Densitometry, Dilatometery, and Gravimetry

Density is the mass per unit volume of a substance. Specific gravity is the density of the substance relative to a standard such as liquid water at 4°C. The density of liquids is monitored by hydrometers, weighing a fixed volume (density balance), the Coriolis method (described in Section 12.2.5), and vibrating methods. Hydrostatic, displacement float, sonic, and radiometric methods are among others also used [4]. 

In the tuning fork densitometer one utilizes the dependency of the frequency of a tuning fork or a vibrating plate, which is excited to produce an under-damped oscillation, on the density of the surrounding medium. The resonant frequency of 

Vibrating and density balance densitometers can also be used for compressed gases (p 2 kg m ). Gases at standard pressure are measured with gas density balances, with which one determines the buoyancy of a closed hollow sphere filled with a comparison gas. 

A sonic densitometer consists of a device for measuring the speed of sound in the liquid [4, 8]. The speed of sound c is given by Eq. (27), where E is the bulk modulus and p is the mass density [4]. Unfortunately the liquid must be clear of particles that might scatter the sound waves and limit the received signal. 

The density of liquids can also be measured radiometrically, by the absorption of gamma-rays in the medium. The intensity of the exiting radiation depends on the density of the medium and is given according to the Beer-Lambert law [Eq. (28), where 7, Iq are the intensities behind and in front of the object, e the absorption coefficient, p the density of the medium and I the path length]. 

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