A Length Comparator is a specialized instrument used to measure dimensional changes in materials like autoclaved aerated concrete (AAC) blocks, calcium silicate bricks, or cement concrete specimens. It is primarily employed to assess drying shrinkage in autoclaved Portland cement and the potential expansive reactivity of cement-aggregate combinations in mortar bars during self-drying storage. The device ensures precise measurements, typically using a high-resolution analogue dial gauge with a sensitivity of 0.001 mm. It comes equipped with a standardization bar to calibrate the instrument for accurate readings.