| Insulation |
Approximate upper useful
temperature |
Approximate melting point |
Remarks |
Properties |
| Magnesium Oxide (MgO) (99.4% min. purity) |
2500°F (1370°C) |
5070°F (2800°C) |
Used primarily with platinum sheathing in
compacted constructions only |
Low impurity levels make this insulation very
useful for all thermocouple calibrations up to 2500°F (1370°C). Above 2500°F
(1370°C) we recommend using beryllium oxide insulation because of MgO's low
resistivity at these elevated temperatures. This material meets the
requirements established in ASTM E235-82 |
| Alumina Oxide (Al2 O3)
(99.6% min. purity) |
2800°F (1540°C) |
3660°F (2015°C) |
Compacted constructions to 2800°F (1540°C).
Un-compacted constructions with vitrified insulators to 3000°F (1650°C) |
Comparable electrical properties to MgO. Used
primarily |
| Beryllium Oxide (BeO) (99.8% min. purity) |
3450°F (1900°C) |
4620°F (2550°C) |
Used with refractory metal sheathing in
compacted and un-compacted constructions |
This excellent high temperature insulation is
used primarily with tungsten-rhenium conductors and either a molybdenum or
tantalum sheath. Beryllia is the only material other than diamond that
combines excellent electrical insulating properties along with high thermal
conductivity. The two disadvantages of this material are cost,
(approximately 10 times greater than MgO), and it's toxicity. Beryllia dust
can cause potential health problems if inhaled. |
| Hafnia Oxide (HfO2) |
4530°F (2500°C) |
5000°F (2760°C) |
Available in compacted and un-compacted
constructions |
Hafnia is replacing BeO in applications can
not be used because of safety concerns. Hafnia can be used up to 4530°F
(2500°C) which is considerably higher than BeO that's rated at 3450°F
(1900°C) |
