The pycnometer (from Greek: ?????? (puknos) meaning "dense"), also called pyknometer or specific gravity bottle, is a flask, usually made of glass, with a close-fitting ground glass stopper with a capillary tube through it, so that air bubbles may escape from the apparatus. This enables the density of a fluid to be measured accurately, by reference to an appropriate working fluid such as water or mercury, using an analytical balance.

If the flask is weighed empty, full of water, and full of a liquid whose specific gravity is desired, the specific gravity of the liquid can easily be calculated. The particle density of a powder, to which the usual method of weighing cannot be applied, can also be determined with a pycnometer. The powder is added to the pycnometer, which is then weighed, giving the weight of the powder sample. The pycnometer is then filled with a liquid of known density, in which the powder is completely insoluble. The weight of the displaced liquid can then be determined, and hence the specific gravity of the powder.

The pycnometer is used in ISO standard: ISO 1183-1:2004, ISO 1014-1985 and ASTM standard: ASTM D854.

<gallery> Image:PycnometerEmpty.jpg|An empty glass pycnometer and stopper Image:Pycnometer full.jpg|A full Pycnometer </gallery>

cs:Pyknometr de:Pyknometer es:Picnómetro fr:Pycnomètre it:Picnometro lt:Piknometras nl:Pyknometer pl:Piknometr pt:Picnometro sl:Piknometer uk:?????????

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