Measurement methods and their characteristics. Types and methods of measurements. Devices. General information
Currently, there are many types of measurements, distinguished by the physical nature of the quantity being measured and the factors that determine varied conditions and measurement modes. The main types of measurements of physical quantities, including linear-angular ones (GOST 16263-70), are direct, indirect, cumulative, joint, absolute and relative.
The most widely used are direct measurements, which consist in the fact that the desired value of the measured quantity is found from experimental data using measuring instruments. The linear dimension can be set directly using the scales of a ruler, tape measure, caliper, micrometer, the acting force - with a dynamometer, temperature - with a thermometer, etc.
Indirect measurements used in cases where the desired quantity is impossible or very difficult to measure directly, i.e. by direct measurement, or when direct measurement gives a less accurate result.
Examples of an indirect type of measurement are establishing the volume of a parallelepiped by multiplying three linear quantities (length, height and width) determined using direct view measurements, calculating engine power, determining the electrical resistivity of a conductor by its resistance, length and cross-sectional area, etc.
Cumulative measurements are carried out by simultaneous measurement of several quantities of the same name, for which the desired value is found by solving a system of equations obtained by direct measurements of various combinations of these quantities. An example of cumulative measurements is the calibration of the weights of a set using the known mass of one of them and the results of direct comparisons of the masses of various combinations of weights.
The letters a, b, c, d are the unknown values of the weights that have to be added or subtracted from the mass of the weight. By solving the system of equations, you can determine the value of each weight.
Joint measurements are simultaneous measurements of two or more different quantities to find the relationship between them, for example, measurements of the volume of a body made with measurements of different temperatures that determine the change in the volume of this body.
The main types of measurements, based on the nature of the measurement results for various physical quantities, include absolute and relative measurements.
Absolute measurements are based on direct measurements of one or more physical quantities. An example of an absolute measurement would be measuring the diameter or length of a roller with a caliper or micrometer, or measuring temperature with a thermometer.
Absolute measurements are accompanied by an assessment of the entire measured value.
Relative measurements are based on measuring the ratio of the measured quantity, which plays the role of a unit, or measurements of a quantity in relation to the quantity of the same name, taken as the initial one. As samples, standard measures in the form of plane-parallel end length measures are often used.
An example of relative measurements can be measurements of the calibers of plugs and staples on horizontal and vertical optimeters with the setting of measuring instruments according to standard measures. When using reference standards or reference parts, relative measurements can improve the accuracy of measurement results compared to absolute measurements.
In addition to the considered types of measurement based on the main feature - the method of obtaining the measurement result - the terms control, testing and diagnostics should be pointed out as physical processes, which are based on the types of measurements that determine the most characteristic principles of compliance with the operational properties of the measured quantity.
To carry out measurements for the purpose of monitoring, diagnosing or testing products, it is necessary to carry out measures that determine the technological process of measurements: analysis of the measurement task, identification of errors, establishment of the number of measurements, selection of a measuring instrument, measurement method, etc.
Measurement technologies include the development of micrometer maps for the main parts of automobile engines during their safety tests.
As noted above, measurement is the process of experimentally obtaining one or more values of a quantity that can be reasonably assigned to it. The value of the measured quantity depends on the measurement conditions, the chosen method, the type of measuring instrument, etc.
Main measurement characteristics include measurement principles, measurement methods, and measurement accuracy.
The measurement principle is a physical phenomenon (effect) that forms the basis for measurements using one or another type of measuring instrument.
A large number of physical effects discovered by scientists during research are used as measurement principles. For example, using the Doppler effect to measure speed; application of the Hall effect to measure induction magnetic field; the use of gravity in measuring mass by weighing.
Examples of the application of different measurement principles - piezoelectric effect, thermoelectric effect and photoelectric effect.
Piezoelectric effect consists in the occurrence of EMF on the surface (faces) of some crystals (quartz, tourmaline, artificial piezoelectric materials) under the influence of external forces. Quartz and piezoceramics (for example, barium titanate), which have fairly high mechanical strength and temperature stability (quartz up to a temperature of 200°C; piezoceramics - up to 115°C), have found the greatest application for measurements.
