Atmosphere of Earth"Air" redirects here. For other uses, see Air (disambiguation).

"Qualities of air" redirects here. It is not to be confused with Air quality.

Blue light is scattered more than other wavelengths by the gases in the atmosphere, giving the Earth a blue halo when seen from space.

Limb view, of the Earth's atmosphere. Colours roughly denote the layers of the atmosphere.

This image shows the moon at centre, with the limb of Earth near the bottom transitioning into the orange-coloured troposphere, the lowest and most dense portion of the Earth's atmosphere. The troposphere ends abruptly at the tropopause, which appears in the image as the sharp boundary between

the orange- and blue- coloured atmosphere. The silvery-blue noctilucent clouds extend far above the Earth's troposphere.

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).Atmospheric stratification describes the structure of the atmosphere, dividing it into distinct layers, each with specific characteristics such as temperature or composition. The atmosphere has a mass of about 5×1018 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. An altitude of 120 km (75 mi) is where atmospheric effects become noticeable during atmospheric reentry of spacecraft. The Kármán line, at 100 km (62 mi), also is often regarded as the boundary between atmosphere and outer space.Air is the name given to atmosphere used in breathing and photosynthesis. Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93%argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. While air content and atmospheric pressure varies at different layers, air suitable for the survival of terrestrial plants and terrestrial animals is currently only known to be found in Earth's troposphere and artificial atmospheres.

Atmospheric pressure is the force per unit area applied into a surface by the weight of air above that surface in the atmosphere of Earth. In most conditions atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low-pressure areas have less atmospheric mass above their location, whereas high-pressure areas have more atmospheric mass above their location.

Similarly, as elevation increases, there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. A column of air one square centimeter in cross-section, measured from sea level to the top of the atmosphere, has a mass of about a kilogram and will exert a force of about 9.8 N (2.2 lb force); a column one square inch in cross-section would have a mass of about 15 lbs, applying a force of 63 N.

Main article: Atmospheric chemistry

Composition of Earth's atmosphere. The lower pie represents the trace gases which together compose 0.039% of the atmosphere. Values normalized for

illustration. The numbers are from a variety of years (mainly 1987, with CO2 and methane from 2009) and do not represent any single source.

Mean atmospheric water vapor

Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases are often referred to as trace gases, among which are the greenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Many natural substances may be present in tiny amounts in an unfiltered air sample, including dust,pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine compounds, elemental mercury, and sulfur compounds such as sulfur dioxide [SO2].

Composition of dry atmosphere, by volume

ppmv: parts per million by volume (note: volume fraction is equal to mole fraction for ideal gas only, see volume (thermodynamics))

Gas Volume

Nitrogen (N2) 780,840 ppmv (78.084%)

Oxygen (O2) 209,460 ppmv (20.946%)

Argon (Ar) 9,340 ppmv (0.9340%)

Carbon dioxide (CO2) 390 ppmv (0.039%)

Neon (Ne) 18.18 ppmv (0.001818%)

Helium (He) 5.24 ppmv (0.000524%)

Methane (CH4) 1.79 ppmv (0.000179%)

Krypton (Kr) 1.14 ppmv (0.000114%)

Hydrogen (H2) 0.55 ppmv (0.000055%)

Nitrous oxide (N2O) 0.3 ppmv (0.00003%)

Carbon monoxide (CO) 0.1 ppmv (0.00001%)

Xenon (Xe) 0.09 ppmv (9×10−6%) (0.000009%)

Ozone (O3) 0.0 to 0.07 ppmv (0 to 7×10−6%)Nitrogen dioxide (NO2) 0.02 ppmv (2×10−6%) (0.000002%)Iodine (I2) 0.01 ppmv (1×10−6%) (0.000001%)Ammonia (NH3) traceNot included in above dry atmosphere:

Water vapor (H2O) ~0.40% over full atmosphere, typically 1%-4% at surface

Structure of the atmosphere

Principal layers

Layers of the atmosphere (not to scale)

