Case Study #144: Checking Cartridge Heaters for Moisture The MgO used as an insulator in cartridge heaters is hygroscopic. That is, it will pull moisture out of the air. If enough moisture is pulled into the heater, then the heater can short out on start- up. The moisture reduces the dielectric strength of the MgO allowing the current to jump to the sheath. Most moisture is trapped inside of the heater as free water vapor. When the temperature inside the heater reaches 212 ° F, this water vapor turns to steam. However, there may be a small part of the moisture that chemically combines with the MgO. MgO + H 2 O = Mg(OH) 2 This moisture requires a temperature of more like 500 ° F to break loose. It has been in my experience that the moisture we and our customers often deal with pri

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