CHAPTER I

GENERAL CONCEPTS

1

1.1 INTRODUCTIONMeasurement is the act or the result of a quantitative

comparison between a predefined standard and an unknown magnitude.

There are two requirements that must be met The standard which is used for comparison must be

accurately defined and commonly accepted (determination of how many times as that of the standard)

The procedure and apparatus employed for obtaining the comparison must be provable.

2

1.2. FUNDAMENTAL METHODS OFMEASUREMENT

There are two basic methods of measurementsa) Direct comparison with either a primary or a

secondary standardb) Indirect comparison with a standard through the

use of a calibrated system (water level by a capacitor, temperature by a resistor). In such types of measurements there is generally an empirical relation established between the measurement actually made and the results that are desired.

3

Measurements are used in three areas. These are Monitoring of processes and operations Control of processes and operations Experimental engineering analysis

1.3 MONITORING OF PROCESSES AND OPERATIONS

This is the simplest aspect of measurement. Instruments used in weather bureau such as thermometers, barometers and anemometers serve the purpose of monitoring. They simply indicate the condition of the environment and the readings do not serve any control function.

4

Water and electric meters for domestic use have similar functions(allows the company to determine the charge). If action is taken based on this measurements, then the measurement serves a control function.

1.4 CONTROL OF PROCESSES AND OPERATIONS

Here the measuring instrument serves as a component of an automatic control system. To control the variable it is first necessary to measure it.

The controller compares the output variable with the desired value of the controlled variable and reacts by sending message to the control element to take corrective action (Fig.1.1)

5

Fig.1.1 Feed-back control system6

1.5 EXPERIMENTAL ENGINEERING ANALYSIS

To solve engineering problems two general methods are available: theoretical and experimental. Many problems require the application of both methods. Normally they complement each other. Complex situations are often dealt by experimental methods, eg. Convection heat transfer relations. Frontier knowledge often require very extensive experimental studies since adequate theories are not available yet.

Types of experimental-analysis problems Testing the validity of theoretical predictions 7

Formulation of generalized empirical relationships Determination of material, component, and system

parameters, variables, and performance indices Study of phenomena with hopes of developing a

theory Solution of mathematical equations by means of

analogies.For any successful experiment, it has to be designed

carefully and this requires specifying the physical variables to be investigated and their role in later analytical works. This will help to procure appropriate instrumentation. It is here where a

8

thorough knowledge of the governing principles of instruments will be required.

In all the above three processes, accurate measurement will include the specification of the degree of accuracy. This indicates the limitations of the instruments that account for certain random and/or regular errors which may be present in the