Chapter 1 - Yidnekachew · PDF fileInstrumentation & Measurement _____ _ AAiT

Instrumentation & Measurement _____________________________________________________ _ AAiT

_____________________________________________________________________________________Compiled by Yidnekachew M. of 16

Chapter 1

Basic Concepts of Measurement and Instrumentation

1.1 Introduction

Measurement techniques have been of immense importance ever since the start of human

civilization, when measurements were first needed to regulate the transfer of goods in barter

trade to ensure that exchanges was fair. The industrial revolution during the nineteenth century

brought about a rapid development of new instruments and measurement techniques to satisfy

the needs of industrialized production techniques. Since that time, there has been a large and

rapid growth in new industrial technology. This has been particularly evident during the last part

of the twentieth century, encouraged by developments in electronics in general and computers in

particular. This, in turn, has required a parallel growth in new instruments and measurement

techniques.

Measurement systems have important vital applications in our everyday lives, whether at home,

in our vehicles, offices or factories.

We use measuring devices in buying our fruits and vegetables. We assume that the measuring

devices are accurate, and we assume that we are all referring to the same units (e.g., kilogram,

meter, liter…). The consequence of inaccurate measuring devices in this case leads to financial

losses on our part.

We check the temperature of our homes and assume that the thermostats reading the temperature

are accurate. If not, then the temperature will be either too high or too low, leading to

inconvenience and discomfort.

We pay for our electricity in units of kWh and we assume that the electricity meter is accurate

and faithfully records the correct number of electricity units that we have used. We pay for the

water we consume in liters, and we also assume that the water meter is correctly measuring the

flow of water in liters. In this case as well, the error will lead to financial loss.



The accuracy of the measurement systems mentioned above is very important, but is more

critical in some applications than others. For example, a pharmacist preparing a medication is

reliant on the accuracy of his/her scales to make sure he/she includes the correct amounts of

ingredients in the medication. Another example is the manufacturing of present-day integrated

circuits and photo-masks that requires a high degree of accuracy. Certain chemical reactions

require high accuracy in the measurement and control of temperature.

The massive growth in the application of computers to industrial process control and monitoring

tasks has spawned a parallel growth in the requirement for instruments to measure, record and

control process variables. As modern production techniques dictate working to tighter and tighter

accuracy limits, and as economic forces limiting production costs become more severe, so the

requirement for instruments to be both accurate and cheap becomes ever harder to satisfy. This

latter problem is at the focal point of the research and development efforts of all instrument

manufacturers. In the past few years, the most cost-effective means of improving instrument

accuracy has been found in many cases to be the inclusion of digital computing power within

instruments themselves. These intelligent instruments therefore feature prominently in current

instrument manufacturers’ catalogues.

1.2 The evolution of measurement

We can look at the evolution of measurement by focusing on invented instruments or by using

the instruments themselves. We will list the steps of progress in measurement, which we define

somewhat arbitrarily, according to human needs as these emerged throughout history:

the need to master the environment (dimensional and geographical aspects);

the need to master means of production (mechanical and thermal aspects);

the need to create an economy (money and trade);

the need to master and control energy (electrical, thermal, mechanical, and hydraulic

aspects);

the need to master information (electronic and optoelectronic aspects).

In addition to these is the mastery of knowledge which has existed throughout history and is

intimately connected:



measurement of time;

measurement of physical phenomena;

measurement of chemical and biological phenomena.

1.3 Functions of Measurement systems

Measurements are made or measurement systems are set up for one or more of the following

functions:

To monitor processes and operations

To control processes and operations

To carry out some analysis

1.3.1 Monitoring

Thermometers, barometers, anemometers, water, gas and electricity meters only indicate

certain quantities. Their readings do not perform any control function in the normal sense.

These measurements are made for monitoring purposes only.

1.3.2 Control

The thermostat in a refrigerator or geyser determines the temperature of the relevant

environment and accordingly switches off or on the cooling or heating mechanism to keep

the temperature constant, i.e. to control the temperature. A single system sometimes may

require many controls. For example, an aircraft needs controls from altimeters, gyroscopes,

angle-of-attack sensors, thermo- couples, accelerometers, etc.

Controlling a variable is rather an involved process and is therefore a subject of study by

itself.

1.3.3 Analysis

Measurement are also made to

test the validity of predictions from theories,

build empirical models, i.e. relationships between parameters and quantities

associated with a problem, and



characterize materials, devices and components.

In general, these requirements may be called analysis.

