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INTRODUCTION
This section gives a historical perspective on gas density measurement techniques, including the mechanical designs, which have been widely used in the past, but have lost their popularity or have been completely discontinued. This is due to their moving parts and their associated high maintenance. This trend toward solid-state instrumentation is a general trend throughout the industry, and in the case of gas densitometers,
it resulted in the proliferation of oscillating and vibrating sensors. These are discussed separately in Sections
6.5 and 6.8, but some of their features and suppliers have been also mentioned in the feature summary above under type G, for the reader’s convenience.
THEORY OF OPERATION
Gas density measurement relies on some of the same laws of physics that are used in liquid density measurement. Liquids have typical densities on the order of 40 to 80 lbm/ft 3 (600 to 1300 kg/m 3 ); gases have densities on the order of 0.05 to 0.5 lbm/ft 3 (0.8 to 8.0 kg/m 3 ). The methods used include the weighing of a known volume either passively or actively. Passive weighing is measuring the static forces created by gravity. An example is a laboratory balance. Active weighing is the process of moving or shaking the volume and determining the density from the required acceleration or the Coriolis forces (Section 6.5) or container displacement, or the oscillation frequency (Section 6.8). Another scheme varies the gas pressure until the buoyancy force is equal to that air at atmospheric pressure; at this condition,the ratio of the absolute pressures equals the specific gravity (SG). The speed of sound through a gas is inversely proportional to the square root of the density and this relationship has been used. Thermodynamic properties, such as heat capacity, have been used as the basis of instruments. Gas jet deflectionhas been used; gas jet impact pressure recovery is also sensitive to gas density.
The centrifugal fan laws can be the basis for measuring density—see any mechanical engineering handbook. The head pressure generated is a function of the gas density. Density is mass per unit volume. Relative density (also called specific gravity) is the ratio of the density of a gas to that of air at the same temperature, pressure, and moisture content. Confusing absolute density with specific density can have disastrous effects on engineering calculations and designs. The relative density is equal to the ratio of molecular weights of the gas and of air, which is usually taken as 28.9644. The density of an ideal gas can be calculated from its specific gravity (or molecular weight), the absolute temperature, and the absolute pressure, because equal numbers of moles of gas occupy equal volumes. See Section 6.1 for details. To avoid redundancy, some of these devices, such as the vibration-based devices (Section 6.8) and Coriolis-type (Section 6.5) designs, are not covered here. Gas density measurement is of interest for several different reasons. Density compensation is used to correct some flowmeters that are sensitive to density. For this purpose, it is preferred to measure the actual gas density at the flowing pressure and temperature. A flow computer is often used for this purpose when the measurement has sufficient value. The other major use is to infer or to determine composition. Specific gravity relative to air is most conveniently measured at or near atmospheric conditions. Some of the designs, such as the displacement, centrifugal, and fluid dynamic designs, may also operate at higher pressures and can measure either specific gravity or actual density. Orientation Table 6.1a provides an overview of the capabilities of the different designs.
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