Replacing a 'Porosint' disc

Note: Figures and PDF version still required.

The Pore Pressure Cell Caps are fitted with sintered phosphor bronze filters. In time these 'Porosint' filters need replacing:-

1. Support the front face of the Pore Pressure Cell Cap and press out the existing filter disc from the back face. A piece of brass rod or similar, less then 10 mm in diameter, can be used as a drift either in a press or tapped lightly with a hammer.

2. Chamfer the new filter stone around one edge of its circumference to provide a lead-in. Press the stone into the Pore Pressure Cell Cap.

3. File the face of the new stone so that the face follows the curvature of the Cell Cap.

4. Drill and tap the centre of the stone I.S.O. metric coarse M2 x 0.4 mm pitch. Countersink at 90° the tapped hole on the front face so that the de-airing screw head sits level with the stone face.

5. As a result of the shaping of the stone (step [3] above) the surface will be smeared. With M2 screw fitted, very lightly grit blast the front face of the Pore Pressure Cell Cap to restore the porosity.

6. Alternatively, the cap can be acid etched.

7. Blow through the stone with compressed air.