Drop (unit)
The drop is a unit of measure of volume, the amount dispensed as one drop from a dropper. It is often used in giving quantities of liquid drugs to patients, and occasionally in cooking.
The volume of a drop is not well-defined: it depends on the device and technique used to produce the drop, on the strength of the gravitational field, and on the density and the surface tension of the liquid. [1]
5 drops of water equals 1 mL and about 150 drops equals 1-oz (US Fluid). A standard unit conversion gives (5.0 drops-water/1.0 mL-water) or (0.2 mL-water/1 drop-water).
There are several exact definitions of a "drop":
- the "metric" drop, 1/20 mL (50 μL).
- the medical drop, 1/12 mL (83⅓ μL).
- the Imperial drop, 1/36 of a fluidram (1/288 of an Imperial fluid ounce, or 1/1440 of a gill) (approximately 99 μL).
- an alternate, possibly apocryphal, definition of the drop is 1/1824 of a gill (approximately 78 μL).
- the U.S. drop, 1/60 of a US fluidram, 1/80 of a teaspoon or 1/480 of a U.S. fluid ounce (approximately 62 μL).
- an alternate definition of the U.S. drop is 1/96 of a teaspoon or 1/576 US fl oz (approximately 51 μL).
In the first decade of the 19th century, the minim, the smallest unit of Apothecary Measure, was promoted by the pharmaceutical and medical establishments as an alternative to the drop.[2] It was noted that the size of a drop can vary considerably depending on the viscosity and specific gravity of the fluid, as well as the size and shape of the vessel from which it is poured. (At the time, surface tension was not well-understood.) The minim came with a set of procedures for ensuring accurate measurement, specifically, diluting powerful medicines that had previously been measured by the drop, then using a "minimometer" or "minim glass" (graduated pipette) with minim marks at regular intervals. The minim was defined as one 60th of a fluid dram or one 480th of a fluid ounce.[3] This is equal to about 61.6 μL (U.S.) or 59.2 μL (Britain).
Pharmacists have since moved to metric measurements, with a drop being rounded to exactly 0.05 mL (that is, 20 drops per milliliter). In hospitals, intravenous tubing is used to deliver medication in drops of various sizes ranging from 10 drops/mL to 60 drops/mL. A drop is abbreviated gtt, with gtts used for the plural. These abbreviations come from gutta, the Latin for drop.[4]
A drop can also be used less formally as a unit of volume in recipes. According to some older kitchen references, 24 drops = ¼ teaspoon. Using U.S. definitions, this makes the drop equal to about 0.051 mL, making it quite comparable to the pharmacist's drop.[4]
[edit] References
- ^ "Drop - size". Physics and Astronomy Online. http://www.physlink.com/education/askexperts/ae681.cfm. Retrieved 2010-03-29.
- ^ Nicholson, William (1809). The British encyclopedia, or Dictionary of arts and sciences comprising an accurate and popular view of the present improved state of human knowledge. Whittingham. p. 264. http://books.google.com/books?id=jYAPAAAAQAAJ&pg=PT264. Retrieved 18 December 2011.
- ^ Royal College of Physicians of London; Richard Powell (1809). The pharmacopoeia of the Royal College of Physicians of London, M. DCCC. IX. Longman, Hurst, Rees, and Orme. p. 6-7. http://books.google.com/books?id=pCPrAAAAMAAJ&pg=PA6. Retrieved 18 December 2011.
- ^ a b "Russ Rowlett". How Many? A Dictionary of Units of Measurement. University of North Carolina. 2005-07-11. http://www.unc.edu/~rowlett/units/. Retrieved 2006-09-07.
