Grain (mass)

In many cultures, a grain is a unit of measurement of mass that is based upon the mass of a single seed of a typical cereal. Historically, in Europe, the average masses of wheat and barley grain were used to define units of mass. Since 1958, the grain or troy grain (Symbol: gr) measure has been defined in the International System of Units as precisely . Thus, there are precisely 7,000 grains per avoirdupois pound in the U.S. customary units. In fact, the grain is the only unit of mass measure common to the traditional three English mass and weight systems (avoirdupois, Apothecaries’, troy). Moreover, the measure for pearls and diamonds — the pearl grain and the metric grain — are equal to of a (metric) carat, i.e. 50 mg (0.77 gr).

Usage in North America

The grain is used, in the U.S. and Canada, to measure the mass of bullets and of gunpowder; it is the measure used by the balances used in handloading; bullets are measured in increments of one grain, gunpowder in increments of 0.1 grains. Moreover, the grain is used to weigh fencing equipment, including the foil; in archery, the grain measures arrows and arrow parts in archery.

Grains are still used occasionally in medicine in the United States, especially in medical prescriptions, usually via the abbreviation "gr." For example, a regular tablet of aspirin is sometimes referred to as "five grain aspirin," or 325 mg. Grains are commonly used for medications that have been included in the United States Pharmacopeia for many decades, such as codeine, opium and phenobarbital combinations. For example, a prescription for tablets containing 325 mg of aspirin and 30 mg of codeine (brand name is Empirin with codeine), is written thus: "ASA gr. v c cod. gr. ss tablets," where "ASA" is short for aspirin (AcetylSalicylic Acid), "v" is the roman numeral for five, "c" is the abbreviation for "with" and "ss" stands for one-half. Likewise, a prescription for B&O Supprettes #15A, which is a compound medication containing belladonna alkaloids and opium, may be written: "Belladonna gr. 1/4 c opium gr ss", as B&O Supprettes #15A contain 16.2 mg (1/4 grain) of powdered belladonna and 30 mg (1/2 grain) of opium. Similarly, a prescription for 60 mg (1 grain) of phenobarbital is often written: "Phenobarb. gr. i". Formulations of these older medications (e.g., Donnatal, Phenobarbital, etc.), often use grains on the product label along with the metric equivalent. For example, Extended-Release Donnatal tablets contain (approximately 48.6 mg) of phenobarbital. Given the potential error in mistaking the abbreviations for "grains" and "grams" (gr and g, respectively), and for consistency with other medical orders, metric units are preferred to avoirdupois or apothecary units; hence, the use of grains in the medical profession is rapidly becoming outmoded.

Grains are also used in environmental permitting to quantify particulate emissions. Grains are used to measure the amount of moisture per cubic foot of air, a measure of absolute humidity.

History

carob seed	~200 mg
barley grain	~65 mg
wheat grain	~50 mg

At least since antiquity, 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 longstanding tradition, 1 carat (the mass of a carob seed) was equivalent to the weight of 4 wheat grains or 3 barleycorns. But since the weights of these seeds are highly variable, especially that of the cereals as a function of moisture, this is a convention more than an absolute law.

The history of the modern troy grain can be traced back to a royal decree in 13th century England: The traditional reading of this text is that it refers to the troy pound, and that the reference to sterling pennies is purely symbolic. According to a more recent reading, however, the pound in question is the Tower pound, and it talks about the actual mass of real sterling pennies.. The Tower pound, abolished in 1527, consisted of 12 ounces like the troy pound, but was lighter. In any case, with both readings one needs to substitute 24 barley grains for the 32 wheat grains of the text, according to the general convention of a 4:3 equivalence, for it to make sense. The weight of the original sterling pennies was 22½ troy grains, or 24 "Tower grains" if the Tower pound was divided in the same way as the troy pound. Regardless of which pound this text originally referred to, a (troy) ounce still equals 20×24 = 480(troy) grains, and a pound consists of 12×20×24 = 5760 grains.

Originally the troy pound was only "the pound of Pence, Spices, Confections, as of Electuaries", and the merchants used different standards, which had to be compatible with those used abroad. One such standard, the avoirdupois pound, was later fixed officially at exactly 7000 troy grains. It consists of 16 avoirdupois ounces of 437½ troy grains each.

See also

• English unit
• Imperial unit
• United States customary units

References