This paper documents the construction details and experimental procedures for the continuously weighed pycnometer used in our laboratory to measure fluid properties such as densities, saturation pressures, phase boundaries. and isothermal compressibilities. This apparatus consists of a cell suspended from an electronic balance at all times and a variable volume bellows cell which is used to adjust the confined sample density within an isothermal enclosure. The principal advantages of this apparatus are rapid measurement and relatively high accuracy. Some experimental results are presented to illustrate capability. The accessible experimental ranges are 100-450 K up to 200 MPa; measurement accuracy is 0.1% or better.