Such models are usually known as vacuum desiccators. Several types of desiccators are available, including standard, automatic, gas purge and vacuum desiccators. A granular form of PO is used in desiccators. The contents of desiccators are exposed to atmospheric moisture whenever the desiccators are opened. In laboratory use, the most common desiccators are circular and made of heavy glass. Desiccators can also be used to store reagents in dry conditions. Desiccators are sometimes used to remove traces of water from an almost-dry sample.