In general, air pressure and density decrease in the atmosphere as height increases. However, temperature has a more complicated profile with altitude. Because the general pattern of this profile is constant and

recognizable through means such as balloon soundings, temperature provides a useful metric to distinguish between atmospheric layers. In this way, Earth's atmosphere can be divided into five main layers. From highest to lowest, these layers are:ExosphereMain article: Exosphere

The outermost layer of Earth's atmosphere extends from the exobase upward. It is mainly composed of hydrogen and helium. The particles are so far apart that they can travel hundreds of kilometers without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the magnetosphere or the solar wind.ThermosphereMain article: Thermosphere

Temperature increases with height in the thermosphere from the mesopause up to the thermopause, then is constant with height. Unlike in the stratosphere, where the inversion is caused by absorption of radiation by ozone, in the thermosphere the inversion is a result of the extremely low density of molecules. The temperature of this layer can rise to 1,500 °C (2,700 °F), though the gas molecules are so far apart that temperature in the usual sense is not well defined. The air is so rarefied, that an individual molecule (of oxygen, for example) travels an average of 1 kilometer between collisions with other molecules. The International Space Station orbits in this layer, between 320 and 380 km (200 and 240 mi). Because of the relative infrequency of molecular collisions, air above the mesopause is poorly mixed compared to air below. While the composition from the troposphere to the mesosphere is fairly constant, above a certain point, air is poorly mixed and becomes compositionally stratified. The point dividing these two regions is known as the turbopause. The region below is the homosphere, and the region above is the heterosphere. The top of the thermosphere is the bottom of the exosphere, called the exobase. Its height varies with solar activity and ranges from about 350–800 km (220–500 mi; 1,100,000–2,600,000 ft).

MesosphereMain article: Mesosphere

The mesosphere extends from the stratopause to 80–85 km (50–53 mi; 260,000–280,000 ft). It is the layer where most meteors burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The mesopause, the temperature minimum that marks the top of the mesosphere, is the coldest place on Earth and has an average temperature around −85 °C (−120 °F; 190 K. At the mesopause, temperatures may drop to −100 °C (−150 °F; 170 K).[5] Due to the cold temperature of the mesosphere, water vapor is frozen, forming ice clouds (or Noctilucent clouds). A type of lightning referred to as either sprites or ELVES, form many miles above thunderclouds in the troposphere.StratosphereMain article: Stratosphere

The stratosphere extends from the tropopause to about 51 km (32 mi; 170,000 ft). Temperature increases with height due to increased absorption of ultraviolet radiation by the ozone layer, which restricts turbulence and mixing. While the temperature may be −60 °C (−76 °F; 210 K) at the tropopause, the top of the stratosphere is much warmer, and may be near freezing.The stratopause, which is the boundary between the stratosphere and mesosphere, typically is at 50 to 55 km (31 to 34 mi; 160,000 to 180,000 ft). The pressure here is 1/1000 sea level.TroposphereMain article: Troposphere

The troposphere begins at the surface and extends to between 9 km (30,000 ft) at the poles and 17 km (56,000 ft) at the equator  with some variation due to weather. The troposphere is mostly heated by transfer of energy from the surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude. This promotes vertical mixing (hence

the origin of its name in the Greek word trope, meaning turn or overturn). The troposphere contains roughly 80% of the mass of the atmosphere. The tropopause is the boundary between the troposphere and stratosphere.Other layersWithin the five principal layers determined by temperature are several layers determined by other properties:

The ozone layer is contained within the stratosphere. In this layer ozone concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from about 15–35 km (9.3–22 mi; 49,000–110,000 ft), though the thickness varies seasonally and geographically. About 90% of the ozone in our atmosphere is contained in the stratosphere.

The ionosphere, the part of the atmosphere that is ionized by solar radiation, stretches from 50 to 1,000 km (31 to 620 mi; 160,000 to 3,300,000 ft) and typically overlaps both the exosphere and the thermosphere. It forms the inner edge of the magnetosphere. It has practical importance because it influences, for example,radio propagation on the Earth. It is responsible for auroras.

