For the measurement of the diameter of the inner hole of steel pipe, precision steel pipe, and some mechanical parts, there are direct measurement, indirect measurement, and comprehensive measurement methods.

Direct measurement
The method of directly measuring the aperture using two or three-point positioning is also the most commonly used aperture measurement method. According to the accuracy level, size, and quantity of the measured aperture, general length measuring tools that can measure the aperture can be used, such as vernier calipers (see calipers), tool microscopes, universal length comparison instruments, horizontal length measuring instruments (see length measuring machines), horizontal optical meters (see comparators) and pneumatic measuring instruments, etc.; special aperture measurement tools can also be used, such as internal diameter micrometers, internal diameter dial indicators and micrometers, internal diameter micrometers, electronic plug gauges and aperture measuring instruments using pneumatic, optical, electrical and other principles.
① Use a lever mechanism to measure the hole: commonly used in hand-held aperture measurement tools, such as internal diameter dial indicators, mechanical or electrical internal diameter micrometers, etc. The difference between the measured aperture size and the calibration ring gauge aperture is read from a dial indicator, or mechanical or electrical micrometer through a lever mechanism. The measuring aperture range of this type of hole measuring tool is generally 10 to 800 mm, among which the measurement accuracy of the internal diameter micrometer can reach 3 to 5 microns.
② Using the wedge principle to measure the aperture: commonly used in hand-held aperture measuring tools. Among them, the internal diameter dial indicator used to measure small holes can measure holes as small as 0.5 mm in diameter. When the measured aperture compression probe moves the measuring rod with a cone, the aperture error can be read from the dial indicator or micrometer. The three-point positioning method is suitable for measuring holes with a diameter of more than 3 mm. When the measuring rod rotates, the measuring rod moves forward by the fixed nut, and the cone with a spiral boss at the top of the measuring rod moves the three measuring heads outward to contact the measured hole. Read the measured aperture size from the scales on the fixed sleeve and the differential cylinder. This type of aperture measuring tool has a three-jaw internal diameter micrometer.
③ The seat-type aperture measuring instrument made of pneumatic, optical, electric, and other principles must be used to measure high-precision apertures under constant temperature conditions close to 20°C. The range of aperture measurement of the light wave interference aperture measuring instrument is 1 to 50 mm, and the accuracy is ±0.5 microns.

Indirect measurement
First measure the function related to the aperture, and then convert the aperture size. There are mainly two methods: ① Using the principle of three points to determine a circle, measure the coordinate values ​​of any three points on the circumference of the measured hole, and then calculate the coefficients D, E, and F in the equation x2＋y2＋Dx＋Ey＋F＝0, and then calculate according to the calculation formula
② Use a roller with a known diameter to roll against the wall of the measured hole to measure the circumference of the measured hole, and then calculate the aperture. This method is suitable for measuring holes with a diameter greater than 500 mm and a continuous surface. The measuring tool that applies this method is called a large-diameter measuring instrument, which is also commonly used to measure the outer diameter of large workpieces.

Comprehensive measurement
Mainly using a smooth plug gauge to check whether the inner hole of a steel pipe or a mechanical workpiece is qualified by the go-no-go method, which will not be explained here.