Deoxyribonucleic acid is a molecule composed of two chains that coil around each other to form a double helix carrying the genetic instructions used in the growth, development and reproduction of all known organisms and many viruses. DNA and ribonucleic acid are nucleic acids; the two DNA strands are known as polynucleotides as they are composed of simpler monomeric units called nucleotides. Each nucleotide is composed of one of four nitrogen-containing nucleobases, a sugar called deoxyribose, a phosphate group; the nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. The nitrogenous bases of the two separate polynucleotide strands are bound together, according to base pairing rules, with hydrogen bonds to make double-stranded DNA; the complementary nitrogenous bases are divided into two groups and purines. In DNA, the pyrimidines are cytosine. Both strands of double-stranded DNA store the same biological information.
This information is replicated as and when the two strands separate. A large part of DNA is non-coding, meaning that these sections do not serve as patterns for protein sequences; the two strands of DNA are thus antiparallel. Attached to each sugar is one of four types of nucleobases, it is the sequence of these four nucleobases along the backbone. RNA strands are created using DNA strands as a template in a process called transcription. Under the genetic code, these RNA strands specify the sequence of amino acids within proteins in a process called translation. Within eukaryotic cells, DNA is organized into long structures called chromosomes. Before typical cell division, these chromosomes are duplicated in the process of DNA replication, providing a complete set of chromosomes for each daughter cell. Eukaryotic organisms store most of their DNA inside the cell nucleus as nuclear DNA, some in the mitochondria as mitochondrial DNA, or in chloroplasts as chloroplast DNA. In contrast, prokaryotes store their DNA only in circular chromosomes.
Within eukaryotic chromosomes, chromatin proteins, such as histones and organize DNA. These compacting structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed. DNA was first isolated by Friedrich Miescher in 1869, its molecular structure was first identified by Francis Crick and James Watson at the Cavendish Laboratory within the University of Cambridge in 1953, whose model-building efforts were guided by X-ray diffraction data acquired by Raymond Gosling, a post-graduate student of Rosalind Franklin. DNA is used by researchers as a molecular tool to explore physical laws and theories, such as the ergodic theorem and the theory of elasticity; the unique material properties of DNA have made it an attractive molecule for material scientists and engineers interested in micro- and nano-fabrication. Among notable advances in this field are DNA origami and DNA-based hybrid materials. DNA is a long polymer made from repeating units called nucleotides.
The structure of DNA is dynamic along its length, being capable of coiling into tight loops and other shapes. In all species it is composed of two helical chains, bound to each other by hydrogen bonds. Both chains are coiled around the same axis, have the same pitch of 34 angstroms; the pair of chains has a radius of 10 angstroms. According to another study, when measured in a different solution, the DNA chain measured 22 to 26 angstroms wide, one nucleotide unit measured 3.3 Å long. Although each individual nucleotide is small, a DNA polymer can be large and contain hundreds of millions, such as in chromosome 1. Chromosome 1 is the largest human chromosome with 220 million base pairs, would be 85 mm long if straightened. DNA does not exist as a single strand, but instead as a pair of strands that are held together; these two long strands coil in the shape of a double helix. The nucleotide contains both a segment of the backbone of a nucleobase. A nucleobase linked to a sugar is called a nucleoside, a base linked to a sugar and to one or more phosphate groups is called a nucleotide.