The homosphere and heterosphere are defined by whether the atmospheric gases are well mixed. In the homosphere the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence. The homosphere includes the troposphere, stratosphere, and mesosphere. Above the turbopause at about 100 km (62 mi; 330,000 ft) (essentially corresponding to the mesopause), the composition varies with altitude. This is because the distance that particles can move without colliding with one another is large compared with the size of motions that cause mixing. This allows the gases to stratify by molecular weight, with the heavier ones such as oxygen and nitrogen present only near the bottom of the heterosphere. The upper part of the heterosphere is composed almost completely of hydrogen, the lightest element.

The planetary boundary layer is the part of the troposphere that is nearest the Earth's surface and is directly affected by it, mainly through turbulent diffusion. During the day the planetary boundary layer usually is well-mixed, while at night it becomes stably stratified with weak or intermittent mixing. The depth of the planetary boundary layer ranges from as little as about 100 m on clear, calm nights to 3000 m or more during the afternoon in dry regions.

The average temperature of the atmosphere at the surface of Earth is 14 °C (57 °F; 287 K) or 15 °C (59 °F; 288 K), depending on the reference.

Physical properties

Comparison of the 1962 US Standard Atmosphere graph of geometric altitude against air density, pressure, the speed of sound and temperature with approximate altitudes of various objects.

Pressure and thicknessMain article: Atmospheric pressure

The average atmospheric pressure at sea level is about 1 atmosphere (atm) = 101.3 kPa (kilopascals) = 14.7 psi (pounds per square inch) = 760 torr =

29.92 inches of mercury (symbol Hg). Total atmospheric mass is 5.1480×1018 kg (1.135×1019 lb), about 2.5% less than would be inferred from the average sea level pressure and the Earth's area of 51007.2 megahectares, this portion being displaced by the Earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and weather.If atmospheric density were to remain constant with height the atmosphere would terminate abruptly at 8.50 km (27,900 ft). Instead, density decreases with height, dropping by 50% at an altitude of about 5.6 km (18,000 ft). As a result the pressure decrease is approximately exponential with height, so that pressure decreases by a factor of two approximately every 5.6 km (18,000 ft) and by a factor of e = 2.718… approximately every 7.64 km (25,100 ft), the latter being the average scale height of Earth's atmosphere below 70 km (43 mi; 230,000 ft). However, because of changes in temperature, average molecular weight, and gravity throughout the atmospheric column, the dependence of atmospheric pressure on altitude is modeled by separate equations for each of the layers listed above. Even in the exosphere, the atmosphere is still present. This can be seen by the effects of atmospheric drag on satellites.In summary, the equations of pressure by altitude in the above references can be used directly to estimate atmospheric thickness. However, the following published data are given for reference:

50% of the atmosphere by mass is below an altitude of 5.6 km (18,000 ft).

90% of the atmosphere by mass is below an altitude of 16 km (52,000 ft). The common altitude of commercial airliners is about 10 km (33,000 ft) and Mt. Everest's summit is 8,848 m (29,029 ft) above sea level.

99.99997% of the atmosphere by mass is below 100 km (62 mi; 330,000 ft), although in the rarefied region above this there are auroras and other atmospheric effects. The highest X-15 plane flight in 1963 reached an altitude of 108.0 km (354,300 ft).

Density and mass

Temperature and mass density against altitude from the NRLMSISE-00 standard atmosphere model (the eight dotted lines in each "decade" are at the eight cubes 8, 27, 64, ..., 729)Main article: Density of air

The density of air at sea level is about 1.2  kg/m3 (1.2 g/L). Density is not measured directly but is calculated from measurements of temperature, pressure and humidity using the equation of state for air (a form of the ideal gas law). Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula. More sophisticated models are used to predict orbital decay of satellites.The average mass of the atmosphere is about 5 quadrillion (5×1015) tonnes or 1/1,200,000 the mass of Earth. According to the American National Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480×1018 kg with an annual range due to water vapor of 1.2 or 1.5×1015 kg depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27×1016 kg and the dry air mass as 5.1352 ±0.0003×1018 kg."

