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NAME
units  unit conversion
Table Of Contents
SYNOPSIS
package require Tcl 8.1
package require units ?2.1?
::units::convert value targetUnits
::units::reduce unitString
::units::new name baseUnits
DESCRIPTION
This library provides a conversion facility from a variety of scientific and engineering shorthand notations into floating point numbers. This allows application developers to easily convert values with different units into uniformly scaled numbers.
The units conversion facility is also able to convert between compatible units. If, for example, a application is expecting a value in ohms (Resistance), and the user specifies units of milliwebers/femtocoulomb, the conversion routine will handle it appropriately. An error will be generated if an incorrect conversion is attempted.
Values are scaled from one set of units to another by dimensional analysis. Both the value units and the target units are reduced into primitive units and a scale factor. Units are checked for compatibility, and the scale factors are applied by multiplication and division. This technique is extremely flexible and quite robust.
New units and new unit abbreviations can be defined in terms of existing units and abbreviations. It is also possible to define a new primitive unit, although that will probably be unnecessary. New units will most commonly be defined to accommodate nonSI measurement systems, such as defining the unit inch as 2.54 cm.
COMMANDS
::units::convert value targetUnits
Converts the value string into a floating point number, scaled to the specified targetUnits. The value string may contain a number and units. If units are specified, then they must be compatible with the targetUnits. If units are not specified for the value, then it will be scaled to the target units. For example,
% ::units::convert "2.3 miles" km 3.7014912 % ::units::convert 300m/s miles/hour 671.080887616 % ::units::convert "1.0 m kg/s^2" newton 1.0 % ::units::convert 1.0 millimeter 1000.0

Returns a unit string consisting of a scale factor followed by a space separated list of sorted and reduced primitive units. The reduced unit string may include a forwardslash (separated from the surrounding primitive subunits by spaces) indicating that the remaining subunits are in the denominator. Generates an error if the unitString is invalid.
% ::units::reduce pascal 1000.0 gram / meter second second

Creates a new unit conversion with the specified name. The new unit name must be only alphabetic (upper or lower case) letters. The baseUnits string can consist of any valid units conversion string, including constant factors, numerator and denominator parts, units with prefixes, and exponents. The baseUnits may contain any number of subunits, but it must reduce to primitive units. BaseUnits could also be the string primitive to represent a new kind of quantity which cannot be derived from other units. But you probably would not do that unless you have discovered some kind of new universal property.
% ::units::new furlong "220 yards" % ::units::new fortnight "14 days" % ::units::convert 100m/s furlongs/fortnight 601288.475303
UNIT STRING FORMAT
Value and unit string format is quite flexible. It is possible to define virtually any combination of units, prefixes, and powers. Valid unit strings must conform to these rules.
A unit string consists of an optional scale factor followed by zero or more subunits. The scale factor must be a valid floating point number, and may or may not be separated from the subunits. The scale factor could be negative.
Subunits are separated form each other by one or more separator characters, which are space (" "), hyphen (""), asterisk ("*"), and forwardslash ("/"). Sure, go ahead and complain about using a minus sign ("") to represent multiplication. It just isn't sound mathematics, and, by rights, we should require everyone to use the asterisk ("*") to separate all units. But the bottom line is that complex unit strings like mkg/s^2 are pleasantly readable.
The forwardslash seperator ("/") indicates that following subunits are in the denominator. There can be at most one forwardslash separator.
Subunits can be floating point scale factors, but with the exception of the leading scale factor, they must be surrounded by valid separators. Subunit scale factors cannot be negative. (Remember that the hyphen is a unit separator.)
Subunits can be valid units or abbreviations. They may include a prefix. They may include a plural suffix "s" or "es". They may also include a power string denoted by a circumflex ("^"), followed by a integer, after the unit name (or plural suffix, if there is one). Negative exponents are not allowed. (Remember that the hyphen is a unit separator.)
Example Valid Unit Strings
Unit String Reduced Unit String

meter 1.0 meter
kilometer 1000.0 meter
km 1000.0 meter
km/s 1000.0 meter / second
/microsecond 1000000.0 / second
/us 1000000.0 / second
kgm/s^2 1000.0 gram meter / second second
30second 30.0 second
30 second 30.0 second
30 seconds 30.0 second
200*meter/20.5*second 9.75609756098 meter / second
SI UNITS
The standard SI units are predefined according to NIST Special Publication 330 . Standard units for both SI Base Units (Table 1) and SI Derived Units with Special Names (Tables 3a and 3b) are included here for reference. Each standard unit name and abbreviation are included in this package.
SI Base Units
Quantity Unit Name Abbr.

Length meter m
Mass kilogram kg
Time second s
Current ampere A
Temperature kelvin K
Amount mole mol
Luminous Intensity candela cd
SI Derived Units with Special Names
Quantity Unit Name Abbr. Units Base Units

