Also known as dehydrators or dehumidifiers, desiccators are a glass container or other apparatus for removing moisture from specimens and protecting them from water vapour in the air. They are also used to protect chemicals such as cobalt chlorine paper which are hygroscopic and react with water from humidity. The simplest form of desiccator has a lower compartment which contains substances like silica to absorb water. The substance is placed in the upper compartment and the lid is greased with a thin layer of petroleum jelly for an airtight seal. This type can be converted into a vacuum desiccator extracting the air with a vacuum pump. Self-regulating desiccators are usually large and expensive and require an external power source. Non-vacuum desiccators with a purge consist of a low humidity atmosphere such as compressed dry air or dry nitrogen. For safety reasons, a pressure relief valve should be installed on each chamber. Static types vary in size from tiny capsules to drum-sized canisters; they are less expensive but require periodic replacement of the desiccating substance. Desiccators are used for the preservation of fragile specimens in laboratories. In industries involving sensitive instruments and components such as electronics and the aerospace industry, desiccators are frequently found.