A biopolymer comprising multiple linked nucleotides is called a polynucleotide. The backbone of the DNA strand is made from alternating sugar residues; the sugar in DNA is 2-deoxyribose, a pentose sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings; these are known as the 3′-end, 5′-end carbons, the prime symbol being used to distinguish these carbon atoms from those of the base to which the deoxyribose forms a glycosidic bond. When imagining DNA, each phosphoryl is considered to "belong" to the nucleotide whose 5′ carbon forms a bond therewith. Any DNA strand therefore has one end at which there is a phosphoryl attached to the 5′ carbon of a ribose and another end a
Atmosphere of Earth
The atmosphere of Earth is the layer of gases known as air, that surrounds the planet Earth and is retained by Earth's gravity. The atmosphere of Earth protects life on Earth by creating pressure allowing for liquid water to exist on the Earth's surface, absorbing ultraviolet solar radiation, warming the surface through heat retention, reducing temperature extremes between day and night. By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, small amounts of other gases. Air contains a variable amount of water vapor, on average around 1% at sea level, 0.4% over the entire atmosphere. Air content and atmospheric pressure vary at different layers, air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres; the atmosphere has a mass of about 5.15×1018 kg, three quarters of, within about 11 km of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space.
The Kármán line, at 100 km, or 1.57% of Earth's radius, is used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km. Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition; the study of Earth's atmosphere and its processes is called atmospheric science. Early pioneers in the field include Richard Assmann; the three major constituents of Earth's atmosphere are nitrogen and argon. Water vapor accounts for 0.25% of the atmosphere by mass. The concentration of water vapor varies from around 10 ppm by volume in the coldest portions of the atmosphere to as much as 5% by volume in hot, humid air masses, concentrations of other atmospheric gases are quoted in terms of dry air; the remaining gases are referred to as trace gases, among which are the greenhouse gases, principally carbon dioxide, nitrous oxide, ozone. Filtered air includes trace amounts of many other chemical compounds.
Many substances of natural origin may be present in locally and seasonally variable small amounts as aerosols in an unfiltered air sample, including dust of mineral and organic composition and spores, sea spray, volcanic ash. Various industrial pollutants may be present as gases or aerosols, such as chlorine, fluorine compounds and elemental mercury vapor. Sulfur compounds such as hydrogen sulfide and sulfur dioxide may be derived from natural sources or from industrial air pollution; the relative concentration of gases remains constant until about 10,000 m. In general, air pressure and density decrease with altitude in the atmosphere. However, temperature has a more complicated profile with altitude, may remain constant or increase with altitude in some regions; because the general pattern of the temperature/altitude profile is constant and measurable by means of instrumented balloon soundings, the temperature behavior provides a useful metric to distinguish atmospheric layers. In this way, Earth's atmosphere can be divided into five main layers.
Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere and thermosphere. From highest to lowest, the five main layers are: Exosphere: 700 to 10,000 km Thermosphere: 80 to 700 km Mesosphere: 50 to 80 km Stratosphere: 12 to 50 km Troposphere: 0 to 12 km The exosphere is the outermost layer of Earth's atmosphere, it extends from the exobase, located at the top of the thermosphere at an altitude of about 700 km above sea level, to about 10,000 km where it merges into the solar wind. This layer is composed of low densities of hydrogen and several heavier molecules including nitrogen and carbon dioxide closer to the exobase; the atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another. Thus, the exosphere no longer behaves like a gas, the particles escape into space; these free-moving particles follow ballistic trajectories and may migrate in and out of the magnetosphere or the solar wind. The exosphere is located too far above Earth for any meteorological phenomena to be possible.
However, the aurora borealis and aurora australis sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere. The exosphere contains most of the satellites orbiting Earth; the thermosphere is the second-highest layer of Earth's atmosphere. It extends from the mesopause at an altitude of about 80 km up to the thermopause at an altitude range of 500–1000 km; the height of the thermopause varies due to changes in solar activity. Because the thermopause lies at the lower boundary of the exosphere, it is referred to as the exobase; the lower part of the thermosphere, from 80 to 550 kilometres above Earth's surface, contains the ionosphere. The temperature of the thermosphere increases with height. Unlike the stratosphere beneath it, wherein a temperature inversion is due to the absorption of radiation by ozone, the inversion in the t
A chuck is a specialized type of clamp used to hold an object with radial symmetry a cylinder. In drills and mills it holds the rotating tool. On a lathe the chuck is mounted on the spindle. For some purposes an additional chuck may be mounted on the non-rotating tailstock. Many chucks have jaws, that are arranged in a radially symmetrical pattern like the points of a star; the jaws are tightened up to hold the workpiece. The jaws will be tightened or loosened with the help of a chuck key, a wrench-like tool made for the purpose. Many jawed chucks, are of the keyless variety, their tightening and loosening is by hand force alone. Keyless designs offer the convenience of quicker and easier chucking and unchucking, but have lower gripping force to hold the tool or workpiece, more of a problem with cylindrical than hexagonal shanks. Collet chucks, rather than having jaws, have collets, which are flexible collars or sleeves that fit around the tool or workpiece and grip it when squeezed. Chucks on some lathes have jaws which move independently, allowing them to hold irregularly shaped objects.
