The need to make accurate measurements of compressed natural gas (CNG) for vehicle refuelling led to an investigation of gas behavior under actual and simulated fill conditions.During the past century, scientists and engineers have developed several equations of state to show how gas properties are related. Two popular equations, the AGA and the BWR equations have been used extensively in the gas industry. With the aid of a computer, these equations have been adapted to determine the amount of natural gas by weight, gas volume or heat content in a CNG automotive fuel cylinder.One of the main problems in using PVT relationships is determining gas temperatures. Unlike pressure, it is difficult to directly measure gas temperature due to the hardware constraints of the fuelling system. Direct temperature measurements, taken during a carefully monitored quick fill experiment, showed that the temperature dropped below ambient then started to increase until the temperature was 10°C to 20°C (approximately 20°f to 40°F) above ambient. A theoretical study, using the first law of thermodynamics, also showed a dip then increase in temperature. By using the results of the experiment and study, a correction factor was incorporated into the computer program to account for the temperature deviation from ambient.By using the correction factor, a close estimate of gas quantity can be made from the initial and final fill pressure, cylinder size, gas composition and ambient temperature. These estimates agreed well with the quantity of gas measured by a commercial mass flow meter.