# Title
udunits supports ppm, but documentation states it does not

# Moderator
None at the moment

# Requirement Summary
The documentation states that udunits does not support dimensionless ratios 
like parts-per-million. But udunits does support such units.

# Technical Proposal Summary
Change sentence to state that ppm, ppb etc are supported, or at least remove 
the statement that these units are not supported.

# Benefits
Every reader working with dimensionless ratios will benefit from the correction.

# Status Quo
The current working draft (1.9) states that ppm etc. are not supported by 
udunits.

# Detailed Proposal
The chapter "Description of the Data" - "Units" (ch03.adoc) states:

> The Udunits package defines a few dimensionless units, such as `percent`, but 
> is lacking commonly used units such as ppm (parts per million).

The last part is wrong. UDUNITS-2 does support ppm, ppb, ppt, and ppq. In 
addition, percent is also supported. Other similar units, like permille are not 
supported. In fact, these units were supported since udunits2 version 2.1.24 
from August 2011. The change occurred here:
https://github.com/Unidata/UDUNITS-2/commit/789147fd3b2e4714e27cd198040ea114ac503861

Please update the documentation to reflect the support of ppm etc.


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