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38-424: The MERV scale is designed to represent the worst-case performance of a filter when dealing with particles in the range of 0.3 to 10 micrometers . The MERV value is from 1 to 16. Higher MERV values correspond to a greater percentage of particles captured on each pass, with a MERV 16 filter capturing more than 95% of particles over the full range. The following table is a grouping of MERV values by particle size: While

76-466: A metre (or one thousandth of a millimetre , 0.001 mm , or about 0.000 04   inch ). The nearest smaller common SI unit is the nanometre , equivalent to one thousandth of a micrometre, one millionth of a millimetre or one billionth of a metre ( 0.000 000 001  m ). The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria , and for grading wool by

114-419: A multiple or submultiple of the unit. All metric prefixes used today are decadic . Each prefix has a unique symbol that is prepended to any unit symbol. The prefix kilo- , for example, may be added to gram to indicate multiplication by one thousand: one kilogram is equal to one thousand grams. The prefix milli- , likewise, may be added to metre to indicate division by one thousand; one millimetre

152-408: A " μ " key, so it is necessary to use a key-code; this varies depending on the operating system, physical keyboard layout, and user's language. The LaTeX typesetting system features an SIunitx package in which the units of measurement are spelled out, for example, \qty{3}{\tera\hertz} formats as "3 THz". The use of prefixes can be traced back to the introduction of the metric system in

190-585: A driver, in order to maintain symmetry. The prefixes from tera- to quetta- are based on the Ancient Greek or Ancient Latin numbers from 4 to 10, referring to the 4th through 10th powers of 10 . The initial letter h has been removed from some of these stems and the initial letters z , y , r , and q have been added, ascending in reverse alphabetical order, to avoid confusion with other metric prefixes. When mega and micro were adopted in 1873, there were then three prefixes starting with "m", so it

228-460: A number of definitions for the non-SI unit, the calorie . There are gram calories and kilogram calories. One kilogram calorie, which equals one thousand gram calories, often appears capitalised and without a prefix (i.e. Cal ) when referring to " dietary calories " in food. It is common to apply metric prefixes to the gram calorie, but not to the kilogram calorie: thus, 1 kcal = 1000 cal = 1 Cal. Metric prefixes are widely used outside

266-483: A reintroduction of compound prefixes (e.g. kiloquetta- for 10 ) if a driver for prefixes at such scales ever materialises, with a restriction that the last prefix must always be quetta- or quecto- . This usage has not been approved by the BIPM. In written English, the symbol K is often used informally to indicate a multiple of thousand in many contexts. For example, one may talk of a 40K salary ( 40 000 ), or call

304-558: Is a homograph of the device's name. In spoken English, they may be distinguished by pronunciation, as the name of the measuring device is often stressed on the second syllable ( / m aɪ ˈ k r ɒ m ɪ t ər / my- KROM -it-ər ), whereas the systematic pronunciation of the unit name, in accordance with the convention for pronouncing SI units in English, places the stress on the first syllable ( / ˈ m aɪ k r oʊ m iː t ər / MY -kroh-meet-ər ). The plural of micron

342-662: Is equal to one thousandth of a metre. Decimal multiplicative prefixes have been a feature of all forms of the metric system , with six of these dating back to the system's introduction in the 1790s. Metric prefixes have also been used with some non-metric units. The SI prefixes are metric prefixes that were standardised for use in the International System of Units (SI) by the International Bureau of Weights and Measures (BIPM) in resolutions dating from 1960 to 2022. Since 2009, they have formed part of

380-476: Is known as filter performance rating (FPR), and is reduced from a 1–10 scale to only 4, 7, 9, and 10. It measures ability to capture large particles, small particles, and weight change over the lifetime of the filter. The 3M scale is known as microparticle performance rating (MPR) and measures the ability to filter particles 0.3 to 1 microns. Although the standards measure slightly different attributes and thus, are not strictly convertible, one retailer reported

418-604: Is normally microns , though micra was occasionally used before 1950. The official symbol for the SI prefix micro- is a Greek lowercase mu . Unicode has inherited U+00B5 µ MICRO SIGN from ISO/IEC 8859-1 , distinct from the code point U+03BC μ GREEK SMALL LETTER MU . According to the Unicode Consortium , the Greek letter character is preferred, but implementations must recognize

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456-781: The ISO/IEC 80000 standard. They are also used in the Unified Code for Units of Measure (UCUM). The BIPM specifies twenty-four prefixes for the International System of Units (SI) . The first uses of prefixes in SI date back to the definition of kilogram after the French Revolution at the end of the 18th century. Several more prefixes came into use, and were recognised by the 1947 IUPAC 14th International Conference of Chemistry before being officially adopted for

494-653: The Year 2000 problem the Y2K problem . In these cases, an uppercase K is often used with an implied unit (although it could then be confused with the symbol for the kelvin temperature unit if the context is unclear). This informal postfix is read or spoken as "thousand", "grand", or just "k". The financial and general news media mostly use m or M, b or B, and t or T as abbreviations for million, billion (10 ) and trillion (10 ), respectively, for large quantities, typically currency and population. The medical and automotive fields in