Optical properties

See also: Sunlight

Solar radiation (or sunlight) is the energy the Earth receives from the Sun. The Earth also emits radiation back into space, but at longer wavelengths that we cannot see. Part of the incoming and emitted radiation is absorbed or reflected by the atmosphere.ScatteringMain article: Scattering

When light passes through our atmosphere, photons interact with it through scattering. If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the Sun. Indirect radiation is light that has been scattered in the atmosphere. For example, on an overcast day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue, you are seeing scattered blue light. This is also why sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.

AbsorptionMain article: Absorption (electromagnetic radiation)

Different molecules absorb different wavelengths of radiation. For example, O2 and O3 absorb almost all wavelengths shorter than 300 nanometers. Water (H2O) absorbs many wavelengths above 700 nm. When a molecule absorbs a photon, it increases the energy of the molecule. We can think of this as heating the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below.

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light.

The combined absorption spectra of the gases in the atmosphere leave "windows" of low opacity, allowing the transmission of only certain bands of light. Theoptical window runs from around 300 nm (ultraviolet-C) up into the range humans can see, the visible spectrum (commonly called light), at roughly 400–700 nm and continues to the infrared to around 1100 nm. There are also infrared and radio windows that transmit some infrared and radio

waves at longer wavelengths. For example, the radio window runs from about one centimeter to about eleven-meter waves.EmissionMain article: Emission (electromagnetic radiation)

Emission is the opposite of absorption, it is when an object emits radiation. Objects tend to emit amounts and wavelengths of radiation depending on their "black body" emission curves, therefore hotter objects tend to emit more radiation, with shorter wavelengths. Colder objects emit less radiation, with longer wavelengths. For example, the Sun is approximately 6,000 K (5,730 °C; 10,340 °F), its radiation peaks near 500 nm, and is visible to the human eye. The Earth is approximately 290 K (17 °C; 62 °F), so its radiation peaks near 10,000 nm, and is much too long to be visible to humans.Because of its temperature, the atmosphere emits infrared radiation. For example, on clear nights the Earth's surface cools down faster than on cloudy nights. This is because clouds (H2O) are strong absorbers and emitters of infrared radiation. This is also why it becomes colder at night at higher elevations. The atmosphere acts as a "blanket" to limit the amount of radiation the Earth loses into space.The greenhouse effect is directly related to this absorption and emission (or "blanket") effect. Some chemicals in the atmosphere absorb and emit infrared radiation, but do not interact with sunlight in the visible spectrum. Common examples of these chemicals are CO2 and H2O. If there are too much of these greenhouse gases, sunlight heats the Earth's surface, but the gases block the infrared radiation from exiting back to space. This imbalance causes the Earth to warm, and thus climate change.Refractive indexThe refractive index of air is close to, but just greater than 1. Systematic variations in refractive index can lead to the bending of light rays over long optical paths. One example is that, under some circumstances, observers onboard ships can see other vessels just over the horizon because light is refracted in the same direction as the curvature of the Earth's surface.

The refractive index of air depends on temperature, giving rise to refraction effects when the temperature gradient is large. An example of such effects is the mirage.Circulation

Main article: Atmospheric circulation

An idealised view of three large circulation cells.

