Preservation Index (PI and TWPI)
PI and TWPI – Indicators of Preservation Quality This is an introduction to the concept, which you can also find by clicking the question mark icon in the top right corner of the graph view when one of the graphs displays PI values.
Introduction
Organic materials degrade due to physical, chemical, and biological processes. The speed at which this degradation occurs is influenced by many factors, with temperature and relative humidity (RH) being the most critical.
At high temperature and humidity, degradation happens faster than under cooler, drier conditions.
At high temperature and humidity, degradation happens faster than under cooler, drier conditions.
To assess the preservation quality of materials intended for long-term storage, monitoring temperature and RH in the storage or display environment is essential.
PI and TWPI are indicators used to evaluate preservation conditions, particularly in relation to chemical degradation.
PI – Preservation Index
The Preservation Index (PI) represents the expected preservation quality of organic materials (such as acidic paper, plastics, color film, magnetic tapes, or herbarium specimens) stored in a specific space—e.g., a storage or exhibition room—at the time of measuring temperature and RH.
A PI of 50 is defined as the expected lifetime in years before visible damage or deterioration occurs in sensitive organic materials, if stored constantly at 20°C and 45% RH. For example, this could be fading of colors in photographs or yellowing of paper. If PI = 100, the same level of degradation would take twice as long to occur compared to PI = 50.
In short: the higher the PI value, the better the preservation conditions.
A PI of 50 is defined as the expected lifetime in years before visible damage or deterioration occurs in sensitive organic materials, if stored constantly at 20°C and 45% RH. For example, this could be fading of colors in photographs or yellowing of paper. If PI = 100, the same level of degradation would take twice as long to occur compared to PI = 50.
In short: the higher the PI value, the better the preservation conditions.
TWPI – Time-Weighted Preservation Index
In the real world, temperature and humidity levels in a room or building change over time. These values may fluctuate with the outdoor temperature depending on the building’s insulation, or they may be influenced by heating or air conditioning systems. Therefore, it is rare to find a room where temperature and humidity remain constant over a longer period. Each combination of temperature and relative humidity affects the lifespan or preservation of materials. The Time Weighted Preservation Index is a formula that calculates the overall effect of different PI value combinations. TWPI thus expresses the preservation quality as a single number, reflecting the weighted impact of all measured temperature and relative humidity values over a given period. This gives you an overall score for the quality of the monitored storage room or building. The quality is typically assessed as follows:
-
Go to TOOLS, Charts.
- Select the sensor you want to use and the time period to be measured.
- Choose PI as parameter 1 and humidity as parameter 2, then click SHOW.
- You will now see a chart displaying the two graphs in relation to each other.
- TWPI appears as a brown dotted line in the chart, and the number of years is shown above the chart.
Related Articles
Chart report
The graph module, found under TOOLS > Graphs, allows you to display a visual comparison of two parameters of your choice using a drop-down menu. You can select from all RoomAlyzer measurement points, or compare your own data with DMI's temperature ...