The correct amount is determined by the equation, described above, and summarized by the above recommendations, based on common assumptions about typical museum conditions. Garry Thomson's book, The Museum Environment, 2nd ed., recommends 20 kilograms of regular density silica gel per cubic meter of case volume (he actually calculated 18.75 kg!) for maintenance-free cases, which is in excellent agreement with the above recommendation. If Thomson's calculation were modified to take the MH value of Art-Sorb, Arten or Rhapid Gel into account, Thomson's results would remain in agreement with the above recommendations.
Art-Sorb recommends 0.5 to 1.0 kilograms per cubic meter, based on short-term changes in RH due to a change in temperature. It does not take long-term leakage into account. Since the primary purpose of using silica gel is protection against leakage, Art-Sorb's recommendation is incorrect. The proper amount of Art-Sorb should be determined on the basis of its MH value, as previously described.