Atmospheric circulation is the large-scale movement of air through the troposphere, and the means (with ocean circulation) by which heat is distributed around the Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant as it is determined by the Earth's rotation rate and the difference in solar radiation between the equator and poles.Evolution of Earth's atmosphere

Earliest atmosphereThe outgassings of the Earth were stripped away by solar winds early in the history of the planet until a steady state was established, the first atmosphere. Based on today's volcanic evidence, this atmosphere would have contained 60% hydrogen, 20% oxygen (mostly in the form of water vapor), 10% carbon dioxide, 5 to 7% hydrogen sulfide, and smaller amounts of nitrogen, carbon monoxide, free hydrogen, methane and inert gases.A major rainfall led to the buildup of a vast ocean, enriching the other agents, first carbon dioxide and later nitrogen and inert gases. A major part

of carbon dioxide exhalations were soon dissolved in water and built up carbonate sediments.Second atmosphereWater-related sediments have been found dating from as early as 3.8 billion years ago. About 3.4 billion years ago, nitrogen was the major part of the then stable "second atmosphere". An influence of life has to be taken into account rather soon in the history of the atmosphere, since hints of early life forms are to be found as early as 3.5 billion years ago. The fact that this is not perfectly in line with the 30% lower solar radiance (compared to today) of the early Sun has been described as the "faint young Sun paradox".The geological record however shows a continually relatively warm surface during the complete early temperature record of the Earth with the exception of one cold glacial phase about 2.4 billion years ago. In the late Archaean eon an oxygen-containing atmosphere began to develop, apparently from photosynthesizing algae which have been found as stromatolite fossils from 2.7 billion years ago. The early basic carbon isotopy (isotope ratio proportions) is very much in line with what is found today, suggesting that the fundamental features of the carbon cycle were established as early as 4 billion years ago.Third atmosphere

Oxygen content of the atmosphere over the last billion years

The accretion of continents about 3.5 billion years ago added plate tectonics, constantly rearranging the continents and also shaping long-term climate evolution by allowing the transfer of carbon dioxide to large land-based carbonate storages. Free oxygen did not exist until about 1.7 billion years ago and this can be seen with the development of the red beds and the end

of the banded iron formations. This signifies a shift from a reducing atmosphere to an oxidising atmosphere. O2 showed major ups and downs until reaching a steady state of more than 15%The following time span was the Phanerozoic eon, during which oxygen-breathing metazoan life forms began to appear.

Pressure Gauges

   Pressure gauges and switches are among the most often used instruments in a plant. But

because of their great numbers, attention to maintenance--and reliability--can be compromised.

As a consequence, it is not uncommon in older plants to see many gauges and switches out of

service. This is unfortunate because, if a plant is operated with a failed pressure switch, the

safety of the plant may be compromised. Conversely, if a plant can operate safely while a gauge

is defective, it shows that the gauge was not needed in the first place. Therefore, one goal of

good process instrumentation design is to install fewer but more useful and more reliable

pressure gauges and switches.

One way to reduce the number of gauges in a plant is to stop installing them on the basis of habit

(such as placing a pressure gauge on the discharge of every pump). Instead, review the need for

each device individually. During the review one should ask: "What will I do with the reading of

this gauge?" and install one only if there is a logical answer to the question. If a gauge only

indicates that a pump is running, it is not needed, since one can hear and see that. If the gauge

indicates the pressure (or pressure drop) in the process, that information is valuable only if one

can do something about it (like cleaning a filter); otherwise it is useless. If one approaches the

specification of pressure gauges with this mentality, the number of gauges used will be reduced.

If a plant uses fewer, better gauges, reliability will increase.

Pressure Gauge Designs

   Two common reasons for gauge (and switch) failure are pipe vibration and water condensation,

which in colder climates can freeze and damage the gauge housing. Figure 1 illustrates the

design of both a traditional and a more reliable, "filled" pressure gauge. The delicate links,

pivots, and pinions of a traditional gauge are sensitive to both condensation and vibration. The

life of the filled gauge is longer, not only because it has fewer moving parts, but because its

housing is filled with a viscous oil. This oil filling is beneficial not only because it dampens

pointer vibration, but also because it leaves no room for humid ambient air to enter. As a result,

water cannot condense and accumulate. Available gauge features include illuminated dials and

digital readouts for better visibility, temperature compensation to correct for ambient temperature

variation, differential gauges for differential pressures, and duplex gauges for dual pressure

indication on the same dial. Pressure gauges are classified according to their precision, from

grade 4A (permissible error of 0.1% of span) to grade D (5% error).

