There are many classifications of measuring tools \x0d\ Classification of measurement \x0d\ (1) Measurement can be divided into the following three categories according to their fields \x0d\ 1, and legal measurement \x0d\ Legal measurement is compulsory management authorized by the government or the official according to the needs of legal system, technology and administrative management, in which legal measurement mainly involves compulsory management with conflicts of interest. For example, about weighing instruments, pressure gauges, electric meters, water meters, gas meters, blood pressure meters, etc. \x0d\ 2。 Scientometrics \x0d\ Scientometrics mainly refers to basic, exploratory and advanced metrological scientific research, such as measurement unit and unit system, measurement benchmark and standard, physical constant, measurement error, measurement uncertainty and data processing. Scientific metrology is usually the main task of metrology research institutions, especially national metrology research institutions. \x0d\ 3。 Industrial measurement \x0d\ Industrial measurement, also known as engineering measurement, refers to the application measurement in various engineering and industrial enterprises. For example, monitoring the measurement and test of energy, raw material consumption, technological process and product quality and performance. Industrial measurement involves a wide range, and it is a kind of measurement widely carried out in all walks of life. \x0d\\x0d\ (II) According to metrology, it can be divided into the following ten categories \ x0d \1,and geometric measurement \ x0d \ Geometric measurement is usually called length measurement, which is the earliest measurement field. Generally speaking, the content of geometric measurement is the geometric size, shape and position of an object, that is, the "three elements" of geometric quantity. The basic parameters of geometric measurement are length and angle, and straightness, surface roughness, roundness, cylindricity, slope, taper, involute, helix and so on, including the verification of general measuring tools and optical instruments and the measurement of special parts in production. The basic unit of geometric measurement is "meter" and the symbol is "m", which is one of the seven basic units of the international system of units. \x0d\ Commonly used measuring instruments for geometric measurement mainly include: gauge block, square, ruler, micrometer, vernier caliper, dial indicator, dial indicator, flat crystal, level meter, measuring microscope, projector, roundness instrument, surface profiler, gear measuring instrument, length measuring instrument, coordinate measuring machine, etc. \x0d\ 2。 Temperature measurement \x0d\ Temperature measurement is to measure the hot and cold degree of an object by using the thermal effects of various substances. The contents include: ultra-low temperature, low temperature, medium temperature, high temperature, ultra-high temperature, high temperature and so on. The unit of temperature measurement is Kelvin, and the symbol is "K". \x0d\ The commonly used measuring instruments for temperature measurement mainly include: mercury thermometer, thermocouple, semiconductor point thermometer, thermometer, moving coil instrument, temperature indicating and adjusting instrument, temperature circuit detector, automatic temperature recorder, drying box, constant temperature and humidity box, incubator, high and low temperature test box, etc. \x0d\ 3。 Mechanical measurement \x0d\ Mechanical measurement includes mass, capacity, density, pressure, vacuum, force measurement, torque, hardness, impact, speed, flow, vibration and acceleration. The commonly used measuring instruments for mechanical measurement mainly include: weights, balances, belt scales, weighing instruments, standard hardness blocks, hardness meters, tensiometers, dynamometers, load sensors, material testing machines, fatigue testing machines, torque meters, torque wrenches, torque wrench verification devices, torque testing machines, dynamometer, accelerometers, speed sensors, manometers, sphygmomanometers and pressure transmitters. \x0d\ 4。 Electromagnetic measurement \x0d\ Electromagnetic measurement is to measure various electromagnetic physical quantities by using various electromagnetic standards and electromagnetic instruments according to electromagnetic principles. Electromagnetic measurement includes current, electromotive force, resistance, inductance, capacitance, magnetic field strength, magnetic flux and so on. The commonly used measuring instruments for electromagnetic measurement mainly include: standard battery, standard voltage source, standard current source, resistor, capacitor, transformer, resistance box, ammeter, voltmeter, power meter, megger, flux meter, watt-hour meter, watt-hour meter verification device, DC potentiometer, DC bridge, AC bridge, multimeter and so on. \x0d\ 5。 