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Module-1

Measurements and Measurement Systems

Measurement:

• Measurement is the process by which unknown quantity is compared with predefined standard values with significant units.

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• Measurement is a technique in which the properties of an object are determined by comparing them to a standard quantity. Also, measurement is the essential metric to express any quantity of objects, things and events.
• Ex: Measurement expressed as 10kg; 10 is the magnitude of the physical quantity and kg is the standard unit used to express mass of the physical quantity.
• Length
• Weight
• Volume
• Time
• Money
• temperture

Measurement Units

We have units of measurement for different types of measurement as mentioned above. Let’s have a look at the below units of measurement.

• Time: Units for expressing time include seconds, minutes, hours, days, weeks, months, years, etc.
• Length: Units for measuring length include millimetres, centimetres, meters, kilometres, etc.
• Weight: Units for expressing the weight of certain objects include grams, kilograms, tons, etc.
• Volume: Units for expressing volume include cm3, m3, litres, etc.
• Temperature: The major units of temperature include centigrade and fahrenheit.
• It is also possible to perform the conversion of units from one unit to another.

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Measurement Requirements:

• Measurement are meaningful there are two basic requirements:
• The standard used for comparison purposes must be accurately defined and should be commonly accepted and
• The apparatus used and the method adopted must be provable.

Significant Of Measurements:

• For proper operation and economical design measurement is important.
• For obtaining good results in engineering measurement is required.
• All electrical quantities need measurement for EX voltage, current, power, energy etc…

Applications of Measurement

• To construct buildings, we need to consider proper dimensions.
• For trading, purchasing, and selling of goods
• For preparing food, we generally take the ingredients in specific quantities
• For conducting scientific experimentations in labs
• To complete a certain task, time is the crucial thing to count.
• For formulating medicines and treating patients

Methods of Measurement:

The methods of measurements classified into two categories.

• Direct Method
• Indirect Method

Direct Method:

• Direct measurement is when you measure the thing you want to measure directly, the value of the physical parameter is determined by comparing it directly with different standards.
• Examples:
• The physical standards like mass, length and time are measured by direct measurement.
• If you want to know your height, you measure it explicitly with measuring tape.
• Direct measurements, measuring instruments such as vernier calipers, micrometers, and coordinate measuring machines are used to measure the dimensions of the target directly. These measurements are also known as absolute measurements.

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• A direct measurement is a measurement of the exact characteristic you want to measure. Here are some examples of direct measurements:
• Measuring height: Use a measuring tape to measure your height
• Measuring temperature: Use a thermometer to measure the temperature of your oven
• Measuring time: Use a stopwatch to measure how long your morning jog takes
• Measuring current: Use an ammeter plugged into a circuit to measure the current flowing through a wire
• Measuring distance: Use a tape to measure a distance
• Measuring length: Use a scale to compare lengths
• Measuring mass: Use a beam balance to compare masses

Indirect Method:

• Indirect measurement is when you measure something else to find the measurement of the thing you want to measure. An indirect measurement is a mathematical method used to find unknown measurements of objects that are difficult to measure.
• the value of the physical parameter is more generally determined by indirect comparison with the secondary through calibration.
• An example of indirect measurement would be finding the height of a 100-year-old oak tree.
• Ex: the measurement is convert into an analog signal which subsequently process and fed to end device at present result of measurements.

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Instruments And Measurement System

• The human senses cannot provide exact quantitative information about the knowledge of events occurring in our environments.
• Measurements involve the use of instruments as a physical means determining quantities or variables.
• The requirements of precise and accurate measurements in the technological fields have, i.e called instruments.
• The technology of using instruments to measure and control the materials is called instrumentation.
• It is common to refer the measuring instrument as a measurement system.
• Ex: wristwatch, is an instrument for measuring time, scale, thermometer…
• To test a product on material for quantity control.
• Monitor a data in the interest of health and safety.
• Ex: fore casting weather it predicting in the earth case.
• To discover effective components.
• To develop new methods.
• To develop new theories.

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Classification of Measuring Instruments:

The measuring instrument categorized into three types;

• Mechanical Instrument
• Electrical Instrument
• Electronic Instrument

Mechanical Instrument:

The mechanical instrument uses for measuring the physical quantities. This instrument is suitable for measuring the static and stable condition because the instrument is unable to give the response to the dynamic condition. This is due to fact that these instruments are rigid, heavy and bulky and consequently have a large mass.