532-534: The year , equal to exactly 31 557 600  seconds ( ⁠365 + 1  / 4 ⁠  days). The unit is so named because it was the average length of a year in the Julian calendar . Long time periods are then expressed by using metric prefixes with the annum, such as megaannum (Ma) or gigaannum (Ga). The SI unit of angle is the radian , but degrees , as well as arc-minutes and arc-seconds , see some scientific use. Common practice does not typically use

570-437: The 11th CGPM conference in 1960. Other metric prefixes used historically include hebdo- (10 ) and micri- (10 ). Double prefixes have been used in the past, such as micromillimetres or millimicrons (now nanometres ), micromicrofarads (μμF; now picofarads , pF), kilomegatonnes (now gigatonnes ), hectokilometres (now 100  kilometres ) and the derived adjective hectokilometric (typically used for qualifying

608-619: The 1790s, long before the 1960 introduction of the SI. The prefixes, including those introduced after 1960, are used with any metric unit, whether officially included in the SI or not (e.g., millidyne and milligauss). Metric prefixes may also be used with some non-metric units, but not, for example, with the non-SI units of time. The units kilogram , gram , milligram , microgram, and smaller are commonly used for measurement of mass . However, megagram, gigagram, and larger are rarely used; tonnes (and kilotonnes, megatonnes, etc.) or scientific notation are used instead. The megagram does not share

646-591: The Latin annus ), is commonly used with metric prefixes: ka , Ma, and Ga. Official policies about the use of SI prefixes with non-SI units vary slightly between the International Bureau of Weights and Measures (BIPM) and the American National Institute of Standards and Technology (NIST). For instance, the NIST advises that "to avoid confusion, prefix symbols (and prefix names) are not used with

684-484: The United States use the abbreviations cc or ccm for cubic centimetres. One  cubic centimetre is equal to one  millilitre . For nearly a century, engineers used the abbreviation MCM to designate a "thousand circular mils " in specifying the cross-sectional area of large electrical cables . Since the mid-1990s, kcmil has been adopted as the official designation of a thousand circular mils, but

722-404: The astronomical unit is mentioned in the SI standards as an accepted non-SI unit. Prefixes for the SI standard unit second are most commonly encountered for quantities less than one second. For larger quantities, the system of minutes (60 seconds), hours (60 minutes) and days (24 hours) is accepted for use with the SI and more commonly used. When speaking of spans of time,

760-406: The diameter of the fibres. The width of a single human hair ranges from approximately 20 to 200 μm . Between 1 μm and 10 μm: Between 10 μm and 100 μm: The term micron and the symbol μ were officially accepted for use in isolation to denote the micrometre in 1879, but officially revoked by the International System of Units (SI) in 1967. This became necessary because the older usage

798-432: The first time in 1960. The most recent prefixes adopted were ronna- , quetta- , ronto- , and quecto- in 2022, after a proposal from British metrologist Richard J. C. Brown. The large prefixes ronna- and quetta- were adopted in anticipation of needs for use in data science, and because unofficial prefixes that did not meet SI requirements were already circulating. The small prefixes were also added, even without such

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836-515: The flexibility allowed by official policy in the case of the degree Celsius (°C). NIST states: "Prefix symbols may be used with the unit symbol °C and prefix names may be used with the unit name degree Celsius . For example, 12 m°C (12 millidegrees Celsius) is acceptable." In practice, it is more common for prefixes to be used with the kelvin when it is desirable to denote extremely large or small absolute temperatures or temperature differences. Thus, temperatures of star interiors may be given with

874-438: The following typical equivalents: Micrometre The micrometre ( Commonwealth English as used by the International Bureau of Weights and Measures ; SI symbol: μm ) or micrometer ( American English ), also commonly known by the non-SI term micron , is a unit of length in the International System of Units (SI) equalling 1 × 10   metre (SI standard prefix " micro- " = 10 ); that is, one millionth of

912-480: The fuel consumption measures). These are not compatible with the SI. Other obsolete double prefixes included "decimilli-" (10 ), which was contracted to "dimi-" and standardised in France up to 1961. There are no more letters of the Latin alphabet available for new prefixes (all the unused letters are already used for units). As such, Richard J.C. Brown (who proposed the prefixes adopted for 10 and 10 ) has proposed

950-422: The hectolitre (100 litres). Larger volumes are usually denoted in kilolitres, megalitres or gigalitres, or else in cubic metres (1 cubic metre = 1 kilolitre) or cubic kilometres (1 cubic kilometre = 1 teralitre). For scientific purposes, the cubic metre is usually used. The kilometre, metre, centimetre, millimetre, and smaller units are common. The decimetre is rarely used. The micrometre is often referred to by

988-418: The length of the day is usually standardised to 86 400  seconds so as not to create issues with the irregular leap second . Larger multiples of the second such as kiloseconds and megaseconds are occasionally encountered in scientific contexts, but are seldom used in common parlance. For long-scale scientific work, particularly in astronomy , the Julian year or annum (a) is a standardised variant of