Protective Accessories

   The most obvious gauge accessory is a shutoff valve between it and the process (Figure 5-2),

which allows blocking while removing or performing maintenance. A second valve is often

added for one of two reasons: draining of condensate in vapor service (such as steam), or, for

higher accuracy applications, to allow calibration against an external pressure source.

   Other accessories include pipe coils or siphons (Figure 5-2A), which in steam service protect

the gauge from temperature damage, and snubbers or pulsation dampeners (Figure 5-2B), which

can both absorb pressure shocks and average out pressure fluctuations. If freeze protection is

needed, the gauge should be heated by steam or electric tracing. Chemical seals (Figure 5-2C)

protect the gauge from plugging up in viscous or slurry service, and prevent corrosive, noxious

or poisonous process materials from reaching the sensor. They also keep the process fluid from

freezing or gelling in a dead-ended sensor cavity. The seal protects the gauge by placing a

diaphragm between the process and the gauge. The cavity between the gauge and the diaphragm

is filled with a stable, low thermal expansion, low viscosity and non-corrosive fluid. For high

temperature applications, a sodium-potassium eutectic often is used; at ambient temperatures, a

mixture of glycerine and water; and at low temperatures, ethyl alcohol, toluene, or silicon oil.

The pressure gauge can be located for better operator visibility if the chemical seal is connected

to the gauge by a capillary tube. To maintain accuracy, capillary tubes should not be exposed to

excessive temperatures and should not exceed 25 feet (7.5 m) in length. The chemical seal itself

can be of four designs: off line, "flow-through" type self-cleaning, extended seal elements, or

wafer elements that fit between flanges.

   The spring rate of the diaphragm in the chemical seal can cause measurement errors when

detecting low pressures (under 50 psig, 350 kPa) and in vacuum service (because gas bubbles

dissolved in the filling fluid might come out of solution). For these reasons, pressure repeaters

often are preferred to seals in such service. Pressure repeaters are available with 0.1% to 1% of

span accuracy and with absolute pressure ranges from 0-5 mm Hg to 0-50 psia (0-0.7 to 0-350

kPa).

Pressure Gauge design for a regular style gauge.

Pressure Gauge design for a filled style gauge.

SOME COMMON PRESSURE GAUGE TYPES

Commercial Gauges

The high reliability of the OMEGA® Commercial Gauge line is chiefly

attributed to the unique OMEGA® Spring Suspended Movement. The entire

movement is suspended between two springs, the Bourdon tube above and

the link below. Wearing parts have been reduced to a minimum.

Furthermore, these movement parts are ultrasonically cleaned and lubricated with silicone oil to

ensure long cycle life. The OMEGA® Spring Suspended Movement is largely resistant to the

effects of shock, pulsation, and vibration. The result of this is longer gauge life. The numerous

applications for OMEGA® Commercial Gauges include installation on pumps, portable

compressors, industrial machinery, hydraulic and pneumatic systems, instrumentation, and

pressurized vessels. For the user, this means greater resistance to mechanical shock and

vibration. This increased resistance to the effects of rough usage contributes to longer gauge life.

Liquid Fillable Utility Gauges

OMEGA’s PGUF series liquid fillable utility gauges are designed for a wide range of

applications on pumps, compressors, hydraulic systems, machine tools and petrochemical

processing equipment. For high shock and vibration applications the PGUF can be liquid filled in

the field to dampen the gauge pointer movement. The PGUF Series liquid filled gauges come in

ranges of (PSI/BAR) 0-30/2, 60/4, 100/7, 160/11, 300/20, 600/40 and

1000/70 (63 mm (21.2") gauge is also available in 2000/140 and 3000/200).

