The small volume plethysmograph. An improved instrument for measuring the setting shrinkage of composite dental restorative materials.

The small volume plethysmograph provides a continuous record of the volume of small specimens of composite polymeric dental restorative material as they set. Decreases, from initial volumes of around 100mm3, which occur over several hours can be measured to within +/- 0.1 mm3. The specimen is immersed in mercury which it displaces into a capillary tube. A decrease in specimen volume reduces the height of the mercury column in the capillary tube, and this change is sensed by a strain gauge pressure transducer whose output is recorded. Experience has proved the instrument to be stable and accurate with a linearity over its working range of +/- 0.06mm3.