Which instrument is suitable to measure the internal diameter of a test tube?

Vernier calipers are the suitable devices to use when measuring the inner diameter of a test tube because they are specifically designed with a pair of jaws, the upper pair, that are meant to fit inside a cylindrical space. They are also precise to an accuracy of 0.1 mm or 0.01 cm.

• (b) Screw Gauge: Also known as a micrometer, this device is designed to measure the external diameter or thickness of very thin objects, such as wires or thin sheets, to a very high degree of accuracy.
• (c) Feeler Gauge: This device is a set of very thin steel blades of varying thickness used to measure the clearance or space between two flat surfaces (for example, the clearance of a valve in an engine) and is not used to measure the diameter of a circle.
• (d) Bore Gauge: This device is used to measure the diameter of a hole, and although it is a very accurate device, it is a specialized piece of equipment designed to measure the diameter of very large engine cylinders where a very high degree of accuracy in roundness is required.