In the Dutch language a term for inch is engelse duim (english thumb). In many other European languages, the word for "inch" is the same as or derived from the word for "thumb", as a man's thumb is about an inch wide (and this was even sometimes used to define the inch ). "Inch" is cognate with " ounce" ( Old English: ynse), whose separate pronunciation and spelling reflect its reborrowing in Middle English from Anglo-Norman unce and ounce. Both were features of Old English phonology see Phonological history of Old English § Palatalization and Germanic umlaut § I-mutation in Old English for more information. The consonant change from the Latin /k/ (spelled c) to English /tʃ/ is palatalisation. The vowel change from Latin /u/ to Old English /y/ (which became Modern English /ɪ/) is known as umlaut. The English word "inch" ( Old English: ynce) was an early borrowing from Latin uncia ("one-twelfth Roman inch Roman ounce"). Standards for the exact length of an inch have varied in the past, but since the adoption of the international yard during the 1950s and 1960s the inch has been based on the metric system and defined as exactly 25.4 mm. Derived from the Roman uncia ("twelfth"), the word inch is also sometimes used to translate similar units in other measurement systems, usually understood as deriving from the width of the human thumb. It is equal to 1 / 36 yard or 1 / 12 of a foot. The inch (symbol: in or ″) is a unit of length in the British imperial and the United States customary systems of measurement. Measuring tape with inches A fire hydrant marked as 3-inch For other uses, see Inch (disambiguation). The manufacturing tolerances are much tighter than in pipes."Inches" redirects here. Tubing is measured by the exact outside diameter and the wall thickness (WT). On the other hand, tubes are structural members and are measured with an exact outside diameter. As stated above, they are measured with nominal outside diameters using NPS and Schedule to define the pipe size. Pipes are used to transport gases or fluids. However, some process piping, utility facilities, and civil piping may require higher schedule numbers depending on the pressures used. The two most common schedule sizes seen in buildings are Schedule 40 and Schedule 80. Stainless Steel pipe schedules append an "S" to the end of the schedule number, for instance Schedule 40S. It was originally intended for these to be phased out, but they occasionally appear. These were original terms used prior to the schedule system that was implemented in 1927. In addition to the schedule numbers, you will often see Standard (STD), Extra Strong (XS), and Double Extra Strong (XXS). The schedule number is roughly calculated as: Schedule = 1000 x (P/S) where P is the internal service pressure of the pipe (psig) and S is the ultimate tensile strength of the pipe material (psi). Wall thickness is important because it determines the amount of internal pressure that the pipe can withstand. A pipe's schedule is the thickness of the pipe wall, which directly affects the interior dimension and weight of the pipe. While the Nominal Pipe Size identifies the outside diameter of the pipe, more information is needed to identify specific pipes. The DN dimensions must conform to ISO Standard 6708. Outside of North America, pipes are identified by Diametre Nominel (DN), which is a dimensionless value that roughly equates to the outside diameter of the pipe in mm. When specific pipes are specified, the NPS, Schedule, and material are required. Nominal Pipe Sizes apply to all materials of pipe. However, from NPS 14 and above, the NPS value matches the outside diameter of the pipe. Technically, the NPS is non-dimensional and only roughly refers to the diameter of the pipe. Nominal Pipe Size (abbreviated NPS) is a North American standard for identifying pipe sizes.
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