A new a posteriori error estimation technique is applied to the sta-
tionary convection-reaction-diffusion equation. In order to estimate
the approximation error in the usual energy norm, the underlying bi-
linear form is decomposed into a computable integral and two other
terms which can be estimated from above using elementary tools of
functional analysis. Two auxiliary parameter-functions are introduced
to construct such a splitting and tune the resulting bound. If these
functions are chosen in an optimal way, the exact energy norm of
the error is recovered, which proves that the estimate is sharp. The
presented methodology is completely independent of the numerical
technique used to compute the approximate solution. In particular,
it is applicable to approximations which fail to satisfy the Galerkin
orthogonality, e.g. due to an inconsistent stabilization, flux limiting,
low-order quadrature rules, round-off and iteration errors etc. More-
over, the only constant that appears in the proposed error estimate is
global and stems from the Friedrichs-PoincarŽe inequality.