[en] Development of a realtime reactivity calculation algorithm, based on recursive solution of the inverse point kinetic equation, and adapted for digital computer calculation, is described. Application of this algorithm to an ''off-the-shelf'' microcomputer is presented in order to produce an extremely low cost piece of equipment that can both alleviate the work of operators as well as serve as an extremely versatile tool for training purposes. Various applications of this device in a day to day work of a research reactor operators are given. A method of control rod worth measurement after core reconfiguration using this digital reactivity meter is described. Demonstration of various effects, such as control rod insertion or extraction, void coefficient measurement, temperature feedback effects, etc., is described. (author)