what is specific gravity?what do you mean by 1pascal?concept of pressure difference?

This page is about the measurement using water as a reference. For a general use of specific gravity, see relative density. See intensive property for the property implied by specific.

Testing specific gravity of fuel.

Specific gravity is the ratio of the density of a substance to the density (mass of the same unit volume) of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for liquids or air for gases. Temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm equal to 101.325 kPa. Temperatures for both sample and reference vary from industry to industry. In British brewing practice the specific gravity as specified above is multiplied by 1000.^[1] Specific gravity is commonly used in industry as a simple means of obtaining information about the concentration of solutions of various materials such as brines, hydrocarbons, sugar solutions (syrups, juices, honeys, brewers wort, must etc.) and acids.

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The pascal (symbol: Pa) is the SI derived unit of pressure, internal pressure, stress, Young''s modulus and tensile strength,^[1] named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square meter.

Common multiple units of the pascal are the hectopascal (1 hPa ≡ 100 Pa), kilopascal (1 kPa ≡ 1000 Pa), megapascal (1 MPa ≡ 1,000,000 Pa), and gigapascal (1 GPa ≡ 1,000,000,000 Pa).

On Earth, standard atmospheric pressure is 101,325 Pa. Meteorological barometric pressure reports typically report atmospheric pressure in hectopascals.^[2] The kilopascal is used in other applications such as inflation guidance markings on bicycle tires.^[3] One hectopascal corresponds to about 0.1% of atmospheric pressure slightly above sea level; one kilopascal is about 1% of atmospheric pressure. One hectopascal is equivalent to one millibar; one standard atmosphere is exactly equal to 101.325 kPa or 1013.25 hPa or 101325 Pa. The corresponding Imperial unit is pounds per square inch (psi).

The "specific gravity" of a meterial is the ratio of its density to the density of water at 4⁰ C, 1000 kg/m³, it is a pure number with out uinits.

The "pascal" can be expressed using SI derived units, or alternatively solely SI base units,and a unit  of pressure equal to one newton per squre meter.

The difference between two levels of pressure. During operation, valves often experience different pressure levels downstream and upstream.