A free, simple and quick calculator for damage functions and environmental deterioration of collections, provided by the UCL Institute for Sustainable Heritage. Below you will find a
Model description
This calculator packs as many useful transformations of a T and RH datapoint as possible. It is designed to be extremely lightweight. It can be downloaded and used offline by saving the website's html code.
It does not include any assessment of uncertainties and error propagation. Users should be aware that there are three important sources of error: T and RH loggers are imprecise, using values averaged over long periods can ignore variability, and the models used have their own inherent statistical uncertainity. The combination of all three means that ouputs should be imagined with error bounds of 10% to 50%.
Therefore, this calculator is best used as an educational and reference tool, to make comparisons rather than predictions, and to understand the relationships between variables.
Psychrometrics
The calculator uses thermodynamic relationships to compute vapour pressure, dew point, absolute humidity, and enthalpy. Vapour pressure indicates how much water vapour is present in the air and is useful for comparing moisture conditions across different temperatures. Absolute humidity provides similar information, expressed as the mass of water vapour per unit volume of air. The dew point indicates the temperature at which moisture will condense, for example on colder surfaces within a room. Enthalpy can be used to estimate the cooling and dehumidification load: the higher the enthalpy, the more energy is required to cool or dehumidify the air.
Wood EMC & Rot Risk
This is based on the Simpson equation to estimate the Equilibrium Moisture Content (EMC) of wood, assuming stable environmental conditions over a long period of time. The risk of fungal growth in wood varies depending on the wood species, so the following thresholds are indicative only.
• < 20%: Safe (No rot risk)
• 20–28%: Caution (Susceptible)
• > 28%: High Risk (Fungal growth likely)
Basic Lifetime Multiplier
A value > 1 indicates an extended lifetime compared to the reference (20°C, 50% RH); < 1 indicates a shortened life. This is calculated as the ratio of reaction rates using the Arrhenius equation, which broadly reflects the temperature dependence of chemical degradation. This equation has been superseded in the case of cellulose, but it remains a useful "no-assumption" metric for a more general indicator of the hydrolysis of organic materials. It is best used to compare two environments rather than to make specific predictions about any given environment.
Paper lifetime using Strlič model
Unlike the basic model, this equation incorporates
paper pH, where lower acidity significantly increases the rate of deterioration.
The underlying damage function calculates the degradation rate, which can yield a Lifetime Multiplier or an estimated lifetime in years.
To estimate a lifetime, the model requires the
initial Degree of Polymerization (DP) (typically ~2000 for fresh paper) and a
critical DP (usually 300), representing the point where paper becomes too brittle to handle.
DP reflects the average chain length of cellulose molecules. For a fuller implementation of this model, see the
the Collections Demography App.
Photographic lifetime multiplier
This calculation uses the damage function by Fenech et al. This damage function predicts color change. It includes Acetic Acid, which is assumed to be absent in this case. The lifetime multiplier is calculated by dividing the rate at 50% RH and 20ºC by the rate at the actual conditions.
Mould growth (Polynomial)
The model by Johansson et al. is one of many methods to convert T and RH into mould risk. Their index indicates mould activity. This means that any value higher than 0 indicates the potential for mould growth. It is calculated by multiplying two indices, one that indicates the sensitivity to T and other the sensitivity to RH. The equations used here are obtained by fitting curves into the visualisations of their original paper. Their model includes a time-effect, ignored here: if the conditions are not favourable in the recent past, the mould index should be penalised. This kind of analysis requires a time-series of T and RH.
PVC storage lifetime
This calculation uses the damage function by Rijavec et al. The function predicts color change. Following the authors, lifetime is defined as the time until the color has changed 15 Delta E units (this is a standard way of measuring color change). Their equation includes the molecular weight of PVC and the percentage of plasticiser, which are taken here as the average values in the author's dataset. The results are not highly sensitive to these values.
Conservation Heating
Calculates the temperature increase required to reduce Relative Humidity to 50% while maintaining constant absolute moisture content. This is a strategy used in places where dehumidifcation is not viable, for example in historic houses. Naturally, the objective is not always 50%RH.
Salt Deliquescence
Lists salts that are likely to deliquesce (absorb moisture and dissolve) at the current Relative Humidity (RH), based on approximate critical RH thresholds at around 20 °C.
• Ca(NO₃)₂ (>55%)
• NaNO₃ (>74%)
• NaCl (>75%)
• Na₂SO₄ (>80%)
• MgSO₄ (>84%)
• KNO₃ (>94%)
In reality, salt behaviour also depends on temperature, salt mixtures and hydration states. Mixed salt systems can behave very differently from pure salts, often lowering deliquescence thresholds. More advanced thermodynamic modelling of these processes can be performed using tools such as
RUNSALT
.
References
- Strlič, Matija, et al. "Damage function for historic paper. Part III: Isochrones and demography of collections." Heritage Science 3.1 (2015): 1-11.
- Rijavec, Tjaša, Matija Strlič, and Irena Kralj Cigić. "Damage function for poly (vinyl chloride) in heritage collections." Polymer Degradation and Stability 211 (2023): 110329.
- Fenech, Ann, et al. "Modelling the lifetime of colour photographs in archival collections." Studies in Conservation 58.2 (2013): 107-116.
- Johansson, Sanne, Lars Wadsö, and Kenneth Sandin. "Estimation of mould growth levels on rendered façades based on surface relative humidity and surface temperature measurements." Building and Environment 45.5 (2010): 1153-1160.
-Simpson, William Turner. Equilibrium moisture content of wood in outdoor locations in the United States and worldwide. Vol. 268. US Department of Agriculture, Forest Service, Forest Products Laboratory, 1998.