Thermal Equilibration

Thermal Equilibration is most often discussed for managing access to materials kept in cold storage; in this context, the rate of thermal equilibration determines how long an object will need to stay in a staging room when coming out of cold storage. Allowing sufficient time for an object to warm above the dew point temperature of the air prevents condensation from forming on the surface of the object. (For more on Dew Point, see the previous volume of Climate Notes.) This article, however, will explore a few facts about the rate of thermal equilibration to understand the impact of environmental changes and various housing situations.

#1) Thermal equilibration is relatively fast.

Unlike moisture equilibration, which can take days, months or even a year depending on the object’s housing situation, most materials will adjust to the temperature of a new environment in a matter of hours.

When researchers at the Image Permanence Institute exposed a variety of photographic materials to temperature changes, they found that most of the materials fully equilibrated with the new temperature conditions within 6 to 12 hours. The time it took for the materials to equilibrate to the new environment varied according to the material and its configuration (such as mass, format, materials). Despite these differences, all the tested materials - 35mm motion-picture film, 4"x5" acetate sheet films, and 3"x5" resin-coated photographic prints – demonstrated a "fast" rate of thermal equilibration. Table 1 shows the warm-up times to for the materials to reach 50% equilibration –the half way point between the initial temperature and the new temperature condition - and 90% thermal equilibration – when the material has almost reached the new temperature condition. The fastest equilibration time was seen in the two 100-ft rolls of motion-picture film, one stored in the metal can and the other in a cardboard box; in 1.25 hours, both rolls of film had reached 90% equilibration. The slowest equilibration time was seen in a stack of six 1000-ft rolls of film stored in metal cans, which took 7.5 hours to reach 90% equilibration. Even the slowest equilibration was measured in a matter of hours, allowing for the general statement that the rate of temperature equilibration is "fast". 

#2) The time needed to adjust to a new temperature condition is influenced by the amount of exposed surface area and the thermal mass of the object.

While all the tested materials equilibrated in a matter of hours, there was still noticeable variation in the rates of equilibration. For example, it took 1.25 hours for a single 100-ft. roll of 35m motion-picture film in a metal can to reach 90% equilibration while it took 7.5 hours for a stack of six 1000-ft. motion-picture film rolls in metal cans to reach 90% equilibrium. If the material is the same (35mm motion-picture film) and the enclosure is the same (a metal film can), what accounts for the difference in equilibration times?

First, the more surface area exposed to the new environmental condition, the faster the object will exchange energy (and thus equilibrate) with the new environment. In this case, the single film can had more surface area exposed to the air than the film can in the middle of the stack. Consider: the single film can comes in contact with the air at the top, bottom, and the sides of the object (the film can was placed on an open wire shelf); the film can in the middle of the stack is blocked on the top and bottom by the other cans of film. Therefore, the film can in the middle of the stack is not exposed to the new temperature conditions until the other film cans surrounding it have equilibrated with the environment. The outermost film cans in the stack delay the equilibration of the inner most film can because they block direct contact with the new environmental conditions.

Secondly, the thermal mass of the object – as determined by the properties of the material and the format of the object - has a similar effect on equilibration time. The greater the thermal mass of the object, the more time it will take for energy to penetrate to the object’s core and thus the slower the object will equilibrate. In this case, the material was the same – 35mm motion-picture film - and the format was the same – a roll of film – but there was more material in the 1000-ft roll of film than in the 100-ft roll of film. The 1000-ft roll of film equilibrated slower because there was more material to adjust to the new temperature.

#3) Enclosures or housing situations do little to alter the time needed to adjust to a new temperature condition.

Because enclosures and housings offer physical protection for the object, it is tempting to assume that the enclosures might also protect the objects from changes in temperature. However, while some enclosures may act as a moisture barrier (and thus slow down moisture equilibration), studies show that enclosures do not significantly block the transfer of heat or reduce the time needed to adjust to new temperature conditions. When two 1000-ft rolls of motion-picture films – one roll in a metal can and one roll in a plastic can - were exposed to temperature changes, there was only a fifteen minute difference in the time it took for the two rolls to reach 90% equilibration.  The relatively insignificant difference – ¼ hour – demonstrates that the rate of thermal equilibration is not significantly altered by enclosures.

#4) The time needed to fully adjust to the new temperature condition is not affected by the magnitude of the temperature change.

It is also tempting to assume that the greater the change in temperature, the longer the time will be for the object to adjust to the new temperature. However, studies show that the magnitude of the temperature change (the difference between the initial temperature and the new temperature) does not alter the time needed for thermal equilibration. Researches at IPI enclosed a stack of photographic prints in a cardboard box. The stack was then exposed to three different temperature changes:
#1) warmed from 3^oF to 70^oF (-16^oC to 21^oC)
#2) warmed from 41^oF to 70^oF (5^oC to 21^oC)
#3) cooled from 122^oF to 70^oF (50^oC to 21^oC)

In each scenario, it took the stack approximately five hours to equilibrate to the new temperature. This demonstrates two characteristics of thermal equilibration. First, the magnitude of the temperature difference does not significantly alter the time for equilibration. Even though the first scenario (warmed from 3^oF to 70^oF) had to warm up significantly more than the second scenario (warmed from 41^oF to 70^oF) to reach the new temperature, the time to reach equilibration was the same. The greater temperature difference did not mean a longer time to reach thermal equilibrium. Secondly, this experiment also demonstrates that the direction of temperature change does not significantly alter the time needed for equilibration. The third scenario, where the stack was cooled from 122^oF to 70^oF, took the same time to equilibrate as the two scenarios where the stack was warmed.

What does this "fast" rate of thermal equilibration mean in respect to environmental fluctuations?

Because temperature equilibration happens relatively fast, collection materials may experience temperature changes due to environmental fluctuations within a short period of time. Some practical storage situations may slow the rate of equilibration – remember the difference between the stack of film cans and the single film can – but enclosures will not significantly alter the rate of thermal equilibration. However, it is important to remember it is not necessarily the fluctuations in temperature that are most likely to cause damage to the materials. Unlike changes in relative humidity, where the magnitude of fluctuation influences the risk of mechanical or physical damage, changes in temperature are significant to the preservation of collection materials because of the level of temperature or amount of heat in the environment. The higher the temperature, the faster chemical reactions will occur; the lower the temperature, the slower chemical reactions will occur. Therefore, as far as temperature is concerned, it is the sustained high temperatures that have the most significant impact on the stability of the collection materials, not the temporary spikes or wide fluctuations of temperature. In other words, it is not the change tat is important, but the length of time spent at high temperatures. When analyzing the quality of collection storage conditions, it is better to ask, "Can I find a way to lower the temperature?" rather than, "Can I find a way to reduce fluctuations in temperature?" The best way to make "flat lines" of temperature significant is to lower the entire "line" of temperature, cooling the storage collections and thus slowing the rate of chemical decay.    
 

Continue to The Who, Why, When and How of Moisture Equilibration... [1]