Radio Metrology \x0d\ Radio Metrology refers to the measurement of all electrical characteristics from ultra-low frequency to microwave in all frequency ranges used by radio technology. There are mainly high-frequency voltage, power, phase, pulse, impedance, noise and distortion. The commonly used measuring instruments for radio measurement mainly include: signal generator, modulation analyzer, audio analyzer, distortion meter, oscilloscope, function generator, electroencephalograph, electrocardiograph, frequency scanner, ECG monitor, etc. \x0d\ 6。 Time and frequency measurement \x0d\ Time and space are basic parameters to describe the development, movement and change of various objective things, and time and frequency are two different aspects to describe periodic phenomena. Time and frequency are mathematically reciprocal, and they use the same benchmark. The unit of measurement is seconds-"s". The commonly used measuring instruments for time and frequency measurement mainly include: frequency synthesizer, quartz crystal oscillator, frequency meter, general electronic counter, stopwatch, time interval generator, electronic timer, telephone timing and charging device, etc. \x0d\ 7。 Ionizing radiation measurement \x0d\ is also called radioactivity measurement. It is called ionizing radiation measurement by measuring the radiation that can directly or indirectly cause ionization (X-ray, γ-ray, Roentgen ray, radium and neutron radiation of uranium and thorium). Measurement of ionizing radiation can be divided into moderate measurement (or intensity measurement) and dose measurement. It is widely used in medical and health care (such as taking isotopes and liver scanning, which require accurate dose diagnosis), environmental monitoring, atomic power generation, prospecting, flaw detection, decontamination positioning of oil pipelines, agricultural breeding and so on. The commonly used measuring instruments for ionizing radiation measurement mainly include: working gamma-ray radiation source, medical CT scanner, radiation processing working dosimeter, X-ray flaw detector, X-ray protector, dose pen, gamma-ray protector, medical diagnostic X-ray radiation source, solid working dosimeter, chemical working dosimeter, Roentgen meter, etc. \x0d\ 8。 Optical metrology \x0d\ Optical metrology mainly includes light intensity, luminous flux, brightness, illuminance, chromaticity, radiance, sensitivity, laser, etc. Optical measurement is widely used, and modern building construction needs to measure the light intensity to meet the specified illumination standard. In spectroscopy, it is necessary to measure the luminosity of spectrum. In addition, the refractive index of film, film, optical glass, printing, pigment, film and television sensitivity all need accurate measurement of luminosity, chromaticity and color temperature. In the fields of national defense such as missile guidance and special photography, it is more necessary to accurately measure laser, ultraviolet and infrared. The basic unit of optical measurement is the luminous intensity threshold [Della], symbol: cd. The commonly used measuring instruments for optical metrology mainly include: illuminance meter, luminance meter, standard color plate, chromatic aberration meter, color spectrum photometer, medical laser light source, focal meter, Abbe refractometer, corneal contact lens, pupillary distance meter, optometrist, standard lens, spectrum analyzer, glossmeter, automobile headlamp detector, etc. \x0d\ 9. Acoustic metrology \x0d\ Acoustic metrology is a special study on the generation, propagation, reception and influence characteristics of sound waves in measuring materials. Sound intensity, sound pressure and sound power are three important basic parameters in acoustic metrology, among which sound pressure is the most widely used. Acoustic metrology involves communication, broadcasting, film, television, housing construction, medicine and health, navigation, coastal defense, language, music, industrial and agricultural production, production, life and science. The commonly used measuring instruments for acoustic measurement mainly include: microphone, sound level meter, ultrasonic flaw detector, ultrasonic thickness gauge, medical ultrasonic source, ultrasonic power source, audiometer, hearing aid, etc. \x0d\ 10, chemometrics \x0d\ Chemometrics, also known as physical chemometrics, refers to the analysis and determination of the composition, physical characteristics and basic physical constants of various substances. Mainly includes: pH, gas analysis, heat of combustion, viscosity, reference material, etc. It is a remarkable feature of chemometrics that the metrological department transmits the value by issuing standard materials. The commonly used measuring instruments in chemical metrology mainly include: acidity meter, turbidity meter, visible spectrophotometer, atomic absorption spectrophotometer, fluorescence spectrophotometer, filter photoelectric colorimeter, smoke meter, viscometer, calorimeter, dust concentration meter, smoke concentration meter, liquid chromatograph, gas chromatograph, conductivity meter, electrolytic moisture meter, carbon monoxide monitor, sulfur dioxide analyzer, carbon sulfur analyzer, automatic potentiometric titrator, toxic and harmful.