Glossary

Below is a glossary of terms sometimes used at the Metric Maven Blog.

Engineering Notation — A term for the separation of prefixes by magnitudes of 1000 which in turn relate to parts designations. Here is a table of prefixes:

Click on image to enlarge

In resistors we have ohms, kiloohms, Megaohms. For capacitors  microfarads, nanofarads and picofarads. Most people in everyday life only need micro, milli, kilo and mega. These directly relate to how numbers are grouped in everyday life with commas:1,000 (Kilo), 1,000,000 (Mega), 1,000,000,000 (Giga).

Gauge — The term gauge in relation to measurement quantities is a proxy unit and a retread unit. The term is used in place of a direct dimensional description. For instance a 12 gauge shotgun, or 19 gauge sheet metal, or American Wire Gauge 18 do not relate the same dimensional value. There is no correlation between these gauge “designations.”

Implied Precision Fallacy — This is the notion that the size of a measurement unit determines the precision which one must use when measuring. For instance if one uses mm to measure a person’s height, say 1715 mm it is often argued this is “too much precision” and 171.5 cm should be used and rounded to 172 cm instead.  This is pedantic measurement folklore, and has been demonstrated as such by the Australian Construction Industry. They use mm exclusively for building construction, not because they must have mm precision (although that’s not undesirable), but because they can use integers and eliminate decimal points. It is the ease of usage of the metric system that drives the use of a given prefix. Naughtin’s 2nd Law: Prefer Measures Without Decimals. (i.e. choose prefixes in groups of 1000 (see Engineering Notation) so that you may use integer comparisons without decimal points) Measurement progresses as measurement values are more precise and accurate. Setting a limit on precision undermines measurement advancement.

Large Number Fallacy — This is the fictitious idea that large integer numbers are hard for humans to comprehend, and that only numbers near unity are understandable. This false assertion may be numerical folklore which has outlived the era when roman numerals were commonly used for computation. Computation using roman numerals with large numbers is intellectually taxing. Because of the Arabic numeral system, large integer numbers are easily understood and computed. 1500 mm is 1.500 meters, and expressed without a decimal point.. The Australian construction industry demonstrates daily, that the use of large integer numbers is a cognitive advantage, and not a difficulty. They build all their houses and buildings, in millimeters, and because of it, save about 10-15% on their construction costs, when compared to the US.

Magnitude Triad — This describes the three digit separations used for prefixes in the modern metric system. The table below illustrates the separation of metric data into magnitude triads with spaces as separators:

Table-7Milligrade — The idea of multiplying Celsius temperature values by 10 to obtain a three digit integer to describe temperature. For instance 32.5 Celsius is 325 milligrade. This would be done to eliminate the original centigrade definition and have values which could not be confused with Fahrenheit temperatures.  For instance is 32 degrees warm or cold?  In Fahrenheit it’s cold, in Celsius it’s warm. With milligrade it’s obvious which scale is under discussion. It’s 32 F or 320 milligrade. The creation of a shorthand milligrade could be of difficulty as it could be confused with mg. We could use the greek word for heat, zesti and have 320 Zesti or 320 degrees Z.

Mongrel Unit — Mongrel units are produced by creating a new unit which is composed of metric and non-SI units. For instance grams per fluid ounce is used by some sports nutritionists.

Normalized Proxy Unit — a normalized proxy unit is one which represents no actual dimension, but is used across industry as a proxy without consistency. The canonical example is the proxy unit called gauge. Gauge has at least 14 definitions. It is a proxy unit which is used for metal thickness, wire diameter, drill bit diameter, and others.  It is a proxy unit which has no direct relationship to the quantity it describes. The use of gauge is generally allowed on metric drawings as it is not considered to be a dimension.

PigFish — This generally refers to two situations: 1) When metric and imperial parts are mixed in an engineered device. An example would be the International Space Station which was build with metric and imperial parts. Incandescent light bulbs are also PigFish.  They have screw bases defined with a metric diameter, but their maximum bulb diameter is described in multiples of 1/8 of an inch. 2) When a drawing contains both metric and imperial dimensions to describe an object which are often mutually exclusive. It is not dual-dimensioning. For instance a screw shown on a drawing with an M5 thread, but which has a standard length designated in inches, and no metric length is shown..

Prefix Cluster Around Unity — This is shorthand for the centi, deci, deka and hecto prefixes that are not part of the 1000 unit separation considered best practice. They are throwbacks to the mistaken belief that humans need measurment units near unity. They are also an indirect example of Unit Proliferation or more accurately prefix proliferation.  For distance, km, m, and mm are all separated by 1000 and considered good practice. In the US we naturally place commas to separate magnitude increases of 1000 (eg. 1,000 10,000 and 100,000 meters = 1 km, 10 km and 100 km) This grouping is also known as Engineering Notation.

Prefix Kludge — When a metric prefix is used to magnify or reduce an imperial/USC/ACSOWM unit rather than SI. This is attempting to kludge the current non-system with multipliers, which only leads to unit proliferation. For instance, in running, kmi  is kilomiles, which could easily be confused with km with an additional  i  tacked on the end. Another example is the use of kips for kilopounds.

Proxy Unit — a proxy unit is one which is used to indirectly describe a measurement which is of interest, using one which has no readily  accessible direct relationship to the dimension of interest. For instance the use of 1/2 ounce copper to describe the thickness of printed circuit boards.

Oxymoron Unit — this is a new measurement unit generally created by use the word metric as a modifier of an existing imperial unit, rather than using a metric unit.. Examples are metric carat, metric ton, metric cup. There is no need for these new “units.” They are redundant and are also an example of unit proliferation. The metric ton is properly called a Megagram.

Retread Unit — This is a unit whose name is utilized over and over to describe differing magnitudes or even quantities. For instance the length known as an ell, can be many different sizes depending on the country of origin. The ell is consistently a length. The inch is also a retread unit and consistently a length. An example of a retread unit which describes more than one type of quantity, is the ounce. It can be mass as in Troy ounce or Avoirdupois ounce, or describe a volume of fluid as in fluid ounces.  The metric system (SI) was developed to eliminate all retread units.

Unit Proliferation — the creation of new non SI units which are redundant descriptions quantities which can be expressed with basic SI units and prefixes. An example would be kips for kilopounds, or Angstroms where nanometers are sufficient.