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Some mechanical measuring tools, such as squares, calipers, etc., were first used in mechanical manufacturing. In the 16th century, smooth gauges were used in artillery manufacture. In 1772 and 1805, British J. Watt and H. Maudsley and others successively produced a Watt micrometer and a calibration length measuring machine using the principle of thread pair. After the mid-19th century, measurement tools similar to modern mechanical outer micrometers and vernier calipers appeared successively. At the end of the 19th century, sets of gauge blocks appeared. Following the emergence of mechanical measuring tools is a group of optical measuring tools. At the end of the 19th century, vertical length measuring instruments appeared, and at the beginning of the 20th century, length measuring machines appeared. By the 1920s, projectors, tool microscopes, optical micrometers, etc. had been used for measurement in mechanical manufacturing. In 1928, the pneumatic gauge appeared, which is a measuring tool suitable for use in mass production. Electrical measuring tools appeared in the 1930s. Initially there were limit gauges and profilers made with inductive length sensors. In the late 1950s, coordinate measuring machines that displayed measurement results digitally appeared. In the mid-1960s, coordinate measuring machines with electronic computer-aided measurement were used in mechanical manufacturing. In the early 1970s, the computer digitally controlled gear measuring instrument appeared again. At this point, the measuring tool entered the stage of applying electronic computers. 1. Gas thermometer: use hydrogen or helium as the temperature measurement material, because the liquefaction temperature of hydrogen and helium is very low, close to absolute zero, so its temperature measurement range is very wide. This thermometer has high accuracy and is mostly used for precise measurements.
2. Resistance thermometer: It is divided into metal resistance thermometer and semiconductor resistance thermometer, which are made according to the characteristic of resistance value changing with temperature. Metal thermometers are mainly made of pure metals such as platinum, gold, copper, and nickel, as well as rhodium-iron and phosphor bronze alloys; semiconductor thermometers are mainly made of carbon, germanium, etc. Resistance thermometers are easy to use and reliable, and have been widely used. It has a measurement range of around -260°C to 600°C.
3. Thermocouple thermometer: It is a temperature measuring instrument widely used in industry. Made by thermoelectric phenomenon. Two different metal wires are welded together to form the working end, and the other ends are connected to the measuring instrument to form a circuit. When the working end is placed at the temperature to be measured, when the temperature of the working end and the free end are different, an electromotive force will appear, so a current flows through the loop. Through the measurement of electrical quantities, using the temperature at a known location, the temperature at another location can be determined. It is suitable for two substances with a large temperature difference, and is mostly used for high temperature and low turbidity measurement. Some thermocouples can measure high temperatures up to 3000°C, and some can measure low temperatures close to absolute zero.
4. High temperature thermometer: refers to a thermometer specially used to measure the temperature above 500 ℃, including optical thermometer, colorimetric thermometer and radiation thermometer. The principle and structure of high-temperature thermometers are more complicated and will not be discussed here. Its measurement range is from 500°C to above 3000°C, and it is not suitable for measuring low temperature.
5. Pointer thermometer: It is a thermometer shaped like a dashboard, also known as a cold and summer watch. It is used to measure the room temperature and is made of the principle of thermal expansion and contraction of metal. It uses a bimetallic sheet as a temperature sensing element to control the pointer. The bimetallic sheet is usually riveted with copper sheet and iron sheet, and the copper sheet is on the left and the iron sheet is on the right. Because the thermal expansion and contraction effect of copper is more obvious than that of iron, when the temperature rises, the copper sheet pulls the iron sheet to bend to the right, and the pointer is deflected to the right (pointing to high temperature) driven by the bimetallic sheet; otherwise , the temperature becomes lower, and the pointer is deflected to the left (pointing to low temperature) driven by the bimetallic sheet.