A few chuck designs are more complex, involving specially shaped jaws, higher numbers of jaws, quick-release mechanisms, or other special features. Magnetic and vacuum chucks are made, with flat surfaces against which workpieces or tools are held by the pressure of their respective force. To chuck a tool or workpiece is to hold it with a chuck, in which case it has been chucked. Chucking individual slugs or blanks on a lathe is called chucking work. In bar work or bar feed work the stock protrudes from the chuck, is worked upon parted off rather than sawn. Automatic lathes that specialize in chucking work are called chuckers. A self-centering chuck known as a scroll chuck, uses dogs, interconnected via a scroll gear, to hold onto a tool or workpiece; because they most have three jaws, the term three-jaw chuck without other qualification is understood by machinists to mean a self-centering three-jaw chuck. The term universal chuck refers to this type; these chucks are best suited to grip circular or hexagonal cross-sections when fast, reasonably accurate centering is desired.
Sometimes this type of chuck has six jaws instead of three. Four-jawed chucks are useful for gripping square or octagon material, while six-jawed chucks hold thin-walled tubing and plastic materials with minimum distortion. There are independent-jaw chucks with three jaws, but they offer few advantages and are rare. There are hybrid self-centering chucks that have adjustment screws that can be used to further improve the concentricity after the workpiece has been gripped by the scroll jaws; this feature is meant to combine the speed and ease of the scroll plate's self-centering with the run-out eliminating controllability of an independent-jaw chuck. The most used name for this type is a brand name, Set-Tru. To avoid undue genericization of that brand name, suggestions for a generic name have included "exact-adjust". Three-jaw chucks are used on lathes and indexing heads. A drill chuck is a specialised self-centering, three-jaw chuck with capacity of 0.5 in or less and greater than 1 in, used to hold drill bits or other rotary tools.
This type of chuck is used on tools ranging from professional equipment to inexpensive hand and power drills for domestic use. Some high-precision chucks use ball thrust bearings to reduce friction in the closing mechanism and maximize drilling torque. One brand name for this type of chuck, genericized in colloquial use although not in catalogs, is Super Chuck. A pin chuck is a specialized chuck designed to hold small drills that could not be held securely in a normal drill chuck; the drill tightened. Pin chucks are used with high-speed rotary tools other than drills, such as die grinders and jig grinders. On an independent-jaw chuck, each jaw can be moved independently; because they most have four jaws, the term four-jaw chuck without other qualification is understood by machinists to mean a chuck with four independent jaws. The independence of the jaws makes these chucks ideal for gripping non-circular cross sections and gripping circular cross sections with extreme precision; the non-self-centering action of the independent jaws makes centering controllable, but at the expense of speed and ease.
Four-jaw chucks are never used for tool holding. Four-jaw chucks can be found on lathes and indexing heads. Self-centering chucks with four jaws can be obtained. Although these are said to suffer from two disadvantages: inability to hold hex stock, poor gripping on stock, oval, only the latter is true. With three jaw self centering chucks, work, not of uniform section along the work should not be gripped, as the jaws can be strained and the accuracy permanently impaired. Four-jaw chucks can hold a workpiece eccentrically if eccentric features need to be machined. A spider is a simple inexpensive, limited-c
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component, contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston acts as a valve by covering and uncovering ports in the cylinder. An internal combustion engine is acted upon by the pressure of the expanding combustion gases in the combustion chamber space at the top of the cylinder; this force acts downwards through the connecting rod and onto the crankshaft. The connecting rod is attached to the piston by a swivelling gudgeon pin; this pin is mounted within the piston: unlike the steam engine, there is no piston rod or crosshead.
