Abstract: Method and apparatus for detecting a limit of a usable portion of a batch of fluent material flowing in a conduit from a container wherein usable and unusable material lie in layers. The embodiment disclosed includes two containers each having a conduit leading from the container to a switching valve. Each conduit is provided with a sensor for sensing the conductivity of fluent material flowing in the conduit. Signals from the sensors are sent to a computer which has in memory a range of values for the conductives of usable material. When the signal from a sensor goes outside that range the computer generates a signal to the switching valve to cause the switching valve to discontinue flow from the one container and allow flow from the other container which contains a fresh batch of fluent material. A conduit leads from the switching valve to a coating hopper for coating a thin layer of photographic emulsion on a continuous web. The unusable material may be froth on top of the usable material or it may be air.