Measurement methods

Absolute methodIt
It determines the simple value of a measured quantity in the appropriate units, without the need to know its value in any special case (eg for a substance or body, or in a certain place). When using this method, the meter shows the value of the quantity directly (eg measuring the weight on an electronic scale, measuring the electrical resistance with an ohmmeter).

Relative method
Compares the change of the measured quantity with respect to the selected reference value or the relevant property of the measured body with unit bodies (eg measurement of mass on isosceles scales, measurement of electrical resistance by the bridge method).

Direct method
It consists in measuring the value of the monitored quantity based on the definition of the measured quantity or by comparison with a meter with the same property (eg measuring the length with a meter, measuring time with a stopwatch).

Indirect method
Determines the value of the monitored quantity indirectly (by calculation, using conversion nomograms, graphs or tables) through directly measured values of auxiliary quantities, ie it consists in measuring the consequence (effect) of the quantity (eg measuring temperature by longitudinal expansion with a thermometer, measuring electric current by magnetic forces).

Substitution (comparison) method
This method consists in comparing the measured quantity with other quantities of the same kind of various known sizes, among which the quantity which is closest in size to the measured quantity is sought. The actual measurement is performed in such a way that the determined quantity is gradually replaced by individual quantities, from which the one in which the state of measuring instruments differs as little as possible from the state when measuring the quantity is selected as suitable. However, the substitution method can provide a sufficiently satisfactory result only if a number of values ​​of known quantities are quite dense, i.e. if they contain a sufficiently large number of quantities in accordance with the chosen sizes. For this purpose, the specified quantities of graduated values ​​are compiled into sets (sets of weights, resistance and capacitance decades, decades of inductances, etc.), the individual members of which are selected so that from themit was possible to assemble by a simple combination all integral multiples of the same basic value, preferably with the smallest number of members. Values ​​are usually arranged in such series: 1, 1, 1, 2, 5, 10, 10, 20, 50, 100, 100, 200, 500, etc.

Compensation methods
The compensation method can be used only in those cases where the measured quantity can acquire positive and negative values ​​(force, moment of force, electrical voltage, etc.). The mentioned method consists in compensating the measured quantity by an equally large quantity of the same kind, but of the opposite sign. The magnitude of the compensating quantity is either known or can be easily determined. The compensation method is, for example, weighing on scales or determining the density according to Archimedes' law (the buoyancy of the liquid is compensated by the weight of the weight). This method is often used in electrical equipment. The compensation method may have some variations. If only a part of the measured quantity is compensated and the remainder is determined from the deviation of the respective measuring instrument, this compensation method is called deviation. If the compensating variable changes until there is no deviation from the zero position on the measuring instrument, it is called zero. It has the advantage that it is not necessary to know the division or accuracy of the instrument's scale. Compensation methods are usually more accurate than substitution methods and are less influenced by external influences.

Limiting method
It is suitable for measuring periodic events. This method makes it possible to obtain a very accurate measurement with an arbitrarily small relative measurement error. Accurate results can be achieved by the restriction method, provided that the measured action can be repeated in a sufficient number. To use it, it is necessary to know the measurement uncertainty for one period. Its great advantage is not only the possibility to reduce the relative uncertainty of the measurement below a predetermined limit, but also the fact that with a larger number of repeated measurements it is not necessary to accurately register the number of periods. The procedure can best be explained by measuring the swing time T. For example, the time for ten swings is measured with a stopwatch with an accuracy of ± 0.3 s. Ten swings last 68.8 s, so the time range is at least 68.8 - 0.3 = 68 , 5 s and a maximum of 68.8 + 0.3 = 69.1 s. One hundred oscillations therefore last a minimum of 685 and a maximum of 691 s. The stopwatch is monitored up to 685 seconds and then stops when the swing that ends first is completed. after 685 s. Measure eg 688.7 s. For one hundred oscillationstherefore, one hundred oscillations last at least 688.7 - 0.3 = 688.4 s and at most 688.7 + 0.3 = 689 s. If the length of one oscillation is calculated from this relationship, we find that one oscillation lasts at least 6.884 s and at most 6.890 s. The uncertainty of the swing time measurement is then only 6.890 - 6.884 s or ± 0.006 s, which is well below the accuracy of the stopwatch. Another advantage of this method is the ability to determine the time and a large number of swings without having to calculate them accurately. However, the essence of the conditions is that only such a multiple is always selected, when the limits of the multiples are smaller than the size of the measured interval. The narrower these limits are in relation to the size of this interval, the larger the multiples that can be used and the faster the sufficiently accurate result can be obtained.

Interpolation method
This method of measurement involves the use of a certain mathematical apparatus, and therefore it is sometimes included among the methods of processing measurements. When using the substitution or compensation method, it is sometimes not possible to set a value of the replaced or compensated quantity that would exactly match the value of the measured quantity. The prerequisite for the use of this method is that the dependence of the deviation on the measured quantity is linear, ie it is a method of linear interpolation. In this case, two measurements shall be made, one for the next lower value and the other for the next higher value. From the deviations of the measuring instrument corresponding to these values ​​and the deviations of the measured quantity, the measured quantity is determined by linear interpolation. For example, consider weighing on analytical balances. Because it is laborious to balance an object with weights exactly to the deflection of unloaded weights, the object is balanced with one weight to one deflection on the scale and a small weight is added (or removed) so that the new weight causes deflection on the other side of the scale. The weight of the weighed object is then determined by linear interpolation - the ratio of the distances of the individual deviations from the weighed object is in the same ratio as the deviations of the weight of the weighed object from the first weight and the weight with the weight. If the deviations of the measuring instrument are not linear for different weights, the graphical interpolation method may be used. For this, the dependence of the deviation of the measuring instrument (or the corresponding physical quantity) on the measured quantity for a number of known values ​​of the measured quantity is determined. This dependence, which does not have to be linear, is plotted and the points are interpolated by a curve. The magnitude of the measured quantity corresponding to the detected deviation is then subtracted from the graph.

Interesting
The way of measuring the amount of liquid flow seems quite trivial. A propeller is placed in the pipe, the flowing liquid spins the propeller, the propeller spins a small generator and according to the current it is determined at what speed the impeller rotates, and thus at what speed the liquid flows. Using the pipe diameter knowledge, the flow rate is calculated. But it has a few hooks. The measuring impeller resists the flowing liquid. If the liquid is thick and flows fast, there is a problem with the strength of the whole mechanism. If the liquid is belowhigh pressure, a problem arises because it is necessary to seal the generator and the passage of the wires through the wall of the tube to the display device very well. And when a liquid with some particles (eg concrete) is transported through the pipeline, it does not take long and the whole impeller is ground and the measurement is completed. An elegant solution is to measure the flow using electromagnetic induction. It is only necessary that the flowing liquid be at least slightly conductive. Then it is enough to replace a piece of pipe with a non-conductive material, attach electromagnetic coils and electrodes and start measuring the voltage on the electrodes, which depends on the speed of the flowing liquid. It is possible to measure the flow of aggressive chemicals, ice cream, concrete or even contaminated water with particles without any problems. And the meter does notno moving part and has no sources of leaks.

Sources: [1] Wikipedia [2] Types of physical quantities. In: Physics practicum - electronic teaching support University of South Bohemia in České Budějovice, Faculty of Education, Department of Physics, Academic year 2009/2010 [3] Artemis.osu.cz [4] projekty.fs .vsb.cz [5] physics.ujep.cz/~ehejnova [6] REICHL, Jaroslav and Martin VŠETIČKA. Encyclopedia of Physics.