[UDUNITS #GXI-877591]: Adding custom unit to XML database

Jason,

I'm afraid you have a problem. The UDUNITS package was created for units of 
physical quantities -- quantities for which 1 kg of salt has the same mass as 1 
kg of anything else. This is not the case for the international unit, however: 
1 IU of vitamin A is equivalent to 0.3 ug of retinol but 0.6 ug of 
beta-carotene (it's also equivalent to 50 ug of L-ascorbic acid -- assuming it 
would make sense to convert between the two vitamins, which it wouldn't). Such 
quantity-dependent definitions violate the fundamental meaning of "unit" -- 
and, consequently, the assumptions built into the UDUNITS package.

The only way I can think of that the UDUNITS package *might* be able to do what 
you want would be to create an "international unit" for each substance. For 
example:

<unit-system>
  <unit>
    <def>0.3 ug</def>
    <aliases>
      <name><singular>vitamin_A_international_unit</singular></name>
      <symbol>vitamin_A_IU</symbol>
    </aliases>
  </unit>
  <unit>
    <def>0.6 ug</def>
    <aliases>
      <name><singular>beta_carotene_international_unit</singular></name>
      <symbol>beta_carotene_IU</symbol>
    </aliases>
  </unit>
  ...
</unit-system>

Also note that
  * The <name> and <symbol> elements have been moved inside the <aliases> 
element;
  * The element </base> has been removed;
  * The singular form has been changed from "international_units" to 
"..._international_unit" (the plural form will take care of itself);
  * The <singular> element is no longer within the <symbol> element (symbols 
can't be singular or plural; only names can); and
  * The symbol "U" has been eliminated because its reserved for the 
"international unit of enzyme activity", which is not the IU according to 
Wikipedia;

The attachment of such information to a unit is anathema to me for the reasons 
given in section 7.4 of 
<https://www.nist.gov/pml/nist-guide-si-chapter-7-rules-and-style-conventions-expressing-values-quantities>.
 In my considered opinion, such matters are better handled by a "quantities" 
package rather than a "units" package.

Good luck.

> I have been using the UDUNITS2 library for about a year now.  I've come
> across a unit that isn't in the library and I would like to add it.
> The unit is "Internation Unit" and is a mass or mole based unit for
> measuring quantities of proteins.  The unit is coming from a database
> file, and I'd like to enable conversion of the various proteins from
> U/L to pmol/mL or ng/mL.  I think if I can define U (alias IU, and iU)
> in the database I should be able to do the conversions by multiplying
> a constant of the form mol/U or mg/U to eliminate the U and move on to
> more standard units.
> 
> I've tried to add a new xml file to the database, which would make it
> pretty convenient to add other units as needed.
> 
> <unit-system>
> <import>udunits2-prefixes.xml</import>
> <import>udunits2-base.xml</import>
> <import>udunits2-derived.xml</import>
> <import>udunits2-accepted.xml</import>
> <import>udunits2-common.xml</import>
> <import>udunits2-custom.xml</import>
> </unit-system>
> 
> In my custom file I have:
> <unit-system>
> <unit>
> <!--
> Custom
> -->
> </base>
> <name><singular>international_units</singular></name>
> <symbol>U</symbol>
> <aliases>
> <symbol><singular>iU</singular></symbol>
> <symbol><singular>IU</singular></symbol>
> </aliases>
> </unit>
> </unit-system>
> 
> Can you tell me where I'm going wrong here?

Regards,
Steve Emmerson

Ticket Details
===================
Ticket ID: GXI-877591
Department: Support UDUNITS
Priority: Normal
Status: Closed
===================
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