Standard meter bar

It is the most corrosion-resistant metal known, and was used in making the standard meter bar of Paris, which is a 90 percent platinum and 10 percent iridium alloy. This meter bar was replaced in 1960 as a fundamental unit of length (see under Krypton). 

Iridium is a hard, brittle, white, metallic substance that is almost impossible to machine. It is neither ductile nor malleable. Iridium will only oxidi2e at high temperatures and is the most corrosive-resistant metal known. This is why it was used to make the standard meter bar that is an alloy of 90% platinum and 10% iridium. 

Iridium — (L. iris, rainbow), Ir at. wt. 192.217(3) at. no. 77 m.p. 2446°C b.p. 4428°C sp. gr. 22.42 (17°C) valence 3 or 4. Discovered in 1803 by Tennant in the residue left when crude platinum is dissolved by aqua regia. The name iridium is appropriate, for its salts are highly colored. Iridium, a metal of the platinum family, is white, similar to platinum, but with a slight yellowish cast. It is very hard and brittle, making it very hard to machine, form, or work. It is the most corrosion-resistant metal known, and was used in making the standard meter bar of Paris, which is a 90% platinum-10% iridium alloy. This meter bar was replaced in 1960 as a fundamental unit of length (see under Krypton). Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by molten salts, such as NaCl and NaCN. Iridium occurs uncombined in nature with... 

Standard meter bar an unchanging, reproducible quantity 1 meter is the distance traveled by light in a vacuum in 1/299,792,458 of a second. Length is one of the seven fundamental quantities in the SI system (see Table 1.1). All other physical quantities have units that can be derived from these seven. SI is a decimal system. Quantities differing from the base unit by powers of ten are noted by the use of prefixes. For example, the prefix kilo means "one thousand" (10 ) times the base unit it is abbreviated as k. Thus 1 kilometer = 1000 meters, or 1 km = 1000 m. The SI prefixes are listed in Table 1.2. 

Standards. Flowrate standards conld be significantly simplified if the fundamental bases of these measnrements were as simple as those for mass, length, etc. These systems of measurement are based npon discrete standards or artifacts. For example, the platinum kilogram known as K-20 is the nltimate artifact to provide the fundamental basis for mass measnrement in the U.S. and the platinum meter bar (or its modern-day wavelength equivalent) is the ultimate artifact to provide the fundamental basis for length measnrement. 

One of the earliest applications of optical interferometiy was in length measurements where it led to the replacement of the meter bar by an optical wavelength as the practical standard of length. Several lasers are now available that emit highly monochromatic light whose wavelength has been measured extremely accurately. With such a laser, optical interferometiy canbe used for veiy accurate measurements of distances of a hundred meters or more. 

The unit of length, or distance, is the meter. Originally conceived of as one ten-millionth of the distance from the north pole to the equator through Paris, the meter is more accurately defined as the distance between two scratches on a platinum-iridium bar kept in Paris. The U.S. standard is the... 

Other common ways of expressing abundances, particularly of solid or liquid particles, is to express them as concentrations in units of micrograms per cubic meter or nanomoles per cubic meter. For purposes of consistency, concentrations expressed in these units should be normalized to standard conditions of temperature and pressure. Because there is some confusion as to what constitutes standard conditions in atmospheric chemistry (273 K and 1.013 bar are commonly used in chemistry and physics and 293 K and 1.013 bar are used in engineering), it is important to define the standard conditions that are assumed when reporting data. This explicit definition is frequently not done. Concentrations expressed in these units can be easily converted to mixing ratios by use of the ideal gas law ... 

The differences between the units can be ignored when the exact numerical values are not under consideration, unless otherwise we need the nature of activation volumes in order to obtain some aspects of the reaction mechanism, e.g., 1 kbar = 100 MPa = 1000 kg/cm2 = 1000 atm = 7.5 x 105 mmHg. This is indeed the case in high-pressure synthetic chemistry or preparation under pressure. In the Systeme International d Unites (SI units) adopted by the Conference Generale des Poids et Mesures and endorsed by the International Organization for Standardization, the unit of force is the Newton (N), which is equal to kilogram x (meter per second) per second and is written as kgm s 2. The SI unit of pressure is one Newton per square meter (Nm 2) which is called a Pascal (Pa) 1 bar = 105 Pa thus, the Pa is used in this chapter as an approximate equivalent to other units (Table 1). 

Figure 6.9. Mean concentrations of dissolved species in Checker Reef pore waters and overlying seawater. Vertical bars indicate standard deviations. "FR" and "BK" refer to seawater samples from the fore and back reef stations. Other symbols refer to l-and-2 meter deep sampling stations across the reef (D is the midpoint of the reef). (After Sansone et al., 1990.)...

At the time that the metric system was set up, in 1799, it was intended that the milliliter be exactly equal to the cubic centimeter (cm ). However, it was later found that the relation between the gram, as given by the prototype kilogram, and the centimeter, one one-hundredth of the distance between two engraved lines on a standard platinum-iridium bar, the prototype meter kept in Paris hy the International Bureau of Weights and Measures, is such that the milliliter is not exactly equal to the cubic centimeter, but is instead equal to [.000027 cm . It is obvious that the distinction between ml and cm is ordinarily unimportant. 

A natural gas contains 95 wt% CH4 and the balance C2H6. Five hundred cubic meters per hour of this gas at 40 C and 1.1 bar is to be burned with 25% excess air. The air flowmeter is calibrated to read the volumetric flow rate at standard temperature and pressure. What should the meter read (in SCMH) when the flow rate is set to the desired value ... 

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