The pin itself is of hardened steel and is fixed in the piston, but free to move in the connecting rod. A few designs use a'fully floating' design, loose in both components. All pins must be prevented from moving sideways and the ends of the pin digging into the cylinder wall by circlips. Gas sealing is achieved by the use of piston rings; these are a number of narrow iron rings, fitted loosely into grooves in the piston, just below the crown. The rings are split at a point in the rim, allowing them to press against the cylinder with a light spring pressure. Two types of ring are used: the upper rings have solid faces and provide gas sealing. There are many detail design features associated with piston rings. Pistons are cast from aluminium alloys. For better strength and fatigue life, some racing pistons may be forged instead. Billet pistons are used in racing engines because they do not rely on the size and architecture of available forgings, allowing for last-minute design changes. Although not visible to the naked eye, pistons themselves are designed with a certain level of ovality and profile taper, meaning they are not round, their diameter is larger near the bottom of the skirt than at the crown.
Early pistons were of cast iron, but there were obvious benefits for engine balancing if a lighter alloy could be used. To produce pistons that could survive engine combustion temperatures, it was necessary to develop new alloys such as Y alloy and Hiduminium for use as pistons. A few early gas engines had double-acting cylinders, but otherwise all internal combustion engine pistons are single-acting. During World War II, the US submarine Pompano was fitted with a prototype of the infamously unreliable H. O. R. Double-acting two-stroke diesel engine. Although compact, for use in a cramped submarine, this design of engine was not repeated. Media related to Internal combustion engine pistons at Wikimedia Commons Trunk pistons are long relative to their diameter, they act both as cylindrical crosshead. As the connecting rod is angled for much of its rotation, there is a side force that reacts along the side of the piston against the cylinder wall. A longer piston helps to support this. Trunk pistons have been a common design of piston since the early days of the reciprocating internal combustion engine.
They were used for both petrol and diesel engines, although high speed engines have now adopted the lighter weight slipper piston. A characteristic of most trunk pistons for diesel engines, is that they have a groove for an oil ring below the gudgeon pin, in addition to the rings between the gudgeon pin and crown; the name ` trunk piston' derives from an early design of marine steam engine. To make these more compact, they avoided the steam engine's usual piston rod with separate crosshead and were instead the first engine design to place the gudgeon pin directly within the piston. Otherwise these trunk engine pistons bore little resemblance to the trunk piston. Their'trunk' was a narrow cylinder mounted in the centre of the piston. Media related to Trunk pistons at Wikimedia Commons Large slow-speed Diesel engines may require additional support for the side forces on the piston; these engines use crosshead pistons. The main piston has a large piston rod extending downwards from the piston to what is a second smaller-diameter piston.
The main piston carries the piston rings. The smaller piston is purely a mechanical guide, it runs within a small cylinder as a trunk guide and carries the gudgeon pin. Lubrication of the crosshead has advantages over the trunk piston as its lubricating oil is not subject to the heat of combustion: the oil is not contaminated by combustion soot particles, it does not break down owing to the heat and a thinner, less viscous oil may be used; the friction of both piston and crosshead may be only half of that for a trunk piston. Because of the additional weight of these pistons, they are not used for high-speed engines. Media related to Crosshead pistons at Wikimedia Commons A slipper piston is a piston for a petrol engine, reduced in size and weight as much as possible. In the extreme case, they are reduced to the piston crown, support for the piston rings, just enough of the piston skirt remaining to leave two lands so as to stop the piston rocking in the bore; the sides of the piston skirt around the gudgeon pin are reduced away from the cylinder wall.
