After deciding on the permissible ranges of relative humidity for each category of material and after a careful tabulation of the seasonal changes by means of a recording hygrothermograph, seek the advice of an engineer in obtaining the proper controlling unit. This will depend on the climate in which the museum is geographically located, the construction of the building and other factors. If the cost of total environmental control is prohibitive, separate controls for specific areas of exhibition and storage may be more practical.