The first type of pressure detector used by the Germans was the single suction-con-tact type. This type closed contact if negative pressure differential of sufficient magni-tude occured. The normal sensitivity is 1.5 to 2.5 cm of water. When used by the Luft-waffe in the AD 104 unit, the detector is designated D 1.

Any negative pressure differential on X, equal to about 2½ inches of water, will cause X to expand and allow the contact to close, provided that the change in pressure occurs quickly enough. Pressure differential caused by natural causes such as tides and seiches occurs over periods too long to be effective, since C allows the pressure to equalize without expanding the diaphragm sufficiently to make contact. However, large pressure differential caused by action of a ship, or even by a large wave or swell, will affect the diaphragm sufficiently to close the contact, since C cannot equalize the pressure rapidly enough. From this point on, the associated electrical circuits of the pressure component govern further operation, with the pressure-detecting deveice serving only to close its switch for the duration of the pressure differential. It was felt that this device was too subject to actuation by nearby explosions and, to eliminate this and other defects, the D 102 and D 2 were developed.

The second type of pressure detector developed by the Germans was filled with two separately adjustable fixed contacts. One contact is mounted to close on suction, similar to the D 1 (D 101) action. The other contact is mounted on the pressure side and is nor-mally closed in its primary application as a passive contact. The pressure-measuring dia-phragm is of smaller diameter, and the entry hole from the rubber bag is fitted with a de-tonation-protecting valve which blocks only then large, sudden surges of air attempt to operate the unit. This type of unit is generally designated D 2 when used by the Navy and D 102 when used by the Luftwaffe. Although D 2 has never been used, D 102 has been used operationally as the dectecting device for DA 102 and D 103, and its applica-tion to all later Luftwaffe developments involving pressure operation as projected. The following designations and their apparent meanings of D 2 and D 102 units were known to exist:

1.

6.

2.

7.

3.

8.

4.

9.

5.

10.

U = Unterdruck = Suction = contact fitted

D 102 UR/KS and D 102 U/UR/KS are operational forms; D 2 U/KS and D 2 U/UR/KS were on the verge of being operational when hostilities ceased; the other types are ex-perimental and testing devices. D 102 K and D 102 L have short and long (equalization) periods, respectively, for pressure-discriminating circuits.