Electrical Instrument:

The electrical instrument is used for measuring electrical quantities likes current, voltage, power, etc. The ammeter, voltmeter, wattmeter are the examples of the electrical measuring instrument. The ammeter measures the current in amps; voltmeter measures voltage and Wattmeter are used for measuring the power. The classification of the electrical instruments depends on the methods of representing the output reading.

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Electronic Instrument:

• Electronic instruments are steadily becoming more reliable on account of improvements in design and manufacturing processes of semi-conductor devices.
• electronic devices is that very weak signals can be detected by using pre amplifiers and amplifiers.
• Electronic instruments is the ability to obtain indication at a remote location which helps in monitoring inaccessible or dangerous locations.
• Electronic instruments are light compact ,have a high degree of reliability and their power consumption is very low.
• The instrument provides the quick response as compared to the electrical and mechanical instrument.
• Ex: computers require a very fast time response and it is only possible with use of electronic instruments.

Absolute instrument :

• Absolute instrument measures the process variable directly from the process without the use of conversion.
• Such instruments do not require comparison with any other standard.
• The tangent galvanometer is an example for the absolute instrument. These instruments are used as standards in labs and institution.

Secondary instrument:

These instruments are so constructed that the quantity being measured can only be the measured by observing the output indicated by the instrument.

These instruments are calibrated by comparision with an absolute instrument or another secondary instrument which is already been calibrated against an absolute instruments.

Indicating Instrument –

• The instrument which indicates the magnitude of the measured quantity is known as the indicating instrument.
• The indicating instrument has the dial which moves on the graduated dial. The voltmeter, ammeter, power factor meter are the examples of the indicating instrument.

Integrating Instrument –

• The instrument which measures the total energy supplied at a particular interval of time is known as the integrating instrument.
• The total energy measured by the instrument is the product of the time and the measures electrical quantities.
• The energy meter, watt-hour meter and the energy meter are the examples of integrating instrument.

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• Recording instruments:
• by the name itself indicate that they record the values measured.
• These instruments are similar to indicating instruments but, the only variation is that the pointer and scale are replaced by a lightweight metal pencil and a carbon paper in the case of an analog meter.
• An example of these instruments is the power consumption recording meters at power stations.
• Let us consider, the recording meters used at power stations. At the power stations, the operator must know the load demand at all times to operate the generators optimally.
• For this, they use a recording instrument that records the power consumption and its variations, with respect to time throughout the day. By knowing the load demand variations the generator's input can be controlled to get maximum efficiency.

Generalized measurement system:

• The generalized measurement system is a set of elements in which the measurement process is carried on by the system.
• There are many measuring instruments, but they exist for measuring some values of some variables.
• In some measuring instruments, the measuring process is done easily, and finally, it gives the output for the input as reading or signal based on the magnitude of the input variable.
• For example, the measuring process in some simple instruments is easy and simple (for measuring length, mass, etc).
• Most of the instruments like indicating thermometer undergo a complex measuring system. There will be each element for each function in these complex measuring instruments.

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Block diagram of the generalized measurement system

The elements of the measurement system are

1. Input variables
2. Primary sensing element
3. Variable conversion element
4. Variable manipulation element
5. Data transmission element
6. Data processing element
7. Data presentation element
8. Observer

1)Input variables:

• Input variables may be any unknown variable. Without any input variables, the final result can not be achieved by the system.
• Inputs should be in a certain amount of measured quantity

2)Primary sensing element:

• The first element of the measurement system is the primary sensing element. The main function of the primary sensing element is to sense the input variable and gives the output according to the measurand.
• This output will be the input of the next element. So this output is converted analogous electrical signal. This is achieved by using transducers. Variable conversion element

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3)Variable conversion element:

• It receives the output of the primary sensing element as input. As the name indicates, the conversion of the variable from one form to another form takes place. The conversion process is done without altering any data contained in the input.
• The requirement of this element depends upon the measuring instruments, some may need and some may not because they are converted into a required form in the previous element (primary sensing element).

4)Variable manipulation element

• This element manipulates the input variable.
• As per required magnification, the variables are manipulated by manipulation otherwise called as amplification. This is done for the required output from the input variable.
• The manipulation process does not depend upon the variable conversion element, so the manipulation of variables can proceed directly without any conversion element in some cases.

5)Data transmission element:

• Transmission of data or information from one element to another element takes place in this data transmission element. Data transmission is the main function of this element.
• Data transmission elements such as data cables, transmitters, and receivers, transmission shafts, etc are used to transmit the data from one element to another element.

