Abstract
The hydrometer for LPG (Liquefied Petroleum Gases) is ordinarily calibrated with reference to the water density under the atmosphere pressure.
However, the hydrometers are used actually under higher pressure (about 0.8MPa) and the following two effects have to be considered to correct their indications.
1) Pressure effect-The volume of hydrometer is reduced by the pressure and consequently the immersion level is changed.
2) Buoyancy effect-The density of gas (LPG) is relatively higher than that of air. The indication of the hydrometer is affected by the buoyancy of the part of the stem emerging out of the sample liquid.
The correction values for five LPG hydrometers are experimentally determined and their indication is compared with that of other methods by measuring the densities of common samples of pure propane and butane.
In the result, the calibration accuracy of hydrometers is enhanced by correcting the indication for the pressure and buoyancy effects.
