Fundamental Concepts & Terminology
For an understanding of heating, cooling, refrigeration and air conditioning, it is necessary to know some fundamental concepts, technical/scientific terms, definitions and measurements. Let's review them.
Page Summary

DensityThe density of a material is the weight per unit volume of the material. Density is expressed as grams per cubic centimeter or pounds per cubic foot. The density of water is 1 g/cc or 62.4 lb./cu.ft.
Specific VolumeThe specific volume of a substance is the volume occupied by unit weight of the material, i.e. cc/g or cu.ft./lb. By definition, it will be clear that specific volume is the reciprocal of density.
Specific Volume = 1 / Density TemperatureTemperature is an indication of the level of heat in a substance. A substance at a temperature of 10 *C has more heat in it than the same substance at a temperature of 0 *C. The temperature of a substance, however, does not give an idea of the amount of heat the substance has.
A thermometer is an instrument used for the measurement of temperature. Two temperature scales are in common use today: The Fahrenheit scale and the Celsius scale. Celsius Scale The melting point of ice is 0 *C and the boiling point of water is 100 *C. The interval between these two points is divided into 100 equal divisions and each division is called one degree Celsius (1 *C). Fahrenheit Scale The melting point of ice is fixed as 32 *F and the boiling point of water is taken as 212 *F. The interval between the two is divided into 180 equal divisions and each division is called one degree Fahrenheit (1 *F). Therefore, for measuring smaller changes in temperature, Fahrenheit scale is more accurate than Celsius scale. Absolute Temperature (Rankine) Scale The absolute zero on this scale (expressed as *R) is 460 *F. So to arrive at the absolute temperature of a substance expressed in *F, 460 should be added to the given temperature, e.g. absolute temperature of ice melting is 32 *F + 460 = 492 *R. Absolute Temperature (Kelvin) Scale The absolute zero on this scale is  273 *C. So the melting point of ice on the Kelvin absolute scale is 0*C + 273 = 273 K. In this scale, absolute temperature is expressed as, K, without a degree sign, e.g. 298 K. EnergyThe energy of a body is its capacity to do work. It is measured by the total amount of work that the body can do. Energy is of tow kinds:
1. Potential Energy. 2. Kinetic Energy. 
Specific GravityThe specific gravity, or relative density, of a substance is the ratio of the density of the substance to the density of water at 4 *C.
S.G. = Density of the substance/Density of water at 4 *C Thus, if the specific gravity of steel is 7.8, its density will be 7.8 x 62.4 = 486.72 lb./cu.ft. in the FPS system or 7.8 x 1 = 7.8 g/cc in the metric system. Since the density of water in the metric system is 1 g/cc, the density and specific gravity of a substance in the metric system are numerically the same. VelocityVelocity is the rate of change in the position of a body along a straight line in a particular direction. This is expressed as the distance covered by the body in unit time.
In the metric system: Velocity is measured in meters per second  m/s In the FPS system: Velocity is measured in feet per second  ft./s ForceWhen an object has to moved, we either push it or pull it, i.e. we use force. Anything that (1) sets a body in motion or (2) brings a moving body to rest or (3) changes the direction of a moving body is "force". It is measured in weight units such as pounds or newton. The unit of force is that which when applied to a body of unit mass (kg or lb.) gives it a unit acceleration (m/s^2) or (ft./s^2). The unit of force is "Newton" in SI system and "Pound" in the FPS system.
WorkWhen a force acting on a body moves the body, it is said that "work" is done. The amount of work done is defined as the product of the force and distance through which the body is moved. The work done is expressed in the same unit of force and distance. If the force used is expressed in pounds, the distance should be in feet, and work then is expressed as feetpound. In the metric system it is N.m.
W = F x d PowerPower is the rate of doing work. So, power is the work done divided by the time required to do the work. Power is expressed in terms of horsepower. One horsepower is defined as the power required to do work at the rate of 33,000 ft.lb./min. or 550 ft.lb./s.
Potential Energy
This is the energy a body possesses by virtue of its position. A body situated at a height has potential energy. The potential energy of a body is given by the equation, P.E. = mgh 

Kinetic EnergyThis is the energy a body possesses by virtue of motion or velocity, such as a speeding projectile, a moving car, the moving parts of a machine, etc. The amount of kinetic energy a body possesses is determined by the equation.
K.E. = (1/2)mv^2 
Other Forms of EnergyAll energy can be classified as either kinetic or potential energy. However, energy may appear in different forms such as mechanical, electrical, chemical, heat, etc. They can be readily converted from one form to another. Electrical energy can be converted to heat energy in an electrical heater; electrical energy is converted to mechanical energy in a motor, solenoid valve, etc. Mechanical energy, chemical energy and heat energy are converted into electrical energy in the generator, battery and thermocouple respectively.

