3.3 Conclusions for the choice of climatic storage conditions
Conclusions for the choice of climatic storage conditions. On the basis of the above discussion, it becomes clear that the choice of storage conditions is mainly determined by two conflicting principles: to keep humidity and temperature low (to retard chemical deterioration), and to avoid climatic changes (to prevent condensation and to minimise mechanical stress, most importantly for tapes and lacquer discs).
The minimum/maximum values to be kept to are:
| Humidity | Absolute maximum for prolonged exposure | 60% RH |
| Minimum | 25% RH | |
| Temperature | Absolute maximum | 35°C |
| Minimum | 8°C for magnetic tapes |
As explained, the precise values between these maxima/minima, have no immediate negative or positive influence. However, in the mid- and long-term they determine the life expectancy of carriers. Of higher importance, however, is the stability of the chosen climatic conditions (3.2.1.1, 3.2.3, and 3.2.4).
For the purpose of storage recommendations, the following definitions are made:
| Humidity (ranges) | Medium | 40-50% RH |
| Low | 25-35% RH | |
| Variability1 | narrow ± 3%RH relaxed ± 5%RH |
|
| Temperature (mean values) |
Room2 | ~20°C |
| Cool | between 8 and12°C | |
| Variability3 | narrow ± 1°C relaxed ± 3°C |
1 Permissible variations around the mean values are low frequency (annual) deviations.
2 This mean value of “room temperature” condition reflects the situation in moderate climatic zones and is not necessarily obligatory for tropical countries. There, it may be prudent to choose a higher mean temperature, e.g. 25°C, and invest the saving in energy costs in effective dehumidification instead. This will also improve the well being of archival staff members, who generally may not find first-world climatic working conditions acceptable.
3 Permissible variations are low frequency (annual) deviations.
Please note: Temperature/humidity ranges and variability must not be added. The chosen mean value should be kept to the permissible variability.
Recommended storage conditions
| Collections | Humidity | Var | Temp | Var |
|---|---|---|---|---|
| Access storage Tape collections in frequent use Mechanical and optical carriers (except lacquer discs) |
low medium |
narrow relaxed | room room | narrow relaxed |
| Preservation storage Tape collections Mechanical and optical carriers (except lacquer discs) |
low medium to low |
narrow relaxed | cool room to cool | narrow relaxed |
| Access and preservation storage Lacquer discs |
medium | narrow | room | narrow |
The climatic conditions of studios and laboratories should be the same as, or very close to storage conditions. Whenever possible, work with carriers (e.g. routine inspection) stored at cool and low humidity conditions, should be carried out on the spot; otherwise, carriers must be adequately acclimatised.
The choice of target values for humidity and temperature is always a compromise between accessibility, comfort, and the health of operators on the one hand, and costs on the other. It must be further noted that even the lowest affordable values do not prevent, but only retard deterioration. Therefore, archives should choose those parameters that they can afford to keep 24 hours a day all year round. Within the permissible ranges, stability is more important than the absolute values of temperature and humidity.
