There are many different types of high temperature extensometers available. The type of instrument is dependent on the type of testing (hot tensile, creep, LCF etc) and the type of heating system used (split tube furnace, straight tube furnace, side entry port furnace or environmental chamber).
The PSH8MS is typically used in hot tensile applications with tube type furnaces and consist of a separate measuring unit and suspension arm assembly. The measuring unit uses an LVDT with a fixed travel that remains outside of the furnace, the arms are located mostly inside of the heating system and attach the specimen to the measuring unit.
The 3448, 3548 and 2632 Series of instruments are strain gauge instruments designed for high temperature tension , compression and fatigue testing using a side entry port furnace. The arms are ceramic rods that pass through the furnace and contact the specimen. Depending on the model type, the extensometer is attached to the specimen with ceramic cord, or spring-loaded / counter-balanced mounting.