Cartridge heaters are used mainly in immersion applications; however, due to their compact cylindrical shape, they can also be used in a variety of other applications. Important parameters to consider when specifying cartridge heaters are maximum operating or sheath temperature, AC voltage required, watts and watt density. Maximum operating temperature is the maximum temperature that the sheath covering the cartridge heater may reach. Note that the maximum sheath temperature is not the hottest temperature that a heated substance may reach. AC voltage is the minimum alternating current (AC) volts required to operate the cartridge heater. Watt density is the amount of wattage per square inch that a cartridge heater can deliver. Suppliers calculate watt density by dividing the available wattage by the heated area. High watt density heaters should not be used with extremely viscous materials, poorly-circulated materials, or explosive/volatile materials due to risk of fire. Important dimensions for cartridge heaters are the nominal diameter and cartridge length or heated length.