Cartridge heaters are reasonably priced, robust and easy to install. As a rule they consist of a cylindrical stainless steel tube and thus provide a high corrosion prevention. There is a heating wire inside which is wound onto a ceramic core. The number of coils varies depending on the power. In order to avoid a short-circuit, the cartridge heater is filled with magnesium oxide and compressed afterwards. Highly compacted they reach a surface watt density of up to 50 W/cm² and withstand highest stress.