Piezoelectric effect reversible: an emf applied to a piezoelectric crystal causes mechanical stress on its surface. Measuring transducers based on the piezoelectric effect are self-generating for dynamic measurements.
Thermoelectric effect used for temperature measurements, and two main ways to realize this effect are used.
In the first case, the property of changing the electrical resistance of metals and semiconductors with temperature changes is used. The metals often used are copper (for routine measurements) and platinum (for high-precision measurements). The corresponding measuring transducer is called a thermistor. Sensitive elements A semiconductor converter - a thermistor - is made from oxides of various metals. As the temperature increases, the resistance of the thermistor decreases, while that of the thermistor increases. The dependence of the resistance of thermistors with temperature changes is nonlinear; for copper thermistors it is linear; for platinum thermistors it is approximated by a square trinomial.
Platinum thermistors allow you to measure temperatures in the range from -200°C to +1000°C.
For measurement purposes, external and internal photoelectric effects are used. The external photoelectric effect occurs in an evacuated cylinder having an anode and a photocathode. When the photocathode is illuminated, electrons are emitted under the influence of light photons. When there is an electrical voltage between the anode and the photocathode, the electrons emitted by the photocathode form an electric current called photocurrent.
In this way, light energy is converted into electrical energy.
Method of measurement– this is a set of techniques (methods) used to compare the measured quantity with its unit (or scale) in accordance with the selected measurement principle.
Measurement methods are divided into methods of direct assessment and methods of comparison with a measure. Methods of comparison with a measure are divided into contrast, differential, zero, substitution and coincidence methods.
Direct assessment method consists in determining the value of a physical quantity using the reading device of a direct-acting measuring device. For example, measuring voltage with a voltmeter. This method is the most common, but its accuracy depends on the accuracy of the measuring instrument.
Comparison method with measure uses a comparison of the measured value with the value reproduced by the measure. The measurement accuracy may be higher than the accuracy of direct assessment.
Contrasting method is based on the simultaneous influence of the measured and reproducible quantity on a comparison device, with the help of which the relationship between quantities is established. For example, measuring weight using a lever scale and a set of weights.
When differential method the measuring device is affected by the difference between the measured quantity and the known quantity reproduced by the measure. In this case, the balancing of the measured value with a known one is not carried out completely. For example, measuring DC voltage using a discrete voltage divider, a reference voltage source, and a voltmeter.
Using null method the resulting effect of the influence of both quantities on the comparison device is brought to zero, which is recorded by a highly sensitive device - a zero indicator. For example, measuring the resistance of a resistor using a four-arm bridge, in which the voltage drop across a resistor of unknown resistance is balanced by the voltage drop across a resistor of known resistance.
Substitution method is based on alternately connecting the measured quantity and a known quantity to the input of the device, and based on the two readings of the device, the value of the measured quantity is estimated, and then by selecting a known quantity, it is ensured that both readings coincide.
With this method, high measurement accuracy can be achieved with high accuracy of the measure of a known quantity and high sensitivity device. For example, the accurate measurement of a small voltage using a highly sensitive galvanometer, to which a source of unknown voltage is first connected and the deflection of the pointer is determined, and then using an adjustable source of known voltage, the same deflection of the pointer is achieved. In this case, the known voltage is equal to the unknown.
By coincidence method determine the difference between the measured value and the value reproduced by the measure, using the coincidence of scale marks or periodic signals. For example, measuring the rotation speed of a part using a flashing strobe lamp: observing the position of the mark on the rotating part at the moments of the lamp flashes, the speed of the part is determined from the known frequency of the flashes and the displacement of the mark.
Verification of compliance with mandatory requirements and rules is carried out in the manner of state control (supervision) over compliance with mandatory requirements.