Rated accuracy is ± 2.5% of span. Gauges feature a corrosion resistant 304

stainless steel case and ring, and a durable polycarbonate window. Phosphor

bronze bourdon tubes with brass movement, socket and NPT connections

are standard. PGUF gauges are available in 38 mm (11.2"), 50 mm (2") and 63 mm (21.2")

designs with center back or lower mount NPT connections.

General Service Gauges

Sustained accuracy and excellent readability are important user features of

the OMEGA® General Service Gauge line. Meeting the requirements of

many industrial applications, General Service Gauges can be used on steam

boilers or other pressurized vessels; on pumps and compressors; on many

types of industrial machinery; in the chemical, petrochemical and allied process industries; in

power plants; in pulp and paper mills. The pressure gauge actuation system in the OMEGA®

General Service Gauge line is the standard 316 SS Bourdon tube system, which is engineered to

precise tolerances for consistent repeatability and response to pressure fluctuations. Warning: All

gauge components should be chosen with an eye to the media and ambient operating conditions

to which they will be exposed, to prevent misapplication. Improper application can be

detrimental to the gauge, cause failure, and possible personal injury or property damage.

Stainless Steel Industrial Pressure Gauges

OMEGA’s PGM series gauges are suitable for corrosive environments in chemical, petro-

chemical, refining, power, marine and food and pharmaceutical processing applications. The

PGM series comes in a 63 mm (21.2") or 100 mm (4") case, features all stainless steel

construction and is environmentally protected to IP65. The PGM series can also be liquid filled

in the field. Liquid filled pressure gauges provide users with a number of advantages in certain

applications. They are ideally suited for uses on equipment where excessive vibration and

pulsation are encountered such as pumps, compressors and machine tools etc. The liquid fill

minimizes the effect of these severe environments, protects the gauge internals and provides

continuous lubrication in the mechanism, all adding up to extended service life. Liquid filling

provides greater protection of the gauge internals from corrosive atmospheres. Gauges can be

filled with a variety of fluids including glycerin, mineral oil and silicone oil.

Industrial Process Gauges

These gauges are a very popular style for the factory floor. Thousands are

installed world wide to monitor process pressures. Available in vacuum,

compound and ranges up to 20,000 psi (1380 bar). A hermetic seal provides

greater protection and efficiency.

Type T: High Accuracy and Monel Wetted

Highly accurate gauges designed for use in instrument shops, plants of all

types, and laboratories throughout industry. They provide sustained

accuracy to 0.25% full scale for most models. Performance, reliability and

precision measurement are coupled with consistent accuracy in meeting the

demanding service needs of numerous test gauge applications. These gauges are often used as

master reference gauges, in test stand measurements, for production inspection, and for verifying

the accuracy of general service gauges. They have a stainless steel mirror ring for pointer

reflection to prevent parallax error. This mirror surface reflects the pointer in any position and

allows the gauge to be read with great accuracy.

Differential Pressure Gauges

Differential pressure gauges are rugged industrial gauges which indicate the difference between

two input connections. Differential ranges provide the maximum resolution for applications

where one input is always at a higher pressure than the other. In cases where one input can be

higher or lower than the other, a bi-directional differential range should be used. The OMEGA

PGD Series is constructed with two independent Bourdon tubes. The opposing Bourdon tubes

are linked to a single pinion gear which rotates a pointer for direct pressure readings. By using

two independent Bourdon tubes, the gauge can handle liquids or gases on either or both ports.

Gauge Pressure

A gauge is often used to measure the pressure difference between a system and the surrounding atmosphere. This pressure is often called the gauge pressure and can be expressed as

pg = ps - patm        

where

pg = gauge pressure

ps = system pressure

patm = atmospheric pressure

1. Unit of thickness of a metal sheet or wire. (1) For sheet metal, a retrogressive scale (higher numbers mean lower thickness) that starts with 10 gauge representing a thickness of 3.416 millimeters or 0.1345 inches. As the gauge number increases, the thickness drops by 10 percent. For example, a 12 gauge sheet is 2.732 millimeters thick, and a 13 gauge sheet is 2.391 millimeters thick. (2) For wire thickness

there are two scales, see American wire Gauge for the first one. The second is a metric scale in which a gauge number is equal to 10 times the diameter of the wire in millimeters. For example, a 5 gauge wire is 0.5 millimeter in diameter and a 6 gauge wire is 0.6 millimeter in diameter. Also spelled as gage.

