What are Measurements and Measuring Instruments? Understand the basic concepts of measurement
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What are Measurements and Measuring Instruments? Understand the basic concepts of measurement

Engaging in quantitative observation cannot rely solely on human senses, such as vision, hearing, touch, etc., and often requires tools, that is, measuring instruments. Measurement refers to the numerical representation of the size of a manufactured object based on a certain standard (unit).
Published: May 18, 2022
What are Measurements and Measuring Instruments? Understand the basic concepts of measurement

What Is Measurement?

To measure the size is to compare the measurement target with the reference object. Measuring instruments used as reference objects cover a wide variety of products according to their measurement purpose, method, and accuracy. By measuring the dimensions correctly, it is possible to check whether the manufacturer meets the required specifications (within tolerance). Therefore, accurate measurement is the basic condition to confirm whether the product is manufactured correctly.

The Importance of Measurement:

Correct dimensional measurement is the basis of manufacturing. From material procurement, processing, assembly, and quality inspection to shipment, all processes are measured using the same benchmarks, to produce products that meet the design and ensure their quality.

If the measurement work is not done at any stage, the quality of the production cannot be guaranteed. If defective products are mixed in, it will not only lead to poor yield but also lead to customer complaints. In other words, measurements must be performed correctly at all stages of the manufacturing process.

The basis of correct measurement is that all participating manufacturers need to have measurement technology and to properly manage and use measuring instruments. Measurement management is also the basis of quality management. In recent years, measurement management has been standardized as a measurement management system (ISO10012).

What Measurement Methods are Available?

Size measurement methods are divided into direct measurement and indirect measurement.

  1. Direct measurement: It is a method of directly measuring the target using measuring instruments such as vernier calipers, centimeters, or three-dimensional measuring instruments, also known as an absolute measurement. This measurement method can perform a wide range of measurements within the scale range of the measuring instrument, but it may lead to erroneous measurements due to misinterpretation of the scale.
  2. Indirect measurement: It is to use a standard device such as a block gauge and a ring gauge to calculate the size of the target object by using a measuring instrument such as a dial gauge to calculate the difference between it and the target. It is a method of comparing objects with a reference size, so it is also called comparative measurement. Since the shape and size of the standard are fixed, it is easy to measure, but it also has a limited measurement range.

Benchmarks for Length Units:

  1. Human benchmark: The method of determining the basic unit of length has changed greatly with the times. A long time ago, the human body was used as a benchmark, such as the distance from the elbow to the fingertip in a cubit. However, this length varies by region and ranges from 450 to 500% 20mm. To this day, countries such as the United States still use length units derived from the human body, such as yard, feet, and inch.
  2. Earth datum: In the era of navigation, the industry began to flourish with Western Europe as the center, so it is necessary to unify the length datum of the whole world. In the 17th century, Europe began to discuss a unified unit. After more than a century of discussion, in 1791 France proposed the Meter (Greek meaning measurement) unit. At that time, the benchmark was the meridian distance from the earth's north pole to the equator, and 1/10 millionth of it was taken as 1 meter. Later, in the late 19th century, due to the need to integrate the world's dimensional benchmarks, France made a metric standard with platinum-iridium alloy, which is less prone to oxidation and wear.
  3. The speed of light benchmark: The unit based on the earth was questioned because it was difficult to measure from the beginning. The metric standard also had problems with manufacturing errors and deterioration over the years. Therefore, discussions began to establish a new benchmark. In the International Conference on Weights and Measures (CGPM) held in 1960, it was stipulated that the orange wavelength emitted by the element krypton 86 in a vacuum is 1 meter. In 1983, due to the advancement of laser technology, the length of 1 meter was determined based on the speed of light and time. At that time, "Light travels a distance of 1/299,792,458th of a second in a vacuum", which is the definition of 1 meter today.

International Unit System:

Measurements such as length are based on the principle of one measure and one unit all over the world. The International System of Units (SI) was established by the International Conference on Weights and Measures (CGPM) in 1960. In the International System of Units, length is measured in meters as the SI base unit. In addition, a symbol such as "k=10 cubed" of km can be added as a prefix of SI.

What Kinds of Errors are There?

The error of the length refers to the difference between the actual value of the target and the measured value, or the difference between the specified value and the measured value, with "Error = Measured value - True value". No matter how high the measurement precision is, it is difficult to obtain the true value. To prevent errors from occurring, various conditions must be considered.

  1. System error: The error caused by the deviation of the measured value due to a specific reason. For example, errors due to individual differences in measuring instruments (device differences), temperature, measurement methods, etc.
  2. Occasional errors: Errors that occur accidentally during measurement. For example, dust adhering to the measuring instrument causes errors, etc.
  3. Negligence error: The error caused by the lack of experience of the measurer or the operation error.

What are the Factors of Error?

  1. Error due to temperature: The volume of the object will change due to temperature changes, and the length will also change. This situation occurs on both the measured target and the measuring instrument. The change in temperature and the length of an object can be expressed by the coefficient of thermal expansion. The thermal expansion coefficient varies depending on the type of material. In addition, the International Standards Organization has set the standard temperature for length measurement as 20°C.
  2. Error due to material deformation: Applying force on the object will cause a certain percentage of change, and it will return to its original state when the force is stopped. This kind of object change is called elastic deformation. The force acting on an object is called stress, and it generally has a proportional relationship with the deformation of the object. The relationship between the two is expressed by the longitudinal elastic coefficient. As the stress increases, the amount of deformation also increases.

Measurement Principle: Abbe's Principle

Abbe's principle is an important guideline for explaining measurement accuracy and designing measuring instruments. Abbe's principle refers to "to improve the measurement accuracy, the scale of the measurement target and the measuring instrument must be on the same line as the measurement direction".

As far as the outer diameter of the actual measuring instrument is concerned, the scale is on the same line as the measurement position, while the scale of the vernier caliper is separated from the measurement position. In other words, the outer diameter cent card conforms to Abbe's principle, while the vernier caliper does not conform to Abbe's principle. Therefore, the measurement accuracy of the outer diameter centimeter is high.

What are Tolerances?

In any case, there will be some error between the measured value and the actual value. But it is important to clearly define the allowable error range. In the field of measurement, the difference between the maximum size and the minimum size of the allowable error is called "tolerance" or "tolerance". In addition, legally recognized error ranges, such as industrial specifications, are also known as tolerances.

In the actual drawing, if "60 (+0.045 -0.000)" is written, it means that the reference size is 60. Therefore, the upper limit is 60.045 and the lower limit is 60.000.

The rationale for setting tolerances in practice is to strike a balance between accuracy and machining costs. To improve the accuracy, the cost will also increase relatively. The balance between quality and cost can be achieved by designing individual tolerance values with individual workpieces.

What is Cooperation?

Another reason to set tolerances is that dimensional differences must be specified when combining multiple parts such as shafts and holes. This is called mating or fitting. When reviewing the fit, the way of thinking about the measurement will be different depending on the axis of the hole. Considering the diameter of the shaft as a reference, if the shaft only needs to be able to pass through the hole, use a clearance fit/clearance fit. Use an interference fit if the shaft is to be inserted into the hole and then secured. If it is a datum between the above two, use a transition fit.

Analog and Digital Measuring Instruments:

In recent years, measuring instruments have continued to be digitalized. For example, vernier calipers and centimeters with digital counters. In the past, it took practice to correctly read the sub-scale scale of a vernier caliper, but a digital vernier caliper can instantly display several 1/100th of a unit. However, digital measuring instruments also have their drawbacks. When the numerical value of the digital measuring instrument exceeds the accuracy range, if the force during operation is slightly increased or decreased, the numerical display will change rapidly. Especially for measuring instruments that can measure to 1/1000th of a unit, depending on the measurement target, it may be impossible to determine which value should be selected because the measurement value cannot be stabilized.

Depending on the job content, it is sometimes easier to intuitively grasp dimensions using an analog measuring instrument. Therefore, appropriate analog and digital measuring instruments should be used, respectively, according to the application and the required accuracy.

Metrological Traceability:

To ensure food safety, the production history system has been continuously strengthened in recent years to track the process of raw material cultivation, transportation, processing, packaging, and shipment. This is called traceability of food (history management). In the field of measurement, the way of thinking about traceability has also begun to be emphasized. This way of thinking is called metrological traceability, and it proves that measurements made daily are within tolerance.

International Standardization is Ongoing:

Metrological traceability has now been incorporated into international standards set by international research institutions such as the International Committee on Weights and Measures (CIPM), as well as national standards set by national research institutions. With the pace of economic globalization, the requirements for compliance with measurement traceability are gradually increasing.

The International Bureau of Weights and Measures (BIPM), a research organization under the International Committee of Weights and Measures, is engaged in basic research on the International System of Units (SI). In the process of manufacturing globalization, compliance with international standards is necessary. For dimensional measurement results to be consistently recognized anywhere in the world, metrological traceability through international mutual recognition is essential.

What are the Classifications of Electronic Instruments?

In a broad sense, electronic measuring instruments refer to instruments that use electronic technology for measurement and analysis. According to the functions of the measuring instruments, electronic measuring instruments can be divided into two categories: dedicated and general. Special electronic measuring instruments are designed and manufactured for specific purposes and are suitable for the measurement of specific objects. General-purpose electronic measuring instruments are designed to measure one or some basic electrical parameters. They are suitable for a variety of electronic measurements and can be subdivided into many types according to their functions.

Dedicated Electronic Measuring Instruments:
  • Audio/video analyzer: Audio/video signal generator, TV analyzer, video analyzer, audio analyzer, bit error analyzer/bit error analyzer.
  • Optical communication tester: Spectrum analyzer, digital transmission analyzer, optical network analyzer, optical time-domain reflectometer, optical power meter/power probe, bit error tester, optical attenuator, light source, optical oscilloscope.
  • RF and microwave instruments: Spectrum analyzer, network analyzer, impedance analyzer, signal generator digital oscilloscope, noise figure analyzer, cable/antenna analyzer, modulation analyzer, power meter/power probe, frequency meter, LCR table.
  • Wireless communication tester: Mobile phone comprehensive tester, TDMA tester, radio comprehensive tester, PDC/PHS tester, antenna feeder tester, 3G tester, DECT tester, Bluetooth comprehensive tester electronic load.
General Electronic Measuring Instruments:
  • Signal generator: It is used to provide signals required for various measurements. According to different uses, there are signal generators with different waveforms, different frequency ranges, and various powers, such as low-frequency signal generators, high-frequency signal generators, and function signals. Generator, Pulse Signal Generator, Arbitrary Waveform Signal Generator, and RF Synthesized Signal Generator.
  • Voltage measuring instruments: Used to measure the voltage, current, level, and other parameters of electrical signals, such as ammeters, voltmeters (including analog voltmeters and digital voltmeters), multimeters, etc.
  • Time and frequency measuring instruments: Used to measure parameters such as frequency, time interval, and phase of electrical signals, such as various frequency meters, phase meters, wavelength meters, and various time and frequency standards.
  • Signal analysis instruments: Used to observe, analyze, and record changes in various electrical signals, such as various oscilloscopes (including analog oscilloscopes and digital oscilloscopes), waveform analyzers, distortion analyzers, harmonic analyzers, spectrum analyzers, and logic analyzers instrument and so on.
  • Electronic component testing instrument: Used to measure the electrical parameters of various electronic components and check whether they meet the requirements. The different test objects can be divided into transistor tester, integrated circuit (analog, digital) tester, and circuit component tester.
  • Radio wave characteristic tester: Used to measure parameters such as radio wave propagation and interference intensity, such as test receivers, field strength meters, interference testers, etc.
  • Network characteristic test instrument: Used to measure the frequency characteristic, impedance characteristic, power characteristic, etc. of the electrical network, such as impedance tester, frequency characteristic tester, network analyzer, noise figure analyzer, etc.
  • Auxiliary instruments: Instruments used in conjunction with the above instruments, such as various amplifiers, attenuators, filters, recorders, and various AC and DC stabilized power supplies.
Published by May 18, 2022 Source :keyence

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