An apparatus configured to determine water activity and weight of a sample. The apparatus may comprise a chamber configured to at least partially enclose a sample. The apparatus may also comprise a sensor configured to measure water activity of a sample in the chamber. Additionally, the apparatus may comprise a moisture content adjustment device connected to the chamber and configured to change moisture content in the chamber. An isotherm generation module is also disclosed. The isotherm generation module may be configured to receive water activity measurements from a sensor device and weight measurements from a weighing device, the isotherm generation module being configured to generate an isotherm for a sample based on the water activity measurements and the weight measurements.