Water permeability can be measured using the Clam, also known as the Autoclam, developed by Queens University Belfast. It uses a circular steel ring bonded to the concrete surface. Bolts are used to attach the instrument to the ring. The ring forms a chamber, which is filled with water maintained at a constant pressure using a piston controlled by a micrometer screw gauge. The volume of water that passes into the concrete is calculated from the readings on the micrometer at the start and finish of a test. The results can then be used to assess the permeability of the concrete.