time-weighted preservation index
n. (TWPI, abbr.)
a measure of organic material's life expectancy (preservation index) based on the cumulative effects of changes in environment.
The concept of time-weighted preservation index was developed by the Image Permanence Institute.
Nishimura 2011, 4 Preservation index (PI) and time-weighted preservation index (TWPI) are based on the science of chemical kinetics, which deals with the rates of chemical reactions. . . . The main advantage of TWPI is its ability to condense a whole period of changing temperature and RH conditions into one value by properly averaging or "weighting" how much each interval of time contributes to the decay rate overall.