Values based on approximations (particularly C P − C V = nR) are in many cases not sufficiently accurate for practical engineering calculations, such as flow rates through pipes and valves at moderate to high pressures. An experimental value should be used rather than one based on this approximation, where possible. A rigorous value for the ratio ⁠ C P / C V ⁠ can also be calculated by determining C V from the residual properties expressed as