The purpose is to reduce the reciprocating mass, thus making it easier to balan
Vacuum is space devoid of matter. The word stems from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists discuss ideal test results that would occur in a perfect vacuum, which they sometimes call "vacuum" or free space, use the term partial vacuum to refer to an actual imperfect vacuum as one might have in a laboratory or in space. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is lower than atmospheric pressure; the Latin term in vacuo is used to describe an object, surrounded by a vacuum. The quality of a partial vacuum refers to how it approaches a perfect vacuum. Other things equal, lower gas pressure means higher-quality vacuum. For example, a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%. Much higher-quality vacuums are possible. Ultra-high vacuum chambers, common in chemistry and engineering, operate below one trillionth of atmospheric pressure, can reach around 100 particles/cm3.
Outer space is an higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average in intergalactic space. According to modern understanding if all matter could be removed from a volume, it would still not be "empty" due to vacuum fluctuations, dark energy, transiting gamma rays, cosmic rays and other phenomena in quantum physics. In the study of electromagnetism in the 19th century, vacuum was thought to be filled with a medium called aether. In modern particle physics, the vacuum state is considered the ground state of a field. Vacuum has been a frequent topic of philosophical debate since ancient Greek times, but was not studied empirically until the 17th century. Evangelista Torricelli produced the first laboratory vacuum in 1643, other experimental techniques were developed as a result of his theories of atmospheric pressure. A torricellian vacuum is created by filling a tall glass container closed at one end with mercury, inverting it in a bowl to contain the mercury.
Vacuum became a valuable industrial tool in the 20th century with the introduction of incandescent light bulbs and vacuum tubes, a wide array of vacuum technology has since become available. The recent development of human spaceflight has raised interest in the impact of vacuum on human health, on life forms in general; the word vacuum comes from Latin, meaning'an empty space, void', noun use of neuter of vacuus, meaning "empty", related to vacare, meaning "be empty". Vacuum is one of the few words in the English language that contains two consecutive letters'u'. There has been much dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers debated the existence of a vacuum, or void, in the context of atomism, which posited void and atom as the fundamental explanatory elements of physics. Following Plato the abstract concept of a featureless void faced considerable skepticism: it could not be apprehended by the senses, it could not, provide additional explanatory power beyond the physical volume with which it was commensurate and, by definition, it was quite nothing at all, which cannot rightly be said to exist.
Aristotle believed that no void could occur because the denser surrounding material continuum would fill any incipient rarity that might give rise to a void. In his Physics, book IV, Aristotle offered numerous arguments against the void: for example, that motion through a medium which offered no impediment could continue ad infinitum, there being no reason that something would come to rest anywhere in particular. Although Lucretius argued for the existence of vacuum in the first century BC and Hero of Alexandria tried unsuccessfully to create an artificial vacuum in the first century AD, it was European scholars such as Roger Bacon, Blasius of Parma and Walter Burley in the 13th and 14th century who focused considerable attention on these issues. Following Stoic physics in this instance, scholars from the 14th century onward departed from the Aristotelian perspective in favor of a supernatural void beyond the confines of the cosmos itself, a conclusion acknowledged by the 17th century, which helped to segregate natural and theological concerns.
Two thousand years after Plato, René Descartes proposed a geometrically based alternative theory of atomism, without the problematic nothing–everything dichotomy of void and atom. Although Descartes agreed with the contemporary position, that a vacuum does not occur in nature, the success of his namesake coordinate system and more implicitly, the spatial–corporeal component of his metaphysics would come to define the philosophically modern notion of empty space as a quantified extension of volume. By the ancient definition however, directional information and magnitude were conceptually distinct. In the medieval Middle Eastern world, the physicist and Islamic scholar, Al-Farabi, conducted a small experiment concerning the existence of vacuum, in which he investigated handheld plungers in water, he concluded that air's volume can expand to fill available space, he suggested that the concept of perfect vacuum was incoherent. However, according to Nader El-Bizri, the physicist Ibn al-Haytham and the Mu'tazili theologians disagreed with Aristotle and Al-Farabi, they supported the existence of a void.
