manometer
    n 1: a pressure gauge for comparing pressures of a gas

Manometer \Ma*nom"e*ter\, n. [Gr. ? thin, rare + -meter: cf. F.
   manom[`e]tre.]
   An instrument for measuring the tension or elastic force of
   gases, steam, etc., constructed usually on the principle of
   allowing the gas to exert its elastic force in raising a
   column of mercury in an open tube, or in compressing a
   portion of air or other gas in a closed tube with mercury or
   other liquid intervening, or in bending a metallic or other
   spring so as to set in motion an index; a pressure gauge. See
   Pressure, and Illust. of Air pump.
   [1913 Webster] Manometric