At present,the effective area of the piston gauge is mostly measured with the initial balance method.This method uses the definite small mass to eliminate the pressure caused by the difference of the liquid(gas)of the two reference plans of the piston gauge.That means the pressure caused by the liquid(gas)column is invariable.But owing to the compressibiliy of the gas,the density of the gas is changing with the change of the pressure.Even though the heigh of the gas column is invariable,but the pressure cau...