Answer: Tangential pressure cells are typically used to monitor stresses in concrete tunnel linings. They are embedded in the concrete, but as the concrete cures, it shrinks and pulls away from the cell. Thus the cell must be repressurized to gain contact with the concrete.
In contrast, radial pressure cells are placed between the concrete lining and the surrounding rock or earth. They are less affected by the curing of the concrete and thus are not typically equipped with a repressurizing tube.