1.4 Measurement units

The very first measurement units were those used in barter trade to quantify the amounts being

exchanged and to establish clear rules about the relative values of different commodities. Such

early systems of measurement were based on whatever was available as a measuring unit. For

purposes of measuring length, the human torso was a convenient tool, and gave us units of the

hand, the foot and the cubit. Although generally adequate for barter trade systems, such

measurement units are of course imprecise, varying as they do from one person to the next.

Therefore, there has been a progressive movement towards measurement units that are defined

much more accurately.

The first improved measurement unit was a unit of length (the meter) defined as 10-7 times the

polar quadrant of the earth. A platinum bar made to this length was established as a standard of

length in the early part of the nineteenth century. This was superseded by a superior quality

standard bar in 1889, manufactured from a platinum–iridium alloy. Since that time, technological

research has enabled further improvements to be made in the standard used for defining length.

Firstly, in 1960, a standard meter was redefined in terms of 1.65076373 x 106 wavelengths of the

radiation from krypton-86 in vacuum. More recently, in 1983, the meter was redefined yet again

as the length of path travelled by light in an interval of 1/299792458 seconds. In a similar

fashion, standard units for the measurement of other physical quantities have been defined and

progressively improved over the years.

The early establishment of standards for the measurement of physical quantities proceeded in

several countries at broadly parallel times, and in consequence, several sets of units emerged for

measuring the same physical variable. For instance, length can be measured in yards, meters, or

several other units. Apart from the major units of length, subdivisions of standard units exist

such as feet, inches, centimeters and millimeters, with a fixed relationship between each

fundamental unit and its subdivisions.

The latest standards for defining the units used for measuring a range of physical variables are

given in Table 1.1.

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(b) Supplementary fundamental units

(c) Derived units



1.5 Elements of a measurement system

Each measurement system consists of five elements. These elements could all be in one item or

could be all in separate five items. They could be adjacent to each other or they could be

separated by a distance. Some simple systems might not contain all of the components. The

components of a typical system are shown in Figure 1.

Figure 1: Components of a measurement system.



Each of these components is discussed in more detail below.

a. Sensor: The sensor is the element that gives an output that is proportional to the input

applied to it. In general the output is in an electrical format as this is the most suitable

format for later use (in processing, transmission and storage). The input format depends

on the variable to be measured (e.g., temperature, pressure, humidity, pH, speed,

acceleration, light…). Sensors usually have a near linear relationship, although this is not

always the case.

b. Signal Conditioning Element (SCE): This is also referred to sometimes as a variable

conversion element: When the output variable of a primary sensor is in an unsuitable (or

inconvenient) format, a signal conditioning element is used to convert it to a suitable

form. For example, the change in resistance of a strain gauge cannot be directly measured

and thus a deflection type bridge circuit is used to convert it to a suitable voltage. Bridge

circuits are examples of signal conditioning elements and are discussed in more detail in

the coming Chapters. Another example is the amplification of a very weak signal such as

a biomedical signal (such as that used in an electrocardiogram ECG).

The combination of the sensor and the signal conditioning element (SCE) is called the

transducer. By definition, a transducer is a device the converts from one form of energy

to another. The term ‘transducer’ is sometimes incorrectly used to mean ‘sensor’.

c. Signal Processing Element (SPE): This component is needed to improve the quality of

the signal. A very common example is filtering a signal that contains mains frequency

noise (i.e., 50 Hz).

Some of the examples of signal processing elements as used in a measurement system

are:

Remove the mean value from an a.c. signal (i.e., dc shift).

Filter out induced noise (example 50 Hz hum/pick-up).

Convert an analogue signal to a digital format.

Convert a time signal into voltage (e.g., an ultrasonic level sensor).



The combination of the sensor, SCE and SPE is called the transmitter. The output signal

from the SPE could be in a number of formats: voltage, current, frequency or on/off (such

as in a switch). In other words, the information about the variable to be measured will be

contained in the voltage of the output signal, its current or its frequency. It could also just

be a yes/no output signal (for example as given by a thermostat that gives a signal stating

whether the variable measured is more or less than a set value). In the case of frequency

for example, the value of the measured variable would be represented as a certain

frequency deviation from a certain mean frequency.

The voltage and current output usually follow a standard format (e.g. 0-10 V in case of

voltage and 4-20 mA in case of current).