6)Data processing element

• Data is modified and processed before the final result comes. The data processing element modifies the data for some reasons like,
• Modification for final output form,
• Modification for some final calculations,
• Modification for errors in the instruments such as positive error, negative error, zero error, temperature error, etc.
• For the following reasons, the data processing element is used in all measurement systems.

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7) Data presentation element:

• Finally, data is present to the observer via the data presentation element.
• The presentation element is such as to monitor, recorders, needle pointers, LCD and LED display, alarms, indicators like the analog indicator and digital indicator, etc.
• Without data presentation element, data cannot be delivered to the observer.

8) Observer:

• The measurement data is finally delivered to the observer via the data presentation element, for further clarification and calculation.
• The observer used to record these data for further clasification in the future. The recorded data are stored either in hard copy or digital copy.

Stages in the generalized measurement system:

• There are four stages in the generalized measurement system, they are listed below
• Sensing stage
• Conversion stage
• Manipulation stage
• Recording stage

Sensing stage:

• The sensing stage is the primary stage. The input variables are sensed by the sensitive sensors in the variable sensing element and converted to the required type of signals.

Conversion stage

• The conversion stage is the second stage. This stage is not required for some instruments because already been done in the sensing stage. Other instruments are passed through this stage for further conversion of signals.

Manipulation stage

• This stage is the tertiary stage. The signals are further manipulated or amplified for the final stage. Manipulation is done for the signals to indicate by the final stage.

Recording stage:

• This is the final stage. In this stage, the manipulated signals which contain data, are transmitted through the data transmission element to the data presentation element.
• The data presentation element delivers the data to the observer via a monitor, recorders, alarms, pointers, led and LCD screens, etc. The observer records the data and saved further clarification and analysis. By this, the recording stage ends.

Input- output configuration of measuring instruments and measurement systems:

• A generalized configuration in instruments and measurement system which brings out a significant input-output relationship present in then is shown in fig:
• Input quantities are classified into three categories:
• Desired inputs
• Interfacing inputs
• Modifying inputs

1) Desired inputs:

• Desired inputs are defined as quantities for which the instrument or the measurement system is specifically designed to measure and respond.

2) Interfacing inputs:

• Represent quantities to which an instrument or a measurement system becomes unintentionally sensitive.
• The instruments or measurement systems are not desired to respond to interfering inputs but they give an output due to interfering inputs on account of their principle of working, design many other factors like the environments in which they are placed.

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3) Modifying inputs:

• The classes of inputs can be included among the interfering inputs. However ,a separate classification is essential since such a classification is more significant.
• Modifying inputs are defined as inputs which cause a change in input-output relationships for either desired inputs or interfering inputs or for both.

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• The block diagram shown that the circle with a cross in it represents the summing point.
• The two plus signs shown indicate that the output of the summing point is the sum of the instantaneous values of its two inputs.

Ex: let us consider a case of measurement of differential pressure of a fluid with the help of an U-tube manometer using mercury as manometric fluid as shown in fig

• The pressures applied at the two ends of the U-tube are P1 and P2.
• The differential pressure is P=P1-P2 and following the expression is
• p=p1-p2=gh(ρm-ρf)
• Where ρm-ρf density of mercury and fluid respectively kg/m3
• g=gravitational constant ms-2
• h=scale regarding m
• Let us consider the fluid is a Gas and the density ρf is negligible as compared with ρm therefore the equation can be written as p=p1-p2=gρmh
• P1 and p2 are the desired inputs while scale reading h , is the output.the product gρm is the relates the output with input.
• In case of p1=p2 the differential pressure is zero and therefore output i.e scale reading h should be zero.
• However,there are examples when the scale reading h is not zero even though p1=p2 the differential pressure i.e desired input is zero.

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• This is on account of presence of interfering inputs ,action of two of which is shown in figures

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• The fig shows manometer which is placed a vehicle that is accelerating. Although the pressures at the two ends of the manometer are equal, a differential pressure is created on a account of acceleration and therefore scale indicates a reading h.thus accelerating acts as an interfering input.

• Another case of interfering input in this case the manometer is not properly aligned with the gravity vector because of the angle of tilt .
• therefore ,there is an output h, even though there is no differential pressure.
• Hence angle of tilt, ϴ, acts as an interfacing input.

• Hence comparing equations 1.5 and 1.6 it is clear that in comparison to the non feedback system the output in the feedback system ,corresponding to change of transfer function because of modifying inputs is reduced by a factor 1+GdH.
• Now if the feedback systems are designed with a high gain feed back H.