Why peacetime ASDIC trials had limits

Why could ASDIC not be fully tested under peacetime conditions?

Testing submarines at wartime risk was dangerous, so exercises could not reproduce every condition that affected wartime contact and false positives.

Driving a destroyer hard in training was different from pushing a submarine to wartime risks, where an error might kill the whole crew. That safety boundary reduced the realism of exercises and contributed to misleadingly favorable results. Submarine depth-keeping and underwater performance also improved greatly between the world wars. ASDIC would therefore face targets in 1939 that were more capable than many exercise assumptions allowed.