The Collaborative International Dictionary of English v.0.48:

Evaporometer \E*vap`o*rom"e*ter\, n. [L. evaporare to evaporate
   + -meter: cf. F. ['e]vaporm[`e]tre.] (Physics)
   An instrument for ascertaining the quantity of a fluid
   evaporated in a given time; an atmometer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

evaporometer
    n 1: an instrument that measures rate of evaporation of water
         [syn: atmometer, evaporometer]