Abstract: A container has a porous support member affixed thereinside defining upper and lower chambers and a liquid partly filling the container. The liquid is frozen in the upper chamber. The container is oriented so that the upper chamber is gravitationally above the lower chamber, wherein the frozen liquid remains in the upper chamber. The container is placed in a cold zone of a freezer. Thereafter, the presence of the liquid in the lower chamber is indicative that a thaw episode has occurred. The presence of the liquid frozen in the lower chamber is indicative that the thaw episode has been followed by a refreeze event.