Using geometry, Ibn al-Haytham mathematically demonstrated that place is the imagined three-dimensional void between the inner surfaces of a containing body. According to Ahmad Dallal, Abū Rayhān al-Bīrūnī states that "there is no observable
A graduated cylinder, measuring cylinder or mixing cylinder is a common piece of laboratory equipment used to measure the volume of a liquid. It has a narrow cylindrical shape; each marked line on the graduated cylinder represents the amount of liquid, measured. Large graduated cylinders are made up of polypropylene for its excellent chemical resistance or polymethylpentene for its transparency, making them lighter and less fragile than glass. Polypropylene is easy to autoclave. A traditional graduated cylinder is narrow and tall so as to increase the accuracy and precision of volume measurement. An additional version is low. Mixing cylinders have ground glass joints instead of a spout, so they can be closed with a stopper or connect directly with other elements of a manifold. With this kind of cylinder, the metered liquid does not pour directly, but is removed using a cannula. A graduated cylinder is meant to be read with the surface of the liquid at eye level, where the center of the meniscus shows the measurement line.
Typical capacities of graduated cylinders are from 10 mL to 1000 mL. Graduated cylinders are used to measure the volume of a liquid. Graduated cylinders are more accurate and precise than laboratory flasks and beakers, but they should not be used to perform volumetric analysis. Graduated cylinders are sometimes used to measure the volume of a solid indirectly by measuring the displacement of a liquid. For accuracy the volume on graduated cylinders is depicted on scales with 3 significant digits: 100mL cylinders have 1ml grading divisions while 10mL cylinders have 0.1 mL grading divisions. Two classes of accuracy exist for graduated cylinders. Class A has double the accuracy of class B. Cylinders can have double scales. Single scales allow to read the volume from top to bottom while double scale cylinders allow reading for filling and pouring. Graduated cylinders are calibrated either “to contain” and marked as "TC" or “to deliver” and marked “TD”; the tolerances for “to deliver” and “to contain” cylinders are distinct.
The international symbols “IN” and “EX” are more to be used instead of “TC” and “TD” respectively. To read the volume the observation must be at an eye level and read at the bottom of a meniscus of the liquid level. For example, in the picture on the bottom of the page, a 100mL graduated cylinder was used to measure the liquid; the cylinder contains 60mL liquid volume. The main reason as to why the reading of the volume is done via meniscus is due to the nature of the liquid in a closed surrounded space. By nature, liquid in the cylinder would be attracted to the wall around it through molecular forces; this forces the liquid surface to develop either a convex or concave shape, depending on the type of the liquid in the cylinder. Reading the liquid at the bottom part of a concave or the top part of the convex liquid is equivalent to reading the liquid at its meniscus. From the picture, the level of the liquid will be read at the bottom of the meniscus, the concave; the most accurate of the reading that could be done here is reduced down to 1 mL due to the given means of measurement on the cylinder.
From this, the derived error would be one tenth of the least figure. For instance, if the reading is done and the value calculated is set to be 36.5 mL. The error, give or take 0.1 mL, must be included too. Therefore, the more precise value equates to 36.5 ± 0.1. Therefore, there are 3 significant figures. Another example, if the reading is done and the value calculated is set to be 40.0 mL. The precise value would be 40.0 ± 0.1.
