Calculates the Preservation Index (PI) to estimate the natural decay speed of objects.
The Preservation Index, developed by the Image Permanence Institute, is a chemical kinetics metric that determines the rate of deterioration of materials based on temperature and relative humidity. The `calcPI` function returns the estimated years to deterioration, with higher values indicating conditions that are more hygro-thermodynamically favorable for an object.
Details
The formula is based on Arrhenius equation (for molecular energy) and an equivalent for E (best fit to the cellulose triacetate deterioration data). The other parameters are integrated to mimic the results from the experiments. The result is an average chemical lifetime at one point in time of chemically unstable object (based on experiments on acetate film). This means it is the expected lifetime for a specific T, RH and theoretical object if this remains stable (no fluctuations). The chosen activation energy (Ea) has a larger impact at low temperature.
Developed by the Image Permance Institute, the model is based on the chemical degradation of cellulose acetate (Reilly et al., 1995):
- Rate, k = [RH%] × 5.9 × 10^12 × exp(-90300 / (8.314 × TempK))
- Preservation Index, PI = 1/k
This metric is an early version of a lifetime multiplier based on chemical deterioration of acetate film. This last object is naturally relatively unstable and there lies the biggest difference with other chemical metrics together with the fact that it is not relative to the lifetime of the object. All lifetime multipliers give similar results between 20% and 60% RH. The results at very low and high RH can be unreliable.
References
Reilly, James M. New Tools for Preservation: Assessing Long-Term Environmental Effects on Library and Archives Collections. Commission on Preservation and Access, 1400 16th Street, NW, Suite 740, Washington, DC 20036-2217, 1995.
Padfield, T. 2004. The Preservation Index and The Time Weighted Preservation Index. https://www.conservationphysics.org/twpi/twpi_01.html
Activation Energy: ASHRAE, 2011.
Image Permanence Institute, eClimateNotebook
Examples
# Preservation Index at 20°C (Temp) and 50% relative humidity (RH)
calcPI(20, 50)
#> [1] 41.76134
# mydata file
filepath <- data_file_path("mydata.xlsx")
mydata <- readxl::read_excel(filepath, sheet = "mydata", n_max = 5)
mydata |> dplyr::mutate(PI = calcPI(Temp, RH))
#> # A tibble: 5 × 6
#> Site Sensor Date Temp RH PI
#> <chr> <chr> <dttm> <dbl> <dbl> <dbl>
#> 1 London Room 1 2024-01-01 00:00:00 21.8 36.8 45.3
#> 2 London Room 1 2024-01-01 00:15:00 21.8 36.7 45.4
#> 3 London Room 1 2024-01-01 00:29:59 21.8 36.6 45.5
#> 4 London Room 1 2024-01-01 00:44:59 21.7 36.6 46.1
#> 5 London Room 1 2024-01-01 00:59:59 21.7 36.5 46.2