Peter Nitschke wrote on Friday, February 18, 2011 - 10:15 am:Pneumatic implies that the active component is either air or an inert gas - as it is in the UZZ31.
So, no hydro (water) and no pneumatic (air pump), ergo no "hydropneumatic". :-)
That said, arguing semantics could provide hours of amusement to those that are so inclined.