Grain (unit of mass)

In metrology, the grain (gr) is an obsolete unit of measurement of mass, and in the Troy, avoirdupois and Apothecary weight systems, equal to exactly 64.79891 milligrams. It is nominally based on the mass of a single ideal seed of a cereal. From the Bronze Age to the Renaissance, the average masses of wheat and barley grains were part of the definitions of units of mass. Expressions such as "thirty-two grains of wheat, taken from the middle of the ear" (eng. "thirty-two grains of wheat, taken from the middle of the ear") appeared to have been ritualistic formulas, essentially, the pre-modern equivalent of "boilerplate". Another source says that it was defined such that 252,458 units would balance 1 cubic inch (16 cm³) of distilled water at an ambient air-water pressure and temperature of 30 inches of mercury (100 kPa) and 62 °F (17 °C), respectively. Another book says that Captain Henry Kater of the British Standards Commission arrived at this value experimentally.

The grain was the legal foundation of the traditional English system of weights, and is the only unit that is equal across the Troy, avoirdupois, and apothecary mass systems. The unit was based on the weight of a single grain of barley, considered equivalent to 1 1⁄3 grains of wheat. The fundamental unit of the pre-1527 English weight system, known as tower weights, was a different type of grain known as the "kernel of wheat" (eng. wheat grain). The Tower wheat kernel was defined as exactly 45⁄64 gr (45 mg).

Since the implementation of the international yard and pound agreement of July 1, 1959, the grain measure or Troy grain (gr) has been precisely defined in terms of units of mass in the International System of Units, as 64,798 91 milligrams. One gram is thus approximately equivalent to 15.432 36 grains. The unit, first, used by jewelers to measure pearls, diamonds, and other precious stones, called the jeweler's grain (eng. jeweler's grain) or pearl grain (eng. pearl grain), is equal to 1⁄4 carats (50 mg; 0.77 gr). The grain was also the name of a traditional French unit equal to 53,115 mg.

In both, the "British Imperial units" and the "United States customary units", there are precisely 7,000 grains per avoirdupois pound, and 5,760 grains per Troy pound or apothecary pound. It is obsolete in the UK and like most other non-SI units, it has no basis in law and cannot be used in trade.

It is used to estimate with more sensitivity and precision the low mass of small objects (medications, drugs, gunpowder, projectiles, pieces of jewelry, etc.).

Current use

The grain is commonly used to measure the mass of bullets and propellants. In archery, the grand is the standard unit used to weigh arrows.

In North America, water hardness is often measured in grains per US gallon (gpg) of calcium carbonate equivalents. Otherwise, water hardness is measured in dimensionless units of parts per million (ppm), numerically equivalent to density measured in milligrams per liter. One large per US gallon is approximately 17.1 ppm. Soft water contains 1 to 4 gpg of calcium carbonate equivalents, while hard water contains 11 to 20 gpg.

Although no longer recommended in the US, the grains were occasionally used in medicine as part of the Apothecary system, especially in prescriptions for older medicines, such as aspirin or phenobarbital. For example, the dose of a standard 325 mg aspirin tablet is sometimes given as 5 grains. In that example, the grain is close to 65 mg, although the grain may also be close to 60 mg, depending on the medication and manufacturer. The Apothecary system has its own system of notation, in which the unit symbol or abbreviation is followed by the amount in lowercase Roman numerals. For amounts less than one, the amount is written as a fraction, or by a half, ss (or variations such as ss., ṡṡ, or s̅s̅). Therefore, a prescription for tablets containing 325 mg of aspirin and 30 mg of codeine can be written in English as "ASA gr. v c̄ cod. gr. ss tablets" (using the ASA medical abbreviation for asprin, c̄ for "with" and cod. for codeine). The apothecary system has gradually been replaced by the metric system, and the use of grain in prescriptions is now rare.

In the U.S., particulate emission levels, used to monitor and regulate pollution, are sometimes measured in grains per cubic foot rather than the more common parts per million (ppm) by volume. This is the same unit commonly used to measure the amount of moisture in the air, also known as absolute humidity. The SI unit used to measure particulate emissions and absolute humidity is mg/m³. One grain per cubic foot is approximately 2288 mg/m³.

History

At least since ancient times, grains of wheat or barley were used by Mediterranean traders to define units of mass, along with other seeds, especially those of the carob tree. According to a long-standing tradition, 1 carate (the mass of a carob seed) was equivalent to the weight of 4 grains of wheat or 3 of barley. Since the weights of these seeds are highly variable, especially that of cereals as a function of moisture, this is a convention rather than an absolute law.

The history of the great modern Briton can be traced back to the royal decree in 13th century England, reiterating decrees going as far back as King Offa (8th century century). The pound of the tower (eng. Tower pound) was one of many monetary pounds of 240 silver pennies (eng.. silver pennies).

By the consent of the whole kingdom the measure of the king was made, so that an English penny, which is called the Sterling, round without cut, shall weigh thirty-two grains of dry wheat in the middle of the ear; twenty pence makes an ounce; and twelve ounces make a pound.

- Tractatus de Ponderibus et Mensuris,

The pound in question is the Tower pound. The Tower pound, abolished in 1527, consisted of 12 ounces like the Troy pound, but was 1⁄16 (≈6%) lighter. The weight of the original Sterling pennies was 22 ½ Troy grains, or 32 "Tower grains".

The great physical weights were made and sold, commercially, at least as late as the early 1900s, and took various forms, from squares of sheet metal to shapes made of wires and weights such as currency.

The Troy pound was only "the pound of Pence, Spices, Confections, as well as Electuaries), as such goods must be measured by a "troy" or small scale. The old troy standard was established by King Offa's currency reform, it was in full use in 1284 (Assize of Weights and Measures, King Edward I), but was restricted to currency (the pound of pence) until abolished in 1527. This pound was progressively replaced by a new pound, based on the weight of 120 dirhams of 48-grain silver. The new pound used one grain of barley, rather than the grand of wheat.

The avoirdupois (weight goods), refers to those things measured by the smaller but quick scales: the bismar or auncel, the Roman scale, and the steelyard scale. The original merchant pound of 25 shillings or 15 ounces from the Tower they were displaced by several the Hanseatic League pound (16 ounces of the Tower) and by the then important wool trade pound (16 ounces of 437 grains.) A new pound of 7680 grains was inadvertently created as 16 ounces. Troy, referring to a new Troy rather than the old Troy Eventually, the pound of wool won.

The avoirdupois pound was defined in prototype, priced at 6,992 to 7,004 grains. In the "Imperial Weights and Measures Act of 1824", the avoirdupois pound was defined as exactly 7000 grains. The 1855 act authorized Miller's new standard to replace that lost in the fire that destroyed Parliament House. The standard was an avoirdupois pound, the grain being defined as

1/7000 of it.

The division of carate into four grains survived in both senses well into the early 20th century. For pearls and diamonds, the weight is quoted in carats, divided into four grains. The carate was eventually set at 205 milligrams (1877), and later at 200 milligrams. For the gold finesse, the gold factor was given as a weight, the total being a solid 24 carats or 96 francs.

Equivalences

Finally:

• 7000 grains are equivalent to an avoirdupois pound
  • 5760 grains equal to one troy pound
  • 5760 grains are equivalent to a pharmaceutical pound

In the old Castilian system another unit was used, called Castilian grain, which was equivalent to 49,914 milligrams.