Use of voltage, current or frequency has implication in terms of the effect of noise. The

effect of noise on current transmission and voltage transmission is discussed in more

detail in the coming chapters.

d. Signal Transmission: The signal is then transmitted to the final location where it is

needed. Most modern measurement system could be distributed over a wide area, and

hence transmission in this case is necessary. There are three reasons why the signal needs

to be transmitted to a remote location:

i. Convenience: It is easier for example to locate the final equipment in a warm

office than on a the roof of the building where the transmitter is located.

ii. Inaccessibility: The transmitter may sometimes be located in an area that cannot

be accessed or reach. The measured variable could be inaccessible because it is

located in a narrow tunnel if it is located in a high position.

iii. Hazardous location: The transmitter might be located in an area that is accessible,

but hazardous to humans. An example of the hazardous situation is where the

measured variable is in a chemical or nuclear plant, or in an area with very high

temperatures.

Transmission can be done by a number of methods, some of which are:



Cable Transmission: This is typically done by screened single core or multi-

core. This method suffers from the problem of losses and attenuation especially

over long distances and from electromagnetic interference. The cable is screened

to reduce noise interference. Where the distance is long and losses become

excessive, repeaters are needed at regular distances to re-amplify the signal.

Fiber optics: Fiber optic cables are now more widely used. They offer the

following advantages (the first two being most important to measurement

systems):

They are resistant to interference by electric and magnetic fields.

They have low losses over long distances (as opposed to copper

cable that might need repeaters at long distances, e.g., 2 km).

They have a large bandwidth and can offer high speeds (up to Tera-

Hz). This is not much of an issue in low speed sampling system used

in most measurement systems and is more relevant to high speed

communication and data systems.

They offer electrical isolation (galvanic isolation) between the

transmitter and receiver. In some cases this is necessary for safety

reasons.

The main disadvantage of fiber optic systems is their high cost. They also need

special equipment for installation, testing and repair and they require highly

trained and specialized technicians.

Wireless transmission: This removes the need for cabling and can be very

attractive in cases where the transmitter is placed in inaccessible or remote

locations. However, it does suffer from the problem of obstacles interrupting the

connection (e.g., reinforced concrete) and from attenuation. Most transmitter

manufacturers offer wireless versions of their systems nowadays. Many of the

home weather stations are equipped with a wireless connection.



Display, recording or analysis: D/R/A or use in automatic feedback systems:

This is where the final signal is utilized. One of the following actions is taken:

It is either fed into the automatic feedback system.

The signal is displayed, recorded or analyzed: The signal can either

be displayed on a screen or industrial display, it could be recorded on

a hard-disk for example over a period of days or months and it could

be analyzed to understand trends or draw conclusions.

Both actions can be taken simultaneously as well: We can feed the signal into an

automatic feedback system and display it on a screen or record it.

Not all measurement system will contain all of the five elements. In some cases it is difficult to

identify the boundaries between different elements.

As an example a simple measurement system is the mercury-in-glass thermometer. In this case

all the items are within the same instrument and it is in fact difficult to separate one component

from another. The system only contains a sensor (effectively the mercury in the tube) and a

display component (the scale on the glass). There is neither an SCE, SPE or transmission system.

On the other hand, an example of a complicated system is a computer controlled remote system

in a chemical plant. In this case the five components can be clearly identified. The system is

distributed, and thus the transmission element in this case is necessary due to the distance

between the variable of the process to be measured and the receiver (e.g., a computer). The

computer receiving the signal would display it, record it and keep available for later analysis if

needed. The signal could also be fed into an automatic control system (e.g., temperature control

of the chemical reaction).

1.6 Measurement systems and measurement devices

A measurement system is the generic term of an instrument or a complex system. A person using

a thermometer to measure his body temperature represents a measurement system. This system is

made of: the human observer, the thermometer and the measured variable (temperature) from the

process (the human body). It is important to note that the human observer is part of the



measurement system in this example. If the observer makes an error in reading the temperature

from the thermometer scale, then an error results from the whole measurement system.

1.7 Overview of variables that are measured

The following is a selection of the most widely measured quantities:

a. Electrical parameters: The basic seven parameters are: voltage, current, resistance,

capacitance, inductance, frequency and phase shift. Other electrical parameters that are

effectively derived from the 7 above in terms of measurement are: power and power

factor.

b. Magnetic: One of the magnetic parameters that can be directly measured is the magnetic

flux density.

c. Environmental variables such as: Temperature, pressure and humidity.

d. Mechanical measurements such as: Mass, force, torque, length, area, volume/capacity,

angle and surface roughness.

e. Fluid measurements such as: Viscosity, level measurement and flow measurement.

f. Motion measurement such as: Translational motion and rotational motion.

g. Others: Sound pressure, gas sensing and PH in solutions.

