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The first TDR was developed to measure diving depths of Weddell seals in Antarctica. The initial TDR design is shown with key parts including housing (1), kitchen timer housing and shaft (2 and 3), smoked glass disc (6), pressure indicator arm with flexible needle (10), bourdon tube with external opening (11 and 14), O-ring (12) and housing brass cover (13). Reprinted with permission [11].
