Determination of the mass per unit volume of a substance. The term density is equally applicable to solids (including powders), liquids, and gases. Usually values of density are given in terms of grams per cubic centimeter or pounds per cubic foot. The density of all substances depends on temperature; in the case of gases, on temperature and pressure. The temperature used as a base for determining or reporting values of density is not the same for all substances. For solids 32°F (0°C) is the preferred temperature, although some tables give values at average room temperature because the effect of temperature on density is relatively small for most solids. The effect of temperature on density is more pronounced in liquids, so that the temperature must always be stated along with the density value. For many liquids the reference temperature is 60°F (15.6°C). For gases 32°F and a pressure of 29.921 in. of mercury (or 0°C and 760 mm of mercury or 101.325 kilopascals) are used for most scientific work and for tables of gas data. For fuel gases 60°F (15.6°C) and a pressure of 14.73 lb/in.2 absolute (29.99 in. mercury or 101.56 kPa) are the values used in the United States.
In the case of a solid, if the sample is of regular shape, such as a cube or a cylinder, its volume may be determined by linear measurement. The mass of the sample is determined by weighing it on a suitable scale or balance; then this weight divided by the volume gives the density. Ordinarily the weighing is done in air, and the density value is the density in air, or apparent density. By adjusting for the buoyant effect of the air upon the weight of the sample, the real density is obtained.
A second procedure, applicable to irregular as well as regular shaped samples, is to weigh the sample in air and then to suspend it in a liquid of known density. The volume of the sample is equal to its loss of weight in the liquid divided by the density of the liquid. This is the method of hydrostatic weighing.
One method to determine the density of a gas is to completely evacuate a light but strong vessel of suitable size, the interior volume of which is known. The evacuated vessel is weighed, filled with a sample of the gas, and then weighed again. Of course the pressure and temperature of this sample of gas must be obtained.
A densitometer or gravitometer may be used to indicate and record the density of a flowing stream of a liquid or a gas. In a densitometer, a spinning propeller produces a pressure difference between inlet and outlet chambers of the device, which is proportional to the density of the gas flowing through it.
Two precision methods, the oscillator or vibrator method and the magnetic method, have emerged which allow more rapid and accurate determinations on liquid systems.
In the oscillator method, the density of a sample is related to the change in resonance frequency of a laterally vibrating tube. This frequency is inversely proportional to the square root of the mass of the tube and its contents. By calibrating the tube with media of known density at a given temperature, the density of unknown solutions may be determined if the volumes are strictly indentical. It is now established, that the accuracy of this method decreases as the viscosity of the medium increases. Hence, accurate viscosity measurements must accompany the density calibrations for a given instrument.
The instruments used in the magnetic method are called magnetic densimeters. This densimeter is a device whereby a tiny ferromagnetic cylinder, encased in a glass or plastic jacket, is held at a precise height within a medium by virtue of a solenoid controlled by a servo system in circuit with a height sensor. The jacket and ferromagnetic material constitute a buoy or float. The solenoid induces a magnetic moment M at the buoy which is proportional to the electric current I to the solenoid. The total magnetic force on the buoy is the product of this moment and the field gradient, dH/dz, where H is the magnetic intensity and z is the vertical distance from the center of the solenoid. The field gradient varies with z and is also proportional to the current. Thus the total magnetic force at a particular distance z in the solution which compensates for the difference in the opposing forces of gravity (downward) and the buoyancy (upward) exerted by the medium, through Archimedes' principle, is M (dH/dz). The magnetic force, under proper conditions, is directly proportional to the square of the current, which can be measured very accurately. Thus M(dH/dz) = k I2, where k is a constant. If the buoyant force on the buoy is sufficient to make it float on the liquids of interest, the force generated by the solenoid must be downward to add to the force of gravity. The equation relating these forces is shown below, where, VB is the volume of the buoy, g is 
the acceleration of gravity, and ρ and ρB are the densities of the solution and the buoy, respectively. By means of a precision resistor and an accurate differential voltmeter, the measurements consist simply of reading or recording the voltage, which is a parabolic function of the density.
