From: C.B. 4318 REPORT ON "U-570" (H.M.S. "GRAPH")
H.P. Air Line
There are six groups of air bottles totalling twelve bottles. Nos. 1, 2 and 6 groups are located externally in the casing, No. 3 in the E.R.A.'s mess, Nos. 4 and 5 groups in the torpedo compartment. The bottles are charged by a Junkers diesel air compressor and a Krupp electric air compressor.
2. The working pressure is 205 atms. and the air line is made of copper, external diameter 11/16 in., internal diameter 7/17 in.
Tore & Maciek, I imagine the copper was replace with steel, in the later war?