2.1 introduction

To understand the theory of flow measurement, some basic knowledge is needed.

Fluid is a kind of material which will deform continuously when it is subjected to shear stress. A fluid can be a liquid, vapor, or gas. For most fluids, some fluid properties can be calculated from other known properties.

Five key fluid properties must be understood in order to use a differential pressure flow meter of the appropriate size:

Density or specific gravity;

Static pressure;

Temperature;

Viscosity;

Isentropic index;

These properties are factors in the calculation of differential pressure flow.

2.2 force, mass and weight

The flowmeter uses the concept of energy conversion to determine the flow rate in the pipe by measuring the physical difference under pressure.

Newton's second law can be used to convert mass into force.

Force, mass and weight

The SI unit of force is Newton n. The force required to move a mass of 1 kg at an acceleration of 1 m / s ². The U.S. unit of force is LBF, which is the force required to move a pound mass one foot at an acceleration of 32.174 ft / s ².

2.3 density

Fluid density is the mass per unit volume. Note that for the same mass, the volume of this mass varies with temperature and pressure. The change of temperature and pressure will cause the fluctuation of density. The fluctuation of density of liquid is usually small, while that of gas is very large. A fluid with a very small density change under moderate temperature and pressure fluctuations is considered incompressible. If the density changes significantly at different pressures and temperatures, it is considered to be a compressible fluid.

For industrial gas measurement applications, the real gas law or ideal gas law is usually used. The ideal gas law is suitable for medium temperature and low pressure conditions. However, the interaction between gas molecules is not considered. If the assumption about the ideal gas law is not applicable, the real gas law is used.

2.4 specific gravity

Specific gravity (SG) is the ratio of one material density to another (or reference) material density. The reference material for liquids is usually water at 68 ° f (20 ° C). The density of distilled water at 68 ° f is 62.316 LBM / ft? Or 998 kg / M? At 20 ° C. A hydrometer is usually used to obtain the specific gravity of a liquid, which is an instrument with a scale reading of specific gravity Baume (° b) or API (American Petroleum organization).

Gas specific gravity is defined as the ratio of target gas molecular weight to air molecular weight (specified as 28.9644). As long as the composition of the gas is constant, the ratio of molecular weight to the molecular weight of the reference gas remains unchanged regardless of temperature, pressure or position.