The test involves injecting mercury into an evacuated and dry core sample under controlled pressures (capillary pressure). Mercury is the non-wetting phase fluid which forces air to be the “wetting” phase. The volume of mercury injected at each pressure step determines the non-wetting phase saturation. This method is very fast, often requiring only hours. High injection pressures are possible with modern equipment enabling all the connected pore volume to be filled, even in low permeability samples.