plane angle radian rad m/m m/m
solid angle steradian sr m^2/m^2 m^2/m^2
frequency hertz Hz /s
force newton N mkg/s^2
pressure pascal Pa N/m^2 kg/ms^2
energy, work joule J Nm m^2kg/s^2
power, radiant flux watt W J/s m^2kg/s^3
electric charge coulomb C sA
electric potential volt V W/A m^2kg/s^3A
capacitance farad F C/V s^4A^2/m^2kg
electric resistance ohm V/A m^2kg/s^3A^2
electric conductance siemens S A/V s^3A^2/m^2kg
magnetic flux weber Wb Vs m^2kg/s^2A
magnetic flux density tesla T Wb/m^2 kg/s^2A
inductance henry H Wb/A m^2kg/s^2A^2
luminous flux lumen lm cdsr
illuminance lux lx lm/m^2 cdsr/m^2
activity (of a
radionuclide) becquerel Bq /s
absorbed dose gray Gy J/kg m^2/s^2
dose equivalent sievert Sv J/kg m^2/s^2
Note that the SI unit kilograms is actually implemented as grams because 1e6 kilogram = 1 milligram, not 1 microkilogram. The abbreviation for Electric Resistance (ohms), which is the omega character, is not supported.
Also note that there is no support for Celsius or Farenheit temperature. The units conversion routines can only scale values with multiplication and division, so it is not possible to convert from thermodynamic temperature (kelvins) to absolute degrees Celsius or Farenheit. Conversion of thermodynamic quantities, such as thermal expansion (per unit temperature), however, are easy to add to the units library.
SI Units can have a multiple or submultiple prefix. The prefix or its abbreviation should appear before the unit, without spaces. Compound prefixes are not allowed, and a prefix should never be used alone. These prefixes are defined in Table 5 of Special Publication 330 .
SI Prefixes
Prefix Name Abbr. Factor

yotta Y 1e24
zetta Z 1e21
exa E 1e18
peta P 1e15
tera T 1e12
giga G 1e9
mega M 1e6
kilo k 1e3
hecto h 1e2
deka da 1e1
deca 1e1
deci d 1e1
centi c 1e2
milli m 1e3
micro u 1e6
nano n 1e9
pico p 1e12
femto f 1e15
atto a 1e18
zepto z 1e21
yocto y 1e24
Note that we define the same prefix with both the USA ("deka") and nonUSA ("deca") spellings. Also note that we take the liberty of allowing "micro" to be typed as a "u" instead of the Greek character mu.
Many nonSI units are commonly used in applications. Appendix B.8 of NIST Special Publication 811 lists many nonSI conversion factors. It is not possible to include all possible unit definitions in this package. In some cases, many different conversion factors exist for a given unit, depending on the context. (The appendix lists over 40 conversions for British thermal units!) Application specific conversions can always be added using the new command, but some well known and often used conversions are included in this package.
NonSI Units
Unit Name Abbr. Base Units

angstrom 1.0E10 m
astronomicalUnit AU 1.495979E11 m
atmosphere 1.01325E5 Pa
bar 1.0E5 Pa
calorie 4.1868 J
curie 3.7E10 Bq
day 8.64E4 s
degree 1.745329E2 rad
erg 1.0E7 J
faraday 9.648531 C
fermi 1.0E15 m
foot ft 3.048E1 m
gauss 1.0E4 T
gilbert 7.957747E1 A
grain gr 6.479891E5 kg
hectare ha 1.0E4 m^2
hour h 3.6E3 s
inch in 2.54E2 m
lightYear 9.46073E15 m
liter L 1.0E3 m^3
maxwell Mx 1.0E8 Wb
mho 1.0 S
micron 1.0E6 m
mil 2.54E5 m
mile mi 1.609344E3 m
minute min 6.0E1 s
parsec pc 3.085E16 m
pica 4.233333E3 m
pound lb 4.535924E1 kg
revolution 6.283185 rad
revolutionPerMinute rpm 1.047198E1 rad/s
yard yd 9.144E1 m
year 3.1536E7 s
Quantities and Derived Units with Special Names
This units conversion package is limited specifically to unit reduction, comparison, and scaling. This package does not consider any of the quantity names for either base or derived units. A similar implementation or an extension in a typed or objectoriented language might introduce user defined types for the quantities. Quantity type checking could be used, for example, to ensure that all length values properly reduced to meters, or that all velocity values properly reduced to meters/second.
A C implementation of this package has been created to work in conjunction with the Simplified Wrapper Interface Generator (http://www.swig.org/). That package (units.i) exploits SWIG's typemap system to automatically convert script quantity strings into floating point quantities. Function arguments are specified as quantity types (e.g., typedef float Length), and target units (expected by the C application code) are specified in an associative array. Default units are also defined for each quantity type, and are applied to any unitless quantity strings.
A units system enhanced with quantity type checking might benefit from inclusion of other derived types which are expressed in terms of special units, as illustrated in Table 2 of NIST Publication 330 . The quantity area, for example, could be defined as units properly reducing to meter^2, although the utility of defining a unit named square meter is arguable.
REFERENCES
The unit names, abbreviations, and conversion values are derived from those published by the United States Department of Commerce Technology Administration, National Institute of Standards and Technology (NIST) in NIST Special Publication 330: The International System of Units (SI) and NIST Special Publication 811: Guide for the Use of the International System of Units (SI) . Both of these publications are available (as of December 2000) from http://physics.nist.gov/cuu/Reference/contents.html
The ideas behind implementation of this package is based in part on code written in 1993 by Adrian Mariano which performed dimensional analysis of unit strings using fixed size tables of C structs. After going missing in the late 1990's, Adrian's code has reappeared in the GNU Units program at http://www.gnu.org/software/units/
AUTHORS
Robert W. Techentin
Bugs, Ideas, Feedback
This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category units of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.
When proposing code changes, please provide unified diffs, i.e the output of diff u.
Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the leftmost button in the secondary navigation bar.
KEYWORDS
angle, constants, conversion, distance, radians, unit
COPYRIGHT
Copyright © 20002005 Mayo Foundation