A graduated pipette is a pipette with its volume, in increments, marked along the tube. It is used to measure and transfer a volume of liquid from one container to another, it has a tapered tip. Along the body of the tube are graduation markings indicating volume from the tip to that point. A small pipette allows for more precise measurement of fluids. Accordingly, pipettes vary with most measuring between 0 and 25.0 millilitres. There are two types of pipettes that differ based on where the markings are located in reference to the pipette tip; these are Mohr pipettes and Serological pipettes, they differ only by the position of the first graduation mark, nearest the tip of the pipette. A Mohr pipette is designed for use as a drain-out pipette, it has a straight graduation marks indicating 0.10 millilitres changes volume. This type of pipette does not have its first graduation mark until well past the base of the tip. An error can occur because of improper use by the person using the pipette or if there is a break or crack in the pipette.
A Serological pipette is designed for use as a blow-out pipette. A Serological pipette has graduation marks, which start nearer the end of the tip; the pipette can be blown out by gravitational air pressure. Rubber bulbs attached to the end opposite the tip are used to "blow out" any remaining solution. Having solution remain in the pipette can affect an experiment by allowing a discrepancy between what is measured and what is transferred; the designation of whether the pipette is "to deliver" or "to contain" is marked on most serological pipettes. Graduated pipettes are classified into three types: Type 1, Type 2, Type 3. Type 1 and Type 3 pipettes have the nominal value at the bottom. For Type 1, the solution is delivered for all volumes. For type 3, the solution is delivered only at the nominal value. Type 2 pipettes have the nominal value at the top and the solution is delivered for any volume. A pipette is worked by creating a partial vacuum above the liquid-holding chamber, to draw up liquid, by releasing the partial vacuum to deliver liquid.
The accuracy of a graduated pipette was not as good as that of a volumetric pipette. Graduated pipettes are considered to be more precise than the Pasteur pipette, they have tolerances that range from ±0.6% to ±0.4% of the nominal volume when measured at 20 °C. Graduated pipettes are manufactured according to ISO specifications for accuracy and the arrangement of the graduations. Grade A and AS pipettes have the highest accuracy, S standing for "swift delivery"; these allowed. Grade B pipettes have twice the allowed error as grade A and AS pipettes; these pipettes come in 5, 10, 25, 50 mL volumes. A variety of propipetters have been developed, both manual and electrically assisted. Pipettes were made of soda-lime glass, but many are made of borosilicate glass; the standard classification of graduated pipettes is according to shape, delivery tips, graduation lines, periods of infill and discharge, calibration. Standard classification The most accurate glass graduated pipettes are classified according to genre and dimension.
The two genres, Genre 1 and Genre 2, denote pipettes with "standard taper tip" and "long taper tip". The two classes are Class A and Class B. Class A pipettes are manufactured to precise tolerances, or "error limits". Class B are allowed twice the error limits of Class A; the class specification or serial number of Class B are not marked. Standard design The Genre 1 and Genre 2 the tapered delivery tips are different: for Genre 1, the tip is between 15 and 30 mm long, for a 5 ml capacity pipette, between 20 and 40 mm, for 10 to 50 ml capacities; the opening at the tip end is perpendicular to the tube axis, an unexpectedly constricted opening is not acceptable for Genre 2 pipettes. Beveling and fire polishing of the external margin of the tip opening are essential; the time allowed for filling and discharge, from the first to last marking, are specified. Such times are measured with a stopwatch and using distilled water at 20 °C; the capacity scale is required to have no less than 90 mm worth of markings, the exception being that 0.5 ml pipettes have not less than 80 mm.
The standard for all pipettes is that markings be indelible and comprehensible, etching markings and using permanent or enamel ink. Each graduation marking is required to not be off by more than 0.40 mm from true, to be perpendicular to the tube axis. Class A: The error limits of this class of glass volumetric equipment is specified by DIN EN ISO 9712, which applies both to Class A itself and to other classes that have a similar designation, such as Class AS. Class AS: Specifies a "swift" delivery pipette, facilitated by the pipette having an expanded tip; the class marking is found near the volume specification. Class B: The different between Class A/AS and Class B is that the error limits for Class B are twice that allowed classes A and AS, Class B pipettes can be made of plastic, the delivery system waiting time isn't specified; the delivery and waiting times represent the e