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Relative Damage Rate KPI

A simple number that tells you how quickly your collection is deteriorating relative to your ideal environmental levels.

Conserv cloud is a free, cloud-based environmental monitoring tool. You can create an account at https://app.conserv.io.

The challenge

 KPIs should help us answer the simple question "How are we doing?" -- read more about our KPI philosophy in our article "Understanding Preservation KPIs".

Take the example of the common temperature and relative humidity chart so common in environmental monitoring reports.


What do these conditions mean for the longevity of the objects in the collection. Are these conditions causing damage or not? How much damage? Hard to tell by just looking at the temperature and relative humidity data.

What if instead of the typical chart you could tell your team, "During the last month our collection deteriorated 20% faster (or slower) than it would have in ideal conditions."

With an immediate sense of "How we're doing?" your team can then address the spaces and conditions causing the most damage to the collection.

The damage rate calculations

The purpose of damage rate calculations is to link environmental conditions with the rate of deterioration in materials. This is a hard problem that requires a lot more research and data to fully understand.

Background: IPI and the TWPI

Conserv's relative damage rate KPI is loosely based on the logic used in the Time-Weighted Preservation Index (TWPI) created by the Image Permanence Institute.

TWPI uses damage rate calculations to express the longevity of an object - "how many years are left for that object?" This is done by calculating an annual damage rate (say 2%) and then taking the reciprocal (1/2 = 50 years).

In conversations, collections tell us they focus on making the TWPI number larger (longer life is better!) without a good sense of the magnitude of the environmental challenge.

It's hard to look at a longevity calculation and answer the basic question, "How are we doing?" For this reason we feel like there's a better way to express damage rates.

The approach to the Time-Weighted Preservation Index (TWPI) is best laid out in IPI's "Understanding Preservation Metrics"

An interesting critique of the TWPI is laid out in Tim Padfield's article, "The Preservation Index and the Time Weighted Preservation Index"

The Conserv approach

The Conserv relative damage rate compares the damage rate in your actual environment with the damage rate in your ideal environment.

Let's start with an oversimplified example. In your collection your ideal environment is 50-70% RH and 68-72°F with set point targets of 60% RH and 70°F. The damage rate in that environment is roughly 1% annually (made up number).

Over the course of a month, you actually achieve 72°F and 65% RH. The damage rate in that environment is roughly 1.2% annually (made up number).

To calculate the relative damage rate we divide the actual rate with the ideal rate (1.2%/1.0% = 1.2x) For this example we would say that your collection is deteriorating 20% faster than in your ideal environment.

This multiple is much easier for people to understand - your goal is a multiple less than or equal to one.

Here's an example of what this looks like in the Conserv application.


Remember that the relative rate is driven by the actual compared to what the user has set as their ideal environment.

For guidance on setting the right RH levels, start with our article "Setting relative humidity (RH) levels for your collection". We have a similar article on temperature

A little more context on the calculation

The above example is oversimplified because the damage rate isn't calculated based on average temperature and RH readings over a period.

The damage rate is calculated for every reading based on a lagged 24 hour temperature and lagged 30 day relative humidity reading - consistent with the IPI methodology.

The results are then weighted across the period to get the actual damage rate.

The relative damage rate approach is best laid out in Donald Sebera's article "Isoperms: an environmental management tool"

Damage rates for different materials

Calculating damage rates from environmental data for particular material types is still poorly understood.

We based our damage rate calculations on IPI's damage rate model which was developed for cellulose acetate film, so both of our calculations suffer from the same bad assumption that damage rates for that particular type of film are generalizable to other material types.

More basic research is necessary to refine these models, and any advances in damage rate models will be quickly adopted in the Conserv platform.

What's next

Collections professionals can now follow the mantra of "these are my numbers I want them to go down." With the relative damage rate KPI you can get your team aligned on "how are we doing?" and begin to lead the conversation on identifying and addressing problematic times of the year/week/day.

The primary purpose of good metrics is not just to answer questions; Good metrics also help your organization ask better questions to address the root cause of problems.