Marine Marker Mk III (Service)

Description: The outer casing of this marker consists of a body and a tail which are connected by a diaphragm. A heavy nose is secured to the body. The nose has a cavity to receive the time valve, and a central hole at the bottom of the cavity which is closed by a transit plug. The valve is a clockwork device which permits the marker to remain in the water for a period up to six hours before it functions. The period of delay is determi-ned by setting the valve before the marker is released from the aircraft. The main filling consists of magnesium-aluminum phosphide, and is packed round a gauze cylinder. Inside the gauze cylinder is a quantity of fasz calcium phoshide, which also extends over the top of the magnesium-aluminum phosphide. The main filling is held in place by a thin steel plate having large holes covered by brass gauze, and a large spring anchored to a spring-supporting plate, which abuts against the diaphragm connecting the tail of the body of the weapon. A layer of charcoal is provided beneath the steel plate to absorb any small quantities of phosphine gas generated by traces of moisture which may be in the body of the marker during filling, and a layer of glass wool beneath the charcoal which acts as a dust filter. The tail constitutes a buoyancy chamber, and a main outlet tube extends between the diaphragm and a tail cap at the outer end of the tail.

Functioning: After entering the water, the marker rises to the surface and floats tail uppermost, water entering the nose cavity through the two small holes in the side of the nose. At the expiration of the delay set on the time valve, the Welvic plug sealing the hole in the side of the time valve is opened and the water passes through the time valve and the central hole at the bottom of the nose cavity, into the water inlet valve in the nose. Some of the water entering the water-inlet valve soaks through the flannel washer and passes through the small hole in the inlet valve into the brass tube to react with the filling in the brass tube. The remaining water, after dissolving the soluble disc covering the nut, passes upwards through the water-inlet tube, percolates through the coarse-mesh cylinder, the flannel sheath, and the finemesh cylinder, and enters the body of the marker. The brass cap prevents water from passing through that portion of the sheat which is above the fine-mesh cylinder.

The water passing through the fine-mesh cylinder, together with the small quantity with the small quantity which passes down the main outlet tube while the marker is sub-merged, reacts with the calcium phosphide in the gauze cylinder and covering the top of the main filling, to produce impure phosphine, which, in contact with air, is spontaneous-ly inflammable. This supply of gas lasts only for a few minutes, and is thereafter maintai-ned by the pure phosphine evolded from the magnesium-aluminum phosphide mixing with the gaseous oxides of nitrogen given off by the interaction of the potassium bisulphate and the sodium nitrite, which are dissolved by the water entering the brass tube.

Remarks: When breaking the rupture disc, to not hold the head directly over the tail cap of the marker. This avoids possible injury to the eyes which might be caused should sufficient pressure be built up in the marker to force phosphide dust particles past the glass wool filter at the top of the main filling.

Figure 95 - Marine Marker Mk III