Accuracy of measurements is determined by the closeness to zero of the measurement error, i.e. closeness of measurement results to the true value of a quantity.
True value of the measured quantity– the value of a physical quantity that would ideally reflect the corresponding property of an object in quantitative and qualitative terms.
The actual value of the measured quantity is a value found experimentally that is so close to the true value that it can be used instead for a given purpose.
Due to the characteristics of our sense organs (vision and hearing) and the imperfection of the measuring instruments we use, it is impossible to determine the true value of the measured value.
One can only indicate that it is between some two values, one of which is taken with a deficiency, and the other with an excess. The closer these values are to each other, the smaller their difference, the more accurate the measurement is, therefore.
The measurement error can be quantitatively expressed in units of the measured value or in relation to the error to the measurement result, but the accuracy of measurements cannot be determined directly from the measurement results. Therefore, they usually talk about high (medium, low) measurement accuracy in a qualitative sense.
That is why it is more convenient to quantify the accuracy of measurements using an error.
Thus, the experimenter’s task is not only to determine this or that desired value, but also to indicate what is the accuracy of determining this value, or, in other words, what is the value of the error allowed.
The following main measurement characteristics are distinguished:
1) the method by which measurements are taken;
2) measurement principle;
3) measurement error;
4) measurement accuracy;
5) correctness of measurements;
6) reliability of measurements.
Measurement method- this is a method or a set of methods by which a given quantity is measured, i.e., a comparison of the measured quantity with its measure according to the accepted principle of measurement.
There are several criteria for classifying measurement methods.
1. According to the methods of obtaining the desired value of the measured quantity, the following are distinguished:
1) direct method (carried out using direct, direct measurements);
2) indirect method.
2. According to measurement techniques, there are:
1) contact measurement method;
2) non-contact measurement method.
Contact measurement method based on direct contact of any part of the measuring device with the measured object.
At non-contact measurement method the measuring device does not come into direct contact with the object being measured.
3. According to the methods of comparing a quantity with its measure, the following are distinguished:
1) direct assessment method;
2) method of comparison with its unit.
Direct assessment method is based on the use of a measuring device that shows the value of the measured quantity.
Comparison method with measure based on comparing the object of measurement with its measure.
Measuring principle– this is a certain physical phenomenon or their complex on which the measurement is based.
Measurement error is the difference between the result of measuring a quantity and the real (actual) value of this quantity.
Accuracy of measurements– this is a characteristic that expresses the degree of correspondence of the measurement results to the real value of the measured quantity.
Correct measurement– this is a qualitative characteristic of a measurement, which is determined by how close to zero a value is constant or fixedly changing with multiple measurements errors (systematic error).
Reliability of measurements is a characteristic that determines the degree of confidence in the obtained measurement results.
4 The concept of physical quantity The meaning of systems of physical units
A physical quantity is a concept of at least two sciences: physics and metrology. By definition, a physical quantity is a certain property of an object or process, common to a number of objects in terms of qualitative parameters, but differing, however, in quantitative terms (individual for each object). There are a number of classifications created according to various criteria. The main ones are divided into:
1) active and passive physical quantities – when divided in relation to measurement information signals. Moreover, the first (active) in this case are quantities that, without the use of auxiliary energy sources, have the probability of being converted into a measurement information signal. And the second (passive) are quantities for which it is necessary to use auxiliary energy sources that create a signal of measurement information;
2) additive (or extensive) and non-additive (or intensive) physical quantities - when dividing on the basis of additivity. It is believed that the first (additive) quantities are measured in parts; in addition, they can be accurately reproduced using a multivalued measure based on the summation of the sizes of individual measures. But the second (non-additive) quantities are not directly measured, since they are converted into a direct measurement of a quantity or a measurement by indirect measurements. In 1791, the first ever system of units of physical quantities was adopted by the French National Assembly. It was a metric system of measures. It included: units of length, area, volume, capacity and weight. And they were based on two now well-known units: the meter and the kilogram.
