Record of Storage Atmosphere

TM 38-260

dust will retain moisture even after nearby clean surfaces have dried by evaporation. Rust and other corrosive products

also are hygroscopic, which partially accounts for the fact that rust spots spread rapidly. Because of such absorption, the

corrosion rate for a given metal increases rapidly above a certain relative humidity, called the critical humidity. The

critical humidities of metals vary. Some are quite high, that of copper being nearly 100 percent, while most ferrous

metals will corrode in a relative humidity of approximately 50 percent when they are bare.

5-4. Record of Storage Atmosphere

a. Hygrothermographs. A continuous record of temperature and relative humidity shall be kept for inspection for 1

year for all controlled humidity storage enclosures (buildings and hutments). In some instances, where humidity is

controlled by the application of heat, the controls are equipped with instruments for recording both temperature and

humidity. Where this type of recorder is present, no further recording instruments are needed. For all other storage

enclosures, a 30 day, vertical-drum hygrothermograph is recommended.

b. Sensing elements. The sensing elements on hygrothermographs should be maintained in accordance with the

manufacturer's recommended procedures.

c. Psychrometry. The properties of the storage atmosphere and the accuracy of the humidistat and

hygrothermographs may be determined by using a sling psychrometer and a psychometric chart or by using a battery-

powered, motor-driven relative humidity indicator. The manufacturer's recommendations for using either of these

instruments must be followed.

5-5. Types of Storage Environment

a. Controlled humidity storage. Types of controlled humidity storage areas are listed below.

(1) Controlled humidity storage. Controlled humidity (CH) storage is a type of storage in which the relative

humidity within an enclosed building or hutment is maintained at 50 percent or less by use of dynamic dehumidification

equipment, and controlled by automatic or semiautomatic regulators.

(2) Heated storage. Heated storage is a type of storage in which relative humidity within an enclosed building

or hutment is maintained at 50 percent or less through the use of temperature-regulated controls.

b. Noncontrolled storage. Noncontrolled storage denotes those storage areas that have no means for controlling

the relative humidity. Examples are: enclosed buildings, outdoor, under cover (shed and lean-to), and outdoor (no

covered structure). Outdoor storage is satisfactory for rough type items such as tanks, furnace shells, conveyors, anvils,

heavy castings, and forgings. Storage under open sheds should be restricted to semi-rough type items. The term

"noncontrolled storage" referenced in this paragraph is used in its generic form "noncontrolled humidity storage" in

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