Gas expansion pycnometer is also known as constant volume gas pycnometer. Specific gravity of rocks is determined by use of a balance and pycnometer. Nor does "RD" depend on the actual volume of the pycnometer. The powder is placed inside a pycnometer of known volume, and weighed. A gas pycnometer is also sometimes referred to as a helium pycnometer. Specific gravity can also be measured by a pycnometer or oscillating U-tube electronic meter. Specific gravity is measured by a hydrometer, refractometer, pycnometer or oscillating U-tube electronic meter. A gas pycnometer can be used to measure the volume of a powder sample.