For submarines in general with analog gauges:

When operating near the surface, a more precise depth gauge scale allows finer control. The shallow water depth gauge must be secured before passing its depth limit (close the sea cock), and one then depends on the deep water gauge.

As we ascend, we must reopen the seawater line to the shallow gauge, if we are to use it.

Assuming gauges are the same diameter, and scale is laid out over the same arc, the 0-25 will be more spread out than a 0-250 (by a 10x factor).

May not be why there were two gauges on "Das Boot."

Dan B. Odenweller