How to stop refer to a cell wich is empty?

I have three columns: A, B and C.

A measures Time.

B measures Temperature.

C measures ∆T.

The cells in Column C are based on the numbers in the cells in column B (ex. B5-B4), this results in misleading information unless the relevant cell in column B do contain a number. The program seems to assume that an empty cell contains the number 0, but I want that an empty cell results in no calculation at all, is that possible?