The rate of many processes, including the diffusion of water into rhyolitic volcanic glass (obsidian), as well as the racemization of amino acids is 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 properties 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 will be described. The cells function because water diffuses through the plastic across a constant vapor-pressure gradient. This vapor-pressure gradient is maintained constant between the substances within the cell and the materials outside the cell. The plastic cells are usually filled with water and surrounded by a dehydrating agent, such as silica gel. A better arrangement is to fill the cell with a mixture of solid sodium chloride (NaCl) and a saturated solution of NaCl, and to surround the cell with pure water. A number of plastics have been investigated, including polycarbonate, polystyrene, tefzel, polyallomer, and methacrylate. The cells have been sealed by various methods including screw caps, room-temperature vulcanizingsilicone rubber sealant, and rubber stoppers. The final design consists of a small cell made of a polycarbonate plastic centrifuge tube containing solid NaCl plus NaCl-saturated solution sealed with a rubber stopper and placed in a polypropylene tube containing pure water. Our 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°, and that will fit into metal fittings that can be screwed into standard 3/4-inch plastic water pipe (approximately 1 inch outside diameter).