—wavelength
328 millimeters (12.9 in)—also heat water and food perfectly
well.
A common misconception is that microwave ovens
cook food "from the inside out".
In reality, microwaves are absorbed in the outer layers
of food in a manner somewhat similar to heat from other
methods.
How Does A Microwave Oven Work?
Basic Theory of Operation:
Microwave ovens use various combinations of electrical circuits and
mechanical devices to produce and control an output of microwave
energy for heating and cooking. Generally speaking the systems of a
microwave oven can be divided into two fundamental sections, the
control section and the high-voltage section.
The Manufacturing Process
Oven cavity and door manufacture
The process of manufacturing a microwave oven starts with the
cavity and the door. First, the frame is formed using automatic
metal-forming presses that make about 12 to 15 parts per minute.
The frame is then rinsed in alkaline cleaner to get rid of any dirt or
oil and further rinsed with water to get rid of the alkaline solution.
Next, each part is treated with zinc phosphate, which prepares it for
electro-deposition. Electro-deposition consists of immersing the
parts in a paint tank at 200 volts for 2.5 minutes. The resulting
coating is about 1.5 mils thick. The parts are then moved through a
paint bake operation where the paint is cured at 300 degrees
Fahrenheit (149 degrees Celsius) for 20 minutes. The chassis or
frame is mounted in a pallet for the main assembly operation. A
pallet is a vise-like device used in conjunction with other tools.
Quality Control:
Extensive quality control during the manufacture of microwave
ovens is essential, because microwave ovens emit radiation that can
burn anyone exposed at high levels for prolonged periods. Federal
regulations, applied to all ovens made after October 1971, limit the
amount of radiation that can leak from an oven to 5 mill watts of
radiation per square centimeter at approximately 2 inches from the
oven surface. The regulations also require all ovens to have two
independent, interlocking switches to stop the production of
microwaves the moment the latch is released or the door is opened.
[attachment=63222]
BASIC DEFINITION
A microwave oven is a kitchen appliance that cooks or heats food
by dielectric heating. This is accomplished by using microwave
radiation to heat water and other polarized molecules within the
food. This excitation is fairly uniform, leading to food being more
evenly heated throughout (except in thick objects) than generally
occurs in other cooking techniques.
Principle
A microwave oven works by passing non-ionizing
microwave radiation, usually at a frequency of 2.45
gigahertz—a wavelength of 122 mm (4.80 in)—through the
food.
Microwave radiation is between common radio and
infrared frequencies.
Water, fat, and other substances in the food absorb
energy from the microwaves in a process called dielectric
heating.
Many molecules (such as those of water) are electric
dipoles, meaning that they have a positive charge at one end
and a negative charge at the other, and therefore rotate as
they try to align themselves with the alternating electric field
of the microwaves.
This molecular movement represents heat which is
then dispersed as the rotating molecules hit other molecules
and put them into motion.
Microwave heating is more efficient on liquid water
than on fats and sugars (which have a smaller molecular
dipole moment), and also more efficient than on frozen water
(where the molecules are not free to rotate).
Microwave heating is sometimes explained as a
resonance of water molecules, but this is incorrect: such
6resonance only occurs in water vapor at much higher
frequencies, at about 20 GHz.
Moreover, large industrial/commercial microwave
ovens operating at the common large industrial-oven
microwave heating frequency of 915 MHz—wavelength
328 millimeters (12.9 in)—also heat water and food perfectly
well.
A common misconception is that microwave ovens
cook food "from the inside out".
In reality, microwaves are absorbed in the outer layers
of food in a manner somewhat similar to heat from other
methods.
How Does A Microwave Oven Work?
Basic Theory of Operation:
Microwave ovens use various combinations of electrical circuits and
mechanical devices to produce and control an output of microwave
energy for heating and cooking. Generally speaking the systems of a
microwave oven can be divided into two fundamental sections, the
control section and the high-voltage section.
The Manufacturing Process
Oven cavity and door manufacture
The process of manufacturing a microwave oven starts with the
cavity and the door. First, the frame is formed using automatic
metal-forming presses that make about 12 to 15 parts per minute.
The frame is then rinsed in alkaline cleaner to get rid of any dirt or
oil and further rinsed with water to get rid of the alkaline solution.
Next, each part is treated with zinc phosphate, which prepares it for
electro-deposition. Electro-deposition consists of immersing the
parts in a paint tank at 200 volts for 2.5 minutes. The resulting
coating is about 1.5 mils thick. The parts are then moved through a
paint bake operation where the paint is cured at 300 degrees
Fahrenheit (149 degrees Celsius) for 20 minutes. The chassis or
frame is mounted in a pallet for the main assembly operation. A
pallet is a vise-like device used in conjunction with other tools.
Quality Control:
Extensive quality control during the manufacture of microwave
ovens is essential, because microwave ovens emit radiation that can
burn anyone exposed at high levels for prolonged periods. Federal
regulations, applied to all ovens made after October 1971, limit the
amount of radiation that can leak from an oven to 5 mill watts of
radiation per square centimeter at approximately 2 inches from the
oven surface. The regulations also require all ovens to have two
independent, interlocking switches to stop the production of
microwaves the moment the latch is released or the door is opened.