Yep Cat! You (we) are right.
Germans didn't have a tool compared BT (bathythermograph). The only way to find out the temperature was to take external water in the boat using salinometer for that. Everyone can imagine the results = water temperature quickly changed so that was far out to be an exact method.
Diving officer could note the heavy layer if the boat was in good trim, but as you wrote, there is many parts affecting to layers so they change "all the time". Also, in North Atlantic conditions layers usually lay well down in maybe 600 - 1000 fts, in the depths out of normal operating depth of the boat, especially in early war time. In Pac the situation was totally different, boats could usually easily achieve the layer. So I was reading, it was common that on approach phase USN boat often used to dive to find out were layer laid and after attack just dived to under it and dissapeared.
But, once again we can see, how the germans did loose the "technical war". BTW, BT was a rather new invention and USN did research to find it to be very useful. By early -43 many of the USN boats got the BT.
-RC-
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