The scientist based his methodology on three main independent quantities: mass, length, time. And the mathematician took the milligram, millimeter and second as the main units of measurement for these quantities, since all other units of measurement can be easily calculated using the minimum ones. Thus, at the present stage of development, the following main systems of units of physical quantities are distinguished:
1) GHS system(1881);
2) MKGSS system(end of the 19th century);
3) MKSA system(1901)
Measurement methods (MI)– a method of obtaining measurement results by using principles and measuring instruments.
MI are divided into
Direct assessment method
The value of the measured quantity is read directly from the reading device of a direct-acting measuring device.
Advantage– speed of measurements, making it indispensable for practical application. Disadvantage: limited accuracy.
Comparison method with measure
The measured value is compared with the value reproduced by the measure. Example: measuring length with a ruler.
Advantage– greater measurement accuracy than with the direct assessment method. The disadvantage is that it takes a lot of time to select measures.
Opposition method
The measured quantity and the quantity reproduced by the measure simultaneously act on the comparison device, with the help of which the relationship between these quantities is established.
For example, weighing on an equal-arm balance, in which mass is measured, is defined as the sum of the mass of the weights that balance it and the readings on the scale.
Advantage– reducing the impact on measurement results of factors influencing the distortion of measurement information signals. Disadvantage: increased weighing time.
Differential (difference) method
It is characterized by the difference between the measured and known (reproducible measure) quantities. For example, measurement by comparison with a working standard on a compator, performed when checking length measures.
Advantage- obtaining results with high accuracy, even when using relatively crude means to measure the difference.
Null method
A method of comparison with a measure in which the resulting effect of exposure to a comparison device is brought to zero.
Match Method
A method of comparison with a measure in which the difference between the values of the sought and reproduced measure of quantities is measured using the coincidence of scale marks or periodic signals.
Advantage– the method allows you to significantly increase the accuracy of comparison with the measure. The disadvantage is the cost of acquiring more complex SRMs and the need for professional skills for the operator.
Substitution method
Based on comparison with a measure, in which the measured quantity is replaced by a known quantity reproduced by the measure, keeping all conditions unchanged. For example, weighing with alternately placing the measured mass and weights on the same pan of scales.
Advantages– the measurement error is small, since it is determined mainly by the measurement error and the dead zone of the device (zero - indicator). The disadvantage is the need to use multiple-valued measures.
Indirect measurement method I
Measurement of a physical quantity of one name associated with another desired quantity, determined by a functional relationship, with subsequent calculation by solving the control. Indirect methods are widely used in chemical testing methods.
Advantages– the ability to measure quantities for which there are no direct assessment methods or they do not provide reliable results or are associated with significant costs. Disadvantages: increased time and money spent on measurement.
1.6. Organization of the State Metrological Service
State Metrological Service of Russia (SMS) is a set of state metrological bodies and is created to manage activities to ensure the uniformity of measurements.
The general management of the HMS is carried out by the State Standard of the Russian Federation, which is assigned the following functions by the Law “On Ensuring the Uniformity of Measurements”:
- Interregional and intersectoral coordination of activities to ensure uniformity of measurements;
- Establishment of rules for the creation, approval, storage and application of standards of units of quantities;
- Determination of general metrological requirements for means, methods and results of measurements;
- State metrological control and supervision;
- Monitoring compliance with the terms of international treaties of the Russian Federation on the recognition of test results and verification of measuring instruments;
- Approval of regulatory documents to ensure the uniformity of measurements;
- Approval of state standards;
- Establishment of verification intervals for measuring instruments;
- Classification of technical devices as measuring instruments;
- Establishing a procedure for the development and certification of measurement techniques;
- Conducting and coordinating the activities of State Scientific Metrology Centers (SSMC).
- Accreditation government centers testing of measuring instruments;
- Approval of the type of measuring instruments;
- Maintaining State Register measuring instruments;
- Establishing a procedure for licensing the activities of legal entities and individuals in the production, repair, sale and rental of measuring instruments;
- Organization of activities and accreditation of metrological services of legal entities for the right to carry out calibration work;
- Planning and organization of metrological work;
The HMS includes seven state scientific metrological centers, the All-Russian Scientific Research Institute of Metrological Service (VNIIMS) and about 100 standardization and metrology centers.
