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# Vernacular differential pressure flow serial (III)

Sword finger industrial control 2021-11-25 20:33:05 阅读数:2 评论数:0 点赞数:0 收藏数:0

To understand flow measurement theory , You need to have some basic knowledge .

Fluid is a kind of material that will continue to deform under shear stress . The fluid can be a liquid 、 Steam or gas . For most fluids , Some fluid properties can be calculated from other known properties .

Five key fluid properties must be understood , To use a properly sized differential pressure flowmeter ：

• Density or specific gravity ;
• static pressure ;
• temperature ;
• viscosity ;
• Isentropic index ;

These properties are factors in differential pressure flow calculation .

force 、 Mass and weight

The flowmeter adopts the concept of energy conversion , 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. Referring to 1m/s² Acceleration movement of 1kg The force required for mass . The American unit of force is pound force lbf, Refers to 32.174 ft/s² The acceleration of will 1 The force required to move a foot of a pound of mass .

The secret degree

Fluid density is the mass per unit volume . Please note that , For the same quality , The volume occupied by this mass varies with temperature and pressure . Changes in temperature and pressure can cause density fluctuations , The density fluctuation of liquid is usually small , And the density of the gas fluctuates very much . Fluids with little change in density under moderate temperature and pressure fluctuations are 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 the ideal gas law is usually used . The ideal gas law is applicable to medium temperature and low pressure conditions . however , Regardless of the interaction between gas molecules . If the assumptions about the ideal gas law are not applicable , The law of real gas .

Than heavy

The proportion of (SG) It's the density of one substance and the density of another （ Or reference ） The ratio of material density . The reference material for liquids is usually 68°F (20°C) Water at temperature .68°F The density of distilled water at temperature is 62.316 lbm /ft³, perhaps 20°C At temperature 998 kg/m³. A hydrometer is usually used to obtain the specific gravity of liquids , The hydrometer is a scale reading of specific gravity Baume (°B) or API（ American Petroleum organization ） An instrument for measuring degrees .

The specific gravity of the gas is defined as the ratio of the molecular weight of the target gas to the molecular weight of the air （ It is stipulated that 28.9644）. As long as the gas composition remains the same , No matter the temperature 、 What is the pressure or position , The ratio of molecular weight to the molecular weight of the reference gas remains unchanged .

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Original publication time ： 2017-07-12

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