The PEM calculates and displays, in real time, values that reflect the decay rate of organic materials stored in that location. These values, known as Preservation Index (PI) and Time-Weighted Preservation Index (TWPI), show how temperature and humidity c

The PEM calculates and displays, in real time, values that reflect the decay rate of organic materials stored in that location. These values, known as Preservation Index (PI) and Time-Weighted Preservation Index (TWPI), show how temperature and humidity combine to influence the rate of decay processes such as paper discoloration, dye fading, deterioration of plastics, and textile embrittlement. The PEM displays PI and TWPI values alternately with temperature and RH on a 15-second cycle. Thus, no computer is needed to immediately determine the overall quality of storage conditions with respect to the 'natural aging' rate of all types of organic objects in collections.