The rates of the diffusion of water into rhyolitic volcanic glass (obsidian) and the racemization of amino acids are temperature dependent, and a knowledge of temperatures integrated over time periods of at least a year is necessary to quantify these processes. The construction and use of simple devices consisting of small plastic containers that change weight at a rate that is a function of temperature and the activity of water is described. A number of plastics have been investigated, including polycarbonate, polystyrene, tefzel, polyallomer, and methacrylate. The final design consists of a small cell made of a polycarbonate plastic centrifuge tube containing solid sodium chloride (NaCl) plus NaCl-saturated solution sealed with a rubber stopper and placed in a polypropylene tube containing pure water. The aim has been to develop cells that are sufficiently sensitive to yield a precision of ±0. 2°C when exposed for one year at temperatures that range from 0° to 40°C.