Principles of Measurement, Accuracy, Precision and Standards

INTRODUCTION

An instrument is a device for determining the value or magnitude of a quantity or variable. As technology expands the demand for more accurate instruments increase and products new developments in instrument design and application. On the other hand, measurement is a process by which one can convert physical parameters to a meaningful number.

METHODS OF MEASUREMENTS

The methods of measurements may be classified according to following types:

Direct Methods.

In the direct method of measurement, we compare the unknown quantity directly with the primary or secondary standard. For example if we want to measure the length of the bar, we will measure it with the help of the standard measuring tape or scale that acts as the secondary standard. Here we compare the unknown quantity directly with the standard scale. The scale is expressed as a numerical number and a unit.

The direct comparison method of measurement is not always accurate. In above example of measuring the length, there is limited accuracy with which our eye can read the readings, which can be about 0.01 inch. Here the error does not occur because of the error in the standards, but because of the human limitations in taking the readings.

Indirect Methods.

There are number of quantities that cannot be measured directly by using some instrument. For instance we cannot measure the strain in the bar due to applied force directly. In such cases indirect methods of measurements are used. In this method the unknown quantity to be measured is converted into some other measurable quantity. Then we measure the measureable quantity. For example the strain can be measured in terms of the electrical resistance of the bar.

CLASSIFICATION OF INSTRUMENTS

The instruments may be classified according to the following types:

Electrical and Electronic Instruments.

The measuring instrument that uses the mechanical movement of electromagnetic meter to measure voltage, current, power, etc. is called an electrical measuring instrument. These instruments use the d'Arsonval meter. While any measurement system that uses d'Arsonval meter with amplifiers to increase the sensitivity of measurements is called electronic instrument.

Analogue and Digital Instruments.

An analogue instrument is the instrument that uses analogue signal to display the magnitude of quantity under measurement. The digital instrument uses digital signal to indicate the results of measurement in digital form.

Absolute and Secondary Instruments.

In absolute instrument the measured value is given in term of instrument constants and the deflection of one part of the instrument e.g. tangent galvanometer. In these instruments no calibrated scale is necessary. While in secondary instruments, the quantity of the measured values is obtained by observing the output indicated by these instruments.

CLASSIFICATION OF SECONDARY INSTRUMENTS

The secondary instrument may be classified into the following categories:

Indicating Instruments.

The magnitude of quantity being measured is obtained by deflection of the pointer on the scale, and the output is indicated either in analogue or digital form like ammeter, voltmeter, and wattmeter. Three forces were acting on the pointer to deflect it in proportional to the quantity being measured, these forces are of the following types:

(a) Deflecting Force. This force gives the pointer the initial force to move it from zero position, it's also called deflecting force.

(b) Controlling Force. This force control and limits the deflection of the pointer on scale which must be proportional to the measured value, and also ensure that the deflection is always the same for the same values.

This force is necessary to bring the pointer quickly to the measured value, and then stop without any oscillation.

Recording Instruments.

An instrument which makes a record in any recorded medium of the quantity being measured in order to save information and use it in another time. The instruments like recording devices, X-Y plotter, and oscilloscope and recording instruments

Accuracy and Precision

Accuracy is a closeness with which the instrument reading approaches the true value of the variable under measurement. Accuracy is the degree to which instrument reading match the true or accepted values. It indicates the ability of instrument to indicate the true value of the quantity.

Accuracy refers to how closely the measured value of a quantity corresponds to its "true" value.

Precision is a measure of the reproducibility of the measurement i.e., its measure of the degree to which successive measurements differ from one other. It is the degree of agreement within a group of measurements or instruments. For example if any resistance has true value 3.385,695 Ω, it always read 3.4 MΩ in scale reading.

Let us consider two voltmeters of the same model, both meters have knife-edged pointers and mirror-backed scales to avoid parallax, and they have calibrated scales. They may therefore be read to the same precision. If the value of the resistance in one meter changes considerably, its reading may be in error by a fairy large amount. Therefore the accuracy of the two meters may be quite different.

The precision is composed of two characteristics

Conformity
Significant Figures

Both conformity and significant figures are discussed one by one in the following pages.

1.Conformity

Consider, for example that a resistor, whose true resistance is 3,385,695 Ω is measured by an ohmmeter. This consistently and repeatedly indicates 3.4 M Ω. The observer cannot read the true value from the scale. He estimates from the scale reading consistently a value of 3.4 MΩ. This is as close to the true value as he can read the scale by estimation. Although there are no deviations from the observed value, the error created by the lamination of the scale reading is a precision error. Conformity is necessary for measurements.

2.Significant Figures

An indication of the precision of the measurement is obtained from the number of significant figures in which the result is expressed. Significant figures convey actual information regarding the magnitude and the measurement precision of a quantity.

Resolution

Resolution is the smallest amount of input signal change that the instrument can detect reliably. If the input is slowly increased from some arbitrary input value, it will again be found that output does not change at all until a certain increment is exceeded. This increment is called resolution or discrimination of the instrument. Thus the smallest increment in input that can be detected with certainty by an instrument is its resolution or discrimination.

Example . A digital voltmeter has a read-out reading from 0 to 9,999 counts. Determine the resolution of the instrument in volt when the full scale reading is 9.999 V.

Solution.

The resolution of this instrument is I or I count in 9,999.

Resolution =

= 10^–3 V = 1 mV Ans.

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