Hydrometer consists essentially of a weighted, sealed, long-necked glass bulb that is immersed in the liquid being measured and the depth of flotation gives an indication of liquid density.
In order to determine the specific gravity of liquefied petroleum gases such as propane and butane and other light hydrocarbons under vapour pressure conditions corresponding to the temperature of the liquid. The cylinder used for measurement is constructed by using a clear plastic barrel. The barrel is held between aluminum end plates with tie-rods.
The working pressure is usually 200 psi. The relief valve is attached and measures this pressure.