There are eight German influence-mine firing components whose designation is pre-fixed by the letter "D". Of these, four are pressure-detecting components which are used to detect pressure differentials for all types of pressure-operated units. These are the D 1, D 101, D 2 and D 102 units. The other four are plain pressure units. To aviod pre-matures in rough waters, pressure units are normally used in combination with influence units of other types. However, for use in rivers or other relatively smooth waters, the German undertook the development of pressure units which could be used alone. These units were designated D 103, D 123, and D 133, and were intended for use in the BM 250 mine case.

The information contained herein on the D 2, D 102 and D 103 is based on captured documents and examination of the specimens captured. Information of the other units was obtained from German prisoners of war and scientists.

Of the four pressure-detecting components herein decribed, the D 1, D 101, and D 102 were used operationally. The D 2 was on the verge of operational use at the end of the war.

Of the four pressure units, the D 103 was completely developed and used operational-ly in the latter stages of the war. The other three units of the series were to be of the same basic type as the D 103, but were never completed.