A Length Comparator is a specialized instrument used to measure dimensional changes, such as drying shrinkage or expansion, in materials like autoclaved aerated concrete (AAC) blocks, calcium silicate bricks, or cement concrete specimens. It is particularly useful for assessing the drying shrinkage of autoclaved Portland cement and the potential expansive reactivity of cement-aggregate combinations in mortar bars during self-drying storage. A micrometer (often an analogue dial gauge with a resolution of 0.001 mm) for precise length measurements. A standardization bar to calibrate the instrument and ensure measurement accuracy.