Issue
In openhtf.units L3648C1-L3651C26, PERCENT is defined twice:
PERCENT = UnitDescriptor('percent', 'P1', '%')
ALL_UNITS.append(PERCENT)
PERCENT = UnitDescriptor('percent', 'P1', 'pct')
ALL_UNITS.append(PERCENT)
Because the second assignment overwrites the first, units.PERCENT always resolves to "pct", and the "%" unit is inaccessible.
Solution
Rename PERCENT "pct" --> PERCENT_PCT