1) Density gauge 
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instrument for measuring weight or density of gas
• (n.) An instrument for ascertaining the weight or density of air and gases.
A`ër·om'e·ter noun [ Aëro- + -meter : confer French éromètre .] An instrument for ascertaining the weight or density of air and gases. 
An Aerometer is an instrument used for ascertaining the weight or density of air and gases. Simpler aerometers measure only the relative weights of liquids. They consist of a tube of glass, terminated in a ball at its lower part, and divided into equal portions through its whole length. Another ball filled with mercury is soldered below to keep it ...
aerometer An instrument for weighing air or for estimating the density of air or other gases.
an instrument for determining the weight, density, etc., of air or other gases.