2.  Instrument with a graduated dial or scale used in measuring a dimension or quantity or for establishing mechanical accuracy.

Absolute pressure

   DefinitionSum of the gauge pressure and atmospheric (barometric) pressure which is about 14.7 pounds per square inch (PSI) at sea level.

A b s o l u t e P r e s s u r e :Absolute Pressure is the sum of the available atmospheric pressure and the gage pressure in the pumping system

Absolute Pressure(PSIA) = Gauge Pressure + Atmospheric Pressure

Absolute Pressure = 150 PSIG (Gauge Pressure) + 14.7 PSI(Atmospheric Pressure) = 164.7 PSIA

Absolute Pressure

Absolute Pressure :

Photo illustrating Absolute Pressure

Absolute pressure is calculated by using a vacuum as the zero point and including the gauge and atmospheric pressure in the calculation.

TERMS RELATED TO ABSOLUTE PRESSURE

PSIA

Abbreviation: pounds per square inch absolute. See absolute pressure.

PRESSURE GRADIENT

1. A scale of pressure differences in which there is a uniform variation of pressure from point to point. For example, the pressure gradient of ...

HYDROSTATIC PRESSURE

The force exerted by a body of fluid at rest. It increases directly with the density and the depth of the fluid and is expressed ...

POUNDS PER SQUARE INCH GAUGE (PSIG)

The pressure in a vessel or container as registered on a gauge attached to thecontainer. This reading does not include the pressure of the ...

NORMAL FORMATION PRESSURE

Formation fluid pressure equivalent to about 0.465 pounds per square foot of depth from the surface. If the formation pressure is 4,650 pounds per square ...

ON-VACUUM

Said of any pressure-tight vessel or container when the internal pressure is lower than atmospheric pressure.

Fluids Pressure

A fluid is a substance that flows easily. Gases and liquids are fluids, although sometimes the dividing line between liquids and solids is not always clear. Because of their ability to flow, fluids can exert buoyant forces, multiply forces in a hydraulic systems, allow aircraft to fly and ships to float.

The topic that this page will explore will be pressure and depth. If a fluid is within a container then the depth of an object placed in that fluid can be measured. The deeper the object is placed in the fluid, the more pressure it experiences. This is because is the weight of the fluid above it. The more dense the fluid above it, the more pressure is exerted on the object that is submerged, due to the weight of the fluid.

The formula that gives the P pressure on an object submerged in a fluid is:

P = r x g x h

Where:

r (rho) is the density of the fluid, g is the acceleration of gravity h is the height of the fluid above the object

If the container is open to the atmosphere above, the added pressure must be included if one is to find the total pressure on an object. The total pressure is the same as absolute pressure on pressure gauges readings,

while the gauge pressure is the same as the fluid pressure alone, not including atmospheric pressure.

Ptotal = Patmosphere + Pfluid

Ptotal = Patmosphere + ( r x g x h )

A Pascal is the unit of pressure in the metric system. It represents 1 newton/m2

Example: Find the pressure on a scuba diver when she is 12 meters below the surface of the ocean. Assume standard atmospheric conditions.

Solution: The density of sea water is 1.03 X 10 3 kg/m3 and the atmospheric pressure is 1.01 x 105 N/m2.

Pfluid = r g h = (1.03 x10 3 kg/m3) (9.8 m/s2) (12 m) = 1.21 x 105 Newtons/m2

Ptotal = Patmosphere + Pfluid = (1.01 x 105) + (1.21 x 105 ) Pa = 2.22 x 10 2 kPa (kilo Pascals)