The curved hook downwards is an artifact of the measuring technique itself. Note the rapid rise in expansion which marks the onset of the glass transition range. A typical plot from a dilatometer (after adding the expansion of the standard specimen) is shown in Fig. Some commercial differential pushrod dilatometers can be accurate to as little as 0.5% which is generally quite adequate. The differential accuracy improves if the unknown and the standard are close in expansion characteristics. The key to getting good data is the uniformity of temperature over the specimen and the standard both of which should have flat, parallel ends. An equal length platinum sample is generally a good standard to use. The measuring system and the pushrods are generally made out of alumina or fused silica. This change relative to either the system tube itself or to a second pushrod against a standard specimen is measured using an LVDT (linear variable differential transducer). The specimen, held against the flat wall of the measuring system, pushes on a horizontal spring-loaded pushrod on heating. The experimental setup for a typical commercial pushrod dilatometer is shown in Fig.
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