A method and device for measuring the thickness of a liner applied onto the inner wall of a pipe, the device comprising a sprayer assembly adapted to deposit a polymer resin onto the inner wall of a pipe to create a liner upon curing, the sprayer assembly being moved through the pipe. The sprayer assembly has a leading sensor assembly that measures the inner diameter of the pipe and trailing sensor assembly that measures the inner diameter of the liner. The trailing sensor assembly has a housing with windows through which sensor devices take readings, and a transparent film is disposed across the windows to protect the sensors from occluding contamination. The transparent film is advanced as the exposed portion of the film becomes excessively contaminated.
