show missing abbreviation tags for `ps/t` abbreviation

duty_expression_id 99, is a special case as it doesn't have any other units, it should show the abbreviation and full description in the abbreviation tag.

![screen_shot_2014-08-05_at_12 04 03_pm](https://cloud.githubusercontent.com/assets/215/3815288/14b55d54-1cc3-11e4-8bc7-06bfe95e675f.png)