1.8 Error in a Measurement System

Any measurement system has an input variable which is the true value of the quantity to be

measured and an output variable which is the measured value. This is shown in Figure 1. Ideally,

we would aim to make these two values identical, but in practice this is not possible.

One of the main aims in designing a measurement system is to minimize the error between the

true value and the measured value. In fact, a large percentage of this textbook addresses the issue

of how to minimize the error between the true value and the measured value. The reason for this

error developing could one of the following:

a. Systematic Errors: These are errors that have a clear understood explanation

within the measurement system. Systematic error can be sub-divided into:



Static errors caused by the static characteristics of the measurement system

(effectively the steady state characteristics).

Dynamic errors caused by the dynamic response of the measurement system

(transient response of the device).

b. Random errors caused by unknown reasons.

c. Internal and external noise disturbances.

1.9 Definition of Terms

The following terms are often employed to describe the quality of an instruments reading.

Range

The region between the limits within which a quantity is measured, received or

transmitted, expressed by starting the lower and upper range values. Example: 0 to 150

oF, 20 to 200 psi.

Span

The algebraic difference between the upper and lower range values.

For example:

a) Range 0 to 150 oF , span 150 oF.

b) Range -20 to 200 oF, span 220 oF.

c) Range 20 to 150 psi, span 130 psi.

Elevated Zero Range

A range in which the zero value of the measured variable, measured signal, is greater

than the lower range value.

Example:

-25 to 50 psi.

Suppressed Zero Range

A range in which the zero value of the measured variable is less than the lower range

value.

Example:

20 to 100 psi.



Measured Variable

A quantity property or condition that is measured. Sometimes referred to as the

measured. Example: Temperature, Pressure, rate of flow.

Measured Signal

The electrical, mechanical, pneumatic or other variable applied to the input of a device. It

is the analog of the Measured Variable produced by a transducer.

Example:

In a thermocouple thermometer, the measured signal is an emf which is the electrical

analog of the temperature applied to the thermocouple.

In a flow meter, the measured signal may be a differential pressure which is the analog of

the rate of flow through the orifice.

In an electric tachometer system, the measured signal may be a voltage which is the

electrical analog of the speed of rotation of the part coupled to the tachometer generator.

Output Signal

A signal delivered by a device, element or system.

Accuracy

The accuracy of an instrument indicates the deviation of the reading from a known value

accuracy is typically expressed as:

a. Percentage of full scale reading (upper range value). Example:

A 100 Kpa pressure gage having an accuracy of ± 1 % would be accurate of ± 1

Kpa over the entire range of the gage.

b. Percentage of span. Example:

A pressure gage has span of 200 Kpa, Accuracy of ± 0.5%.

To one reading of 150 Kpa is taken, then the true value of measurement will be

between 0.5 200

150 150 1 149 151 100

xor kPa and kPa

c. Measured Variable Accuracy of ± 1 Kpa, over all ranges of the Instrument.

d. Percentage of the actual reading. Thus, for a ± 2% of reading voltmeter, we would

have an inaccuracy of ± 0.04 volts for a reading of 2 volts.



Precision

The difference between the instruments reported values during repeated measurements of

the same quantity. Typically, this value is determined by statistical analysis of repeated

measurement.

Repeatability

Is the ability of an instrument to reproduce the same measurement each time the same set

of conditions is repeated. This does not imply that the measurement is correct, but rather

that the measurement is the same each time.

Poor Repeatability means poor Accuracy.

Good Accuracy means good repeatability.

Good Repeatability does not necessarily mean good Accuracy.

Sensitivity

The change of an instrument or transducer output per unit change in the measured

quantity. A more sensitive instrument reading changes significantly in response to

smaller changes in the measured quantity. Typically an instrument with higher sensitivity

will also have better repeatability and higher accuracy.

Resolution

The smallest increment of change in the measured valve that can be determined from the

instrument readout scale.

Dead Band

In process instrumentation the range through which an input signal may be varied upon

reversal of direction, without initiating an observable change in output signal. Dead band

is usually expressed in percent of span.



Hysteresis

An instrument is said to exhibit hysteresis when there is a difference in readings

depending on whether the value of the measured quantity is approached from above or

below. Hysteresis results from the inelastic quantity of an element or device. In other

word, it may be the result of mechanical friction, magnetic effects, elastic deformation, or

thermal effects. Hysteresis is expressed in percent of span. Dead band term is included in

the hysteresis.

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