Can we really measure low pressures

As the title suggests, though most people would know how to measure low pressures, there exists some doubt about what they really do measure quite apart from the question of accuracy. While the pressure is undoubtedly the most widely quoted parameter in vacuum technology, because of difficulties in accurate low-pressure measurement the term is generally used rather loosely to express the degree of vacuum or the cleanliness of the vacuum environment. It may serve as a means of roughly comparing different systems, but when used to express related physical parameters the need for accurate measurements becomes apparent.