1026-405: The letter u for the symbol if the Greek letter μ was not available, as in " 15 um ". The Unicode CJK Compatibility block contains square forms of some Japanese katakana measure and currency units. U+3348 ㍈ SQUARE MIKURON corresponds to ミクロン mikuron . Metric prefix A metric prefix is a unit prefix that precedes a basic unit of measure to indicate

1064-641: The lowest MERV value in each row has no minimum requirement for filtering the particle size for that row, it does have stricter requirements for all larger particle sizes than any lower MERV value. For example, MERV 13 has no minimum requirement for removing 0.3–1.0 μm particles (the standard specifies <75%) but has higher minimum requirements for removing 1.0–3.0 μm, 3.0–10.0 μm, and > 10 μm particles than MERV 12 and all lower MERV values. All other MERV values on each row do have minimum removal percentages for that row's particle size. Two proprietary scales are used to rate sub-HEPA air filters. The Home Depot scale

1102-429: The metric SI system. Common examples include the megabyte and the decibel . Metric prefixes rarely appear with imperial or US units except in some special cases (e.g., microinch, kilofoot, kilopound ). They are also used with other specialised units used in particular fields (e.g., megaelectronvolt , gigaparsec , millibarn , kilodalton ). In astronomy, geology, and palaeontology, the year , with symbol 'a' (from

1140-447: The micro sign as well for compatibility with legacy character sets . Most fonts use the same glyph for the two characters . Before desktop publishing became commonplace, it was customary to render the symbol μ in texts produced with mechanical typewriters by combining a slightly lowered slash with the letter u . For example, "15 μm" would appear as " 15 / um ". This gave rise in early word processing to substituting just

1178-453: The older non-SI name micron , which is officially deprecated. In some fields, such as chemistry , the ångström (0.1 nm) has been used commonly instead of the nanometre. The femtometre , used mainly in particle physics, is sometimes called a fermi . For large scales, megametre, gigametre, and larger are rarely used. Instead, ad hoc non-metric units are used, such as the solar radius , astronomical units , light years , and parsecs ;

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1216-423: The prefixes formerly used in the metric system have fallen into disuse and were not adopted into the SI. The decimal prefix for ten thousand, myria- (sometimes spelt myrio- ), and the early binary prefixes double- (2×) and demi- ( ⁠ 1 / 2 ⁠ ×) were parts of the original metric system adopted by France in 1795, but were not retained when the SI prefixes were internationally adopted by

1254-477: The risk of confusion that the tonne has with other units with the name "ton". The kilogram is the only coherent unit of the International System of Units that includes a metric prefix. The litre (equal to a cubic decimetre), millilitre (equal to a cubic centimetre), microlitre, and smaller are common. In Europe, the centilitre is often used for liquids, and the decilitre is used less frequently. Bulk agricultural products, such as grain, beer and wine, often use

1292-440: The time-related unit symbols (names) min (minute), h (hour), d (day); nor with the angle-related symbols (names) ° (degree), ′ (minute), and ″ (second)", whereas the BIPM adds information about the use of prefixes with the symbol as for arcsecond when they state: "However astronomers use milliarcsecond, which they denote mas, and microarcsecond, μas, which they use as units for measuring very small angles." Some of

1330-415: The unit of MK (megakelvin), and molecular cooling may be given with the unit mK (millikelvin). In use the joule and kilojoule are common, with larger multiples seen in limited contexts. In addition, the kilowatt-hour , a composite unit formed from the kilowatt and hour, is often used for electrical energy; other multiples can be formed by modifying the prefix of watt (e.g. terawatt-hour). There exist

1368-546: Was created, it included the " μ " symbol for micro at codepoint 0xB5 ; later, the whole of ISO 8859-1 was incorporated into the initial version of Unicode . Many fonts that support both characters render them identical, but because the micro sign and the Greek lower-case letter have different applications (normally, a Greek letter would be used with other Greek letters, but the micro sign is never used like that), some fonts render them differently, e.g. Linux Libertine and Segoe UI . Most English-language keyboards do not have

1406-421: Was incompatible with the official adoption of the unit prefix micro- , denoted μ, during the creation of the SI in 1960. In the SI, the systematic name micrometre became the official name of the unit, and μm became the official unit symbol. In American English , the use of "micron" helps differentiate the unit from the micrometer , a measuring device, because the unit's name in mainstream American spelling

1444-520: Was necessary to use some other symbol besides upper and lowercase 'm'. Eventually the Greek letter "μ" was adopted. However, with the lack of a "μ" key on most typewriters, as well as computer keyboards, various other abbreviations remained common, including "mc", "mic", and "u". From about 1960 onwards, "u" prevailed in type-written documents. Because ASCII , EBCDIC , and other common encodings lacked code-points for " μ ", this tradition remained even as computers replaced typewriters. When ISO 8859-1

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