Pressure
Pressure is the force exerted per unit area. Whenever a force is evenly distributed over an area, the pressure at any point on the contact surface is the same and can be calculated by dividing the force by the total area over which the force is applied. Units for pressure is pounds per square inch (psi), pounds per square foot (psf), Newton per meter (Nm). We express Nm as Pascal. (Pa)
Atmospheric Pressure
We live at the bottom of the a sea of air mixed with water vapor, more than fifty miles deep. Both air and water vapor have weight. A column of air, one square inch in cross section extending upwards about 50 miles, weighs 14.696 lbs. Thus, we say that the atmospheric pressure at the sea level is 14.7 lb. per square inch (14.7 psi). However, the weight of the airwater vapor mixture does not remain constant but varies somewhat, depending on the temperature, amount of water vapor, etc. But for all practical purposes, the atmospheric pressure at sea level is taken as 14.7 psi. Consider a man climbing up a mountain. The weight of the air exerted on him becomes less, thus the atmospheric pressure becomes less as he go up the mountain. Using the same argument, if he going down below the sea level, for an example, deeper inside a mine, the atmospheric pressure increases. Roughly, for every thousand feet change in altitude from sea level, the atmospheric pressure varies by 0.5 psi. Atmospheric pressure is measured with the help of a barometer. A Barometer is a glass tube about 36 inches long, closed at one end and filled with mercury. The open end of the tube is covered with a cap, and then inverted into an open container filled with mercury. When the cap is removed from the tube end, the mercury column in the tube falls down to a certain height leaving a nearly perfect vacuum in the closed end of the tube. The pressure exerted by atmospheric air on the surface of the mercury in the container causes the column of mercury in the tube to stand. Therefore, the height of the mercury column in the barometer gives a measure of the atmospheric pressure. At sea level, the mercury column will be 29.921 inches (76 cm) high. The weight of the 29.92 cu.in. (76 cc) of mercury is 14.696 lb. (65.371 N). Thus, the atmospheric pressure at sea level can push up a column of mercury in a vacuum tube to a height of 29.921 inches or 760 mm high. Therefore, we say that the atmospheric pressure at sea level is 14.7 psi or 760 mm (29.921") of mercury. (760 mmHg, 29.921"Hg). "Hg" stands for the Latin name hydrargyrum for mercury. Thus, 1 psi pressure equals 29.921 / 14.696 = 2.036" (51 .7 mm) of the mercury column. Density of Mercury (Hg) = 13,594 kg/m^3 (848.75 lbs./ft^3) Specific Gravity of Hg = 13.6 =(848.75 lbs.^3)/(62.4 lbs./ft^3) Mercury Barometer

Vacuum
We find that the space above the mercury column in the tube of a barometer, is devoid of anything (air or any other material) which is said to be under vacuum. However, in scientific and technical parlance, the word vacuum is used to indicate how low the pressure inside a closed space is with respect to atmospheric pressure. By some means, let us allow a very slight amount of air or gas into the space at the top of the barometer. This air or gas will exert some pressure. Therefore, the mercury column which was at 760 mm (29.921") height will now fall slightly  to the extent to the pressure of air and gas admitted to into the top of the tube. Let us assume that the pressure of the air admitted is one psi, which is equal to 51. 7 mm (2.036") due to the pressure of the air admitted, and the height of the mercury column will fall down by 51. 7 mm (2.036") due to the pressure of the air admitted. The height of the mercury column will now be 760  51. 7 = 708mm. (29.921  2.036 = 27.885"). The space above the mercury column in the tube is no more a perfect vacuum, but has some air or gas though the pressure of the air or gas inside is still very much below atmospheric pressure. The space, in technical terms, is said to be under a 'vacuum of 708.3 mm (27.885") of mercury.' This means that the space has a pressure less than atmospheric pressure by 708.3 mm (27.885") Hg; or in other words, the space has a pressure of 51. 7 mm (2.036") Hg only as against the atmospheric pressure of 760 mm (29.921") Hg. So when we use the word 'vacuum' in refrigeration trade, we make a comparison between the pressure of a space and the atmosphere. For example, we draw a vacuum in a refrigeration system and read on the vacuum gauge a figure of 711. 2 mm (28") Hg. This means the system is not under a perfect vacuum but there is still slight air or gas left inside which exerts a pressure of 29.921  28 = 1. 921" Hg. (48.8 mmHg), or 1.921/2.036 = 0.94 psi. In the technical field, we use the word 'vacuum' to denote the pressure of a space which is below atmospheric pressure and the word 'gauge pressure' for pressures above the atmospheric pressure. In both, atmospheric pressure is referred to as the base at 'zero'. So when we say that a system has a pressure of 30 psi gauge, we mean that the system has a pressure of 30 psi above atmospheric, and the actual pressure or true pressure is 30 psi plus the atmospheric pressure of 14.7 psi. Therefore, the 'true pressure' is (30 +14.7) = 44.7 psi. The 'true' pressure is referred to as the absolute pressure in order to distinguish it from gauge pressure. It is expressed as "psia". The gauge pressure is expressed abbreviated psig. The commercial gauges which are commonly used, show the atmospheric pressure as 'zero', the pressure readings as pounds per square inch (psi) or Pascal (Pa). Vacuum gauges indicate inches of Hg (inHg) or millimeters of Hg (mmHg). There are gauges which have vacuum and pressure readings incorporated. These gauges are called Compound Gauges. 
Micron
In refrigeration and many other scientific and industrial applications, very high vacuums have to drawn in systems. To facilitate expressing extremely small absolute pressures (vacuum) correctly, the smaller unit 'micron' is used. A micron is a millionth part of a meter which in turn is equal to 1000 mm. So one micron is equal to 0.001 mm.
By employing the small unit 'micron', we avoid the use of decimals. For example, instead of saying that the system has an absolute pressure of 0.1 mmHg, we can express it as 100 micronHg. There are mercury (McCleod gauge) and electronic gauges to measure very low absolute pressures in systems under vacuum. These gauges show the absolute pressure inside a system in microns. These micron gauges are used to measure the vacuum of a system during and after evacuation.
By employing the small unit 'micron', we avoid the use of decimals. For example, instead of saying that the system has an absolute pressure of 0.1 mmHg, we can express it as 100 micronHg. There are mercury (McCleod gauge) and electronic gauges to measure very low absolute pressures in systems under vacuum. These gauges show the absolute pressure inside a system in microns. These micron gauges are used to measure the vacuum of a system during and after evacuation.