Test and Measurement: Know It All

Jon S. Wilson, Stuart Ball, Creed Huddleston, Edward Ramsden, Dogan Ibrahim

• Publisher: Newnes
• Year: 2008
• Print ISBN: 9781856175302
• eBook ISBN: 9780080949680
• Topic: Design
• Subtopic: Evaluation & Assessment in Education

The Newnes Know It All Series takes the best of what our authors have written to create hard-working desk references that will be an engineer's first port of call for key information, design techniques and rules of thumb. Guaranteed not to gather dust on a shelf!Field Application engineers need to master a wide area of topics to excel. The Test and Measurement Know It All covers every angle including Machine Vision and Inspection, Communications Testing, Compliance Testing, along with Automotive, Aerospace, and Defense testing.- A 360-degree view from our best-selling authors- Topics include the Technology of Test and Measurement, Measurement System Types, and Instrumentation for Test and Measurement- The ultimate hard-working desk reference; all the essential information, techniques and tricks of the trade in one volume

Part I. Measurement Technology and Techniques

Chapter 1. Fundamentals of Measurement

1.1. Introduction

Metrology, or the science of measurement, is a discipline that plays an important role in sustaining modern societies. It deals not only with the measurements that we make in day-to-day living, such as at the shop or the petrol station, but also in industry, science, and technology. The technological advancement of the present-day world would not have been possible if not for the contribution made by metrologists all over the world to maintain accurate measurement systems.

The earliest metrological activity has been traced back to prehistoric times. For example, a beam balance dated to 5000 bc has been found in a tomb in Nagada in Egypt. It is well known that Sumerians and Babylonians had well-developed systems of numbers. The very high level of astronomy and advanced status of time measurement in these early Mesopotamian cultures contributed much to the development of science in later periods in the rest of the world. The colossal stupas (large hemispherical domes) of Anuradhapura and Polonnaruwa and the great tanks and canals of the hydraulic civilization bear ample testimony to the advanced system of linear and volume measurement that existed in ancient Sri Lanka.

There is evidence that well-established measurement systems existed in the Indus Valley and Mohenjo-Daro civilizations. In fact the number system we use today, known as the Indo-Arabic numbers, with positional notation for the symbols 1–9 and the concept of zero, was introduced into western societies by an English monk who translated the books of the Arab writer Al-Khawanizmi into Latin in the 12th century.

In the modern world metrology plays a vital role to protect the consumer and to ensure that manufactured products conform to prescribed dimensional and quality standards. In many countries the implementation of a metrological system is carried out under three distinct headings or services, namely, scientific, industrial, and legal metrology.

Industrial metrology is mainly concerned with the measurement of length, mass, volume, temperature, pressure, voltage, current, and a host of other physical and chemical parameters needed for industrial production and process control. The maintenance of accurate dimensional and other physical parameters of manufactured products to ensure that they conform to prescribed quality standards is another important function carried out by industrial metrology services.

Industrial metrology thus plays a vital role in the economic and industrial development of a country. It is often said that the level of industrial development of a country can be judged by the status of its metrology.

1.2. Fundamental Concepts

The most important fundamental concepts of measurement, except the concepts of uncertainty of measurement, are explained in this section.

1.2.1. Measurand and Influence Quantity

The specific quantity determined in a measurement process is known as the measurand. A complete statement of the measurand also requires specification of other quantities, for example, temperature, pressure, humidity, and the like, that may affect the value of the measurand. These quantities are known as influence quantities.

For example, in an experiment performed to determine the density of a sample of water at a specific temperature (say 20° C), the measurand is the “density of water at 20° C.” In this instance the only influence quantity specified is the temperature, namely, 20° C.

1.2.2. True Value (of a Quantity)

The true value of a quantity is defined as the value consistent with its definition. This implies that there are no measurement errors in the realization of the definition. For example, the density of a substance is defined as mass per unit volume. If the mass and volume of the substance could be determined without making measurement errors, then the true value of the density can be obtained. Unfortunately, in practice, both these quantities cannot be determined without experimental error. Therefore the true value of a quantity cannot be determined experimentally.

1.2.3. Nominal Value and Conventional True Value

The nominal value is the approximate or rounded-off value of a material measure or characteristic of a measuring instrument. For example, when we refer to a resistor as 100Ω or to a weight as 1 kg, we are using their nominal values. Their exact values, known as conventional true values, may be 99.98Ω and 1.0001 kg, respectively. The conventional true value is obtained by comparing the test item with a higher-level measurement standard under defined conditions. If we take the example of the 1-kg weight, the conventional true value is the mass value of the weight as defined in the OIML (International Organization for Legal Metrology) International Recommendation RI 33, that is, the apparent mass value of the weight, determined using weights of density 8000 kg/m³ in air of density 1.2 kg/m³ at 20° C with a specified uncertainty figure. The conventional value of a weight is usually expressed in the form 1.001 g ± 0.001 g.

1.2.4. Error and Relative Error of Measurement

The difference between the result of a measurement and its true value is known as the error of the measurement. Since a true value cannot be determined, the error, as defined, cannot be determined as well. A conventional true value is therefore used in practice to determine an error.

The relative error is obtained by dividing the error by the average of the measured value. When it is necessary to distinguish an error from a relative error, the former is sometimes called the absolute error of measurement. As the error could be positive or negative of another term, the absolute value of the error is used to express the magnitude (or modulus) of the error.

As an example, suppose we want to determine the error of a digital multimeter at a nominal voltage level of 10V DC. The multimeter is connected to a DC voltage standard supplying a voltage of 10V DC and the reading is noted down. The procedure is repeated several times, say five times. The mean of the five readings is calculated and found to be 10.2V.

The error is then calculated as 10.2 – 10.0 = +0.2V. The relative error is obtained by dividing 0.2V by 10.2V, giving 0.02. The relative error as a percentage is obtained by multiplying the relative error (0.02) by 100; that is, the relative error is 0.2% of the reading.

In this example a conventional true value is used, namely, the voltage of 10V DC supplied by the voltage standard, to determine the error of the i...

Table of contents

• Brief Table of Contents
• Table of Contents
• Copyright
• About the Authors
• Part I. Measurement Technology and Techniques
• Part II. Measurement Systems
• Part III. Instrumentation Design Techniques for Test and Measurement
• Part IV. Circuit and Board Testing
• Part V. EMC and RF Emissions Testing and Measurement
• Part VI. Accelerated Testing
• Appendix . Standard Interfaces
• Index