Abstract: A device and method for determining the time-temperature history of a solid, liquid or gaseous medium where the temperature is expressed as a single value of the total temperature conditions to which the medium has been subjected over a given time, the invention involving use of a fluid tight container having a spherical cell of constant wall thickness and permeable to water vapor, the cell containing a dry desiccant and the container being filled with water. Similarly, the vapor pressure history of the medium may be determined by use of a second, non-containerized, spherical cell together with the containerized cell.