The activities of these services are led by Gosstandart of the Russian Federation, which coordinates their work with the work of the State Migration Service on the basis of a unified technical policy.
Rights and obligations
Rights and obligations structural units of the metrological service in the central office, in the parent and base organizations of the metrological service, as well as in enterprises and organizations are determined by the Regulations on the metrological service of the state governing body or legal entity, approved by their head.
The activities of metrological services are supported by legislative and regulatory documents regulating various areas, including metrological support of production and certification of quality systems; standards and means of measurement, control and testing; specialists with professional special training, qualifications and experience in performing metrological work and services.
Financing
Financing of work for the fulfillment of tasks by the parent organization is carried out from the centralized funds of the relevant state governing body, and for the base organization - from specially created extra-budgetary funds.
Metrological services of enterprises can be accredited for the right to calibrate measuring instruments on the basis of agreements concluded with state scientific metrological centers or State Migration Service bodies.
Measurement is the most important concept in metrology. This is an organized human action performed for quantitative knowledge of the properties of a physical object by empirically determining the value of any physical quantity.
There are several types of measurements. When classifying them, they usually proceed from the nature of the dependence of the measured quantity on time, the type of measurement equation, the conditions that determine the accuracy of the measurement result and the methods of expressing these results.
According to the nature of the dependence of the measured value on time, measurements are divided into:
static, in which the measured quantity remains constant over time;
dynamic, during which the measured quantity changes and is not constant over time.
Static measurements are, for example, measurements of body size, constant pressure, dynamic measurements are measurements of pulsating pressures, vibrations.
According to the method of obtaining measurement results, they are divided into
Straight;
Indirect;
Aggregate;
Joint.
Direct- these are measurements in which the desired value of a physical quantity is found directly from experimental data. Direct measurements can be expressed by the formula Q=X, where Q is the desired value of the measured quantity, and X is the value directly obtained from experimental data.
In direct measurements, the measured quantity is subjected to experimental operations, which is compared with the measure directly or using measuring instruments calibrated in the required units. Examples of straight lines are measuring body length with a ruler, mass using scales, etc.
Direct measurements are widely used in mechanical engineering, as well as in the control of technological processes (measurement of pressure, temperature, etc.).
Indirect- these are measurements in which the desired quantity is determined on the basis of a known relationship between this quantity and quantities subjected to direct measurements, i.e. They measure not the actual quantity being determined, but others that are functionally related to it. The value of the measured quantity is found by calculating using the formula Q=F(x 1,x 2,…,x N), where Q is the desired value of the indirectly measured quantity; F- functional dependence, which is known in advance, x 1,x 2,…,x N are the values of quantities measured directly.
Examples of indirect measurements: determining the volume of a body by direct measurements of its geometric dimensions, finding the electrical resistivity of a conductor by its resistance, length and cross-sectional area.
Indirect measurements are widely used in cases where the desired quantity is impossible or too difficult to measure directly, or when direct measurement gives a less accurate result. Their role is especially great when measuring quantities that are inaccessible to direct experimental comparison, for example, dimensions of the astronomical or subatomic order.
Aggregate- these are measurements of several quantities of the same name made simultaneously, in which the desired one is determined by solving a system of equations obtained by direct measurements of various combinations of these quantities.
An example of cumulative measurements is the determination of the mass of individual weights in a set (calibration using the known mass of one of them and the results of direct comparisons of the masses of various combinations of weights).
