We first briefly describe an algebraic model of classical and modal many-valued logics due to the authors and introduced in previous works. A similar approach, also using Computer Algebra techniques (Gröbner bases) and oriented to perform effective calculus in a continuous logic: Minimal Polynomial Logic (MPL) is presented. The implementation has been developed in the Computer Algebra System Maple. The possibility to perform knowledge extraction and to check consistency in Rule Based Expert Systems (RBES) whose underlying logic is MPL, has also been explored. The article is illustrated with examples of very simple RBES.