Relative density
A pycnometer (from Greek: πυκνός (puknos) meaning "dense"), also called pyknometer or specific gravity bottle, is a device used to determine the density of a liquid. A pycnometer is 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 device enables a liquid's density to be measured accurately by reference to an appropriate working fluid, such as water or mercury, using an analytical balance.
Annular velocity
The other major variable is the rheology of the drilling fluid. Rheology is sometimes thought of as viscosity to the uninitiated, though improperly. Viscosity (sometimes thought of as its thickness) is a very basic measurement of the fluids resistance to change in movement or flow. The viscosity of a fluid can be measured with a Marsh Funnel. Rheology is the study of viscosity and requires more precise and complicated procedures and equipment for its determination. For drilling fluid applications a rheometer is used.