Hello people, Please, can you help me understand meaning of few definitions like: First, there is so called specific gravity, defined as relative quantity: SG = rho/rho(H2O) at temperature of 4 °C. Since water's density at 4 °C is 1000 kg/m3 = 1 g/cm3, it seems that specific gravity is always rho/1000 if rho is in kg/m3. What I don't know is if this definition is applicable to any substance, so for example, if we speak about SG of iron, it is rho(iron)/1000 if calculated at 4°C. Is this always defined according to ref. tmperature of 4°C or not? What confuses me is that I found in one table that SG of water at 0°C is 1.0. I belive this is mistake since water's density at 4°C is 1000 kg/m3 and not at 0°C. Is this correct? Also, specific weight is defined as gamma = rho * g? What is practical benefit of using these quantities? Isn't plain ordinary density sufficient?