Example. It is necessary to calibrate the weight, consisting of weights of 1, 2, 2*, 5, 10 and 20 kg (the asterisk indicates a weight that has the same nominal value, but a different true value). Calibration consists of determining the mass of each weight using one reference weight, for example, a weight weighing 1 kg. To do this, we will carry out measurements, changing the combination of weights each time (the numbers show the mass of individual weights, 1 arr. means the mass of a standard weight of 1 kg):
Letters a, b, c, d means the weights that have to be added to or subtracted from the mass of the weight indicated on the right side of the equation to balance the scales. By solving this system of equations, you can determine the mass of each weight.
Joint- these are measurements of two or several quantities of different names made simultaneously to find dependencies between them.
An example is the measurement of electrical resistance at 20 0 C and the temperature coefficients of a measuring resistor based on direct measurements of its resistance at different temperatures.
According to the conditions that determine the accuracy of the result, measurements are divided into three classes:
1. ^ Measurements with the highest possible accuracy , achievable with the existing level of technology.
These include, first of all, standard measurements related to the highest possible accuracy of reproducing established units of physical quantities, and, in addition, measurements of physical constants, primarily universal ones (for example, the absolute value of the acceleration of gravity, the gyromagnetic ratio of a proton, etc.).
This class also includes some special measurements that require high accuracy.
2. ^ Control and verification measurements , the error of which with a certain probability should not exceed a certain specified value.
These include measurements performed by state laboratories for supervision of the implementation and compliance with standards and the state of measuring equipment and factory measurement laboratories, which guarantee
the error of the result with a certain probability not exceeding a certain predetermined value.
3. ^ Technical measurements , in which the error of the result is determined by the characteristics of the measuring instruments.
Examples of technical measurements are measurements performed during the production process at machine-building enterprises, on switchboards of power plants, etc.
According to the method of expressing measurement results, a distinction is made between absolute and relative measurements.
Absolute are called measurements that are based on direct measurements of one or more basic quantities or on the use of values of physical constants.
An example of absolute measurements is the determination of length in meters, force electric current in amperes, acceleration of gravity in meters per second squared.
Relative are called measurements of the ratio of a quantity to a quantity of the same name, which plays the role of a unit, or measurements of a quantity in relation to a quantity of the same name, taken as the initial one.
An example of relative measurements is the measurement of relative air humidity, defined as the ratio of the amount of water vapor in 1 m3 of air to the amount of water vapor that saturates 1 m3 of air at a given temperature.
The main characteristics of measurements are: measurement principle, measurement method, error, accuracy, correctness and reliability.
^ Measuring principle - physical phenomenon or set physical phenomena, which form the basis of the measurements. For example, measuring body weight using weighing using gravity proportional to mass, measuring temperature using the thermoelectric effect.
Measurement method- a set of techniques for using principles and measuring instruments. Measuring instruments are the technical means used that have standardized metrological properties.
Measurement error - the difference between the values of the measured quantity obtained during measurement X" and the true Q values:
The error is caused by the imperfection of measurement methods and instruments, the variability of observation conditions, as well as the insufficient experience of the observer or the characteristics of his senses.
^ Accuracy of measurements is a characteristic of measurements that reflects the closeness of their results to the true value of the measured value.
Quantitatively, accuracy can be expressed as the reciprocal of the relative error modulus:
For example, if the measurement error is 10 -2%=10 -4, then the accuracy is 10 4.
^ Correct measurement is defined as the quality of measurement, reflecting the closeness to zero of systematic errors in the results (i.e., such errors that remain constant or naturally change with repeated measurements of the same quantity). The accuracy of measurements depends, in particular, on how much the actual size of the unit in which the measurement is made differs from its true size (by definition), i.e. on the extent to which the measuring instruments used for a given type of measurement were correct (correct).
The most important characteristic of the quality of measurements is their reliability; it characterizes confidence in measurement results and divides them into two categories:
reliable and unreliable, depending on whether the probabilistic characteristics of their deviations from the true values of the corresponding quantities are known or unknown. Measurement results whose reliability is unknown are of no value and in some cases can serve as a source of misinformation.
The presence of error limits the reliability of measurements, i.e. imposes a limit on the number of valid significant figures numerical value measured value and determines the accuracy of measurements.
