Museum climate data checker: RH and temperature logger analysis, dew point, mould risk spells and preservation index

What it does

Museums, archives, libraries and historic houses log temperature and relative humidity (RH) to protect their collections. Damp air lets mould grow and metals corrode; fast RH swings make wood, ivory and paint swell and shrink until they crack; warmth speeds up the chemical decay of paper, film and plastics. This checker reads a logger’s export and turns thousands of readings into the numbers a conservator needs:

• Dew point of every reading (Magnus formula), to see where cold walls or cases might get condensation.
• Hours above and below your RH and temperature limits, and the share of the time.
• Mould risk: hours at or above the RH you set, and the longest unbroken spell at that level, with its start and end. A long spell matters more than scattered hours.
• Daily RH swings: the range in each calendar day, the mean, the largest, and the days over your swing limit.
• Preservation index: a time-weighted measure of chemical decay. The rate at each reading is exp(−Ea/RT) × RHⁿ, relative to the reference temperature and RH. An index of 0.86 means objects decay as if they had 0.86 times the life they would have at the reference.

For a single reading of air (wet bulb, humidity ratio, enthalpy), use the psychrometric calculator.

How to use it

1. Export the logger’s readings as CSV, TSV or Excel and press Open a file, or paste them. The page finds the time, temperature and RH columns from the header; change them if needed. Pick the date order if your dates are day/month/year or month/day/year, and °F if needed.
2. Set your limits. The values on the page are examples only: use the targets your institution or loan agreement sets.
3. For the preservation index, set the activation energy for the material (the example is 100 kJ/mol) and the reference conditions.

Worked example

The page opens with a made-up month of hourly readings (720 readings over 30.0 days) from a room with a daily cycle and a damp spell in the middle. With the example limits it reports 91.0 h (3.8 days) at or above 70 % RH, all in one spell from 2026-06-13 16:00 to 2026-06-17 11:00. RH ran from 47.9 to 83.0 % (mean 55.8 %); it was above 65 % for 98.0 h (13.6 % of the time). The temperature averaged 20.2 °C and was above 22 °C for 73.0 h. The dew point averaged 10.9 °C. The mean daily swing was 8.9 % RH; the largest was 27.6 % RH on 2026-06-17, and 2 days were over the 10 % limit. Being warmer and damper than 20 °C and 50 % on average, the room’s decay rate is 1.16 × the reference and the preservation index is 0.864.

Tips

• Each reading counts for the time until the next one. A gap of more than three usual steps (a flat battery, a download) counts as one step and breaks a damp spell, and the page reports how many gaps there were.
• Check the logger’s clock and calibration; an RH sensor that drifts by a few % can move hours across a limit.
• Download the readings with dew point and relative decay rate to plot them with other data.

FAQ

Which activation energy should I use? It depends on the material and the reaction; values found in the literature for paper and film are often given in the region of 100 kJ/mol. Treat the index as a way to compare rooms and seasons, not as a lifetime forecast.

Does it predict mould? No. It shows when the air was damp enough, for long enough, to need attention. Mould also depends on the surface, dust, air movement and the temperature.

Is my data uploaded? No. The file is read in your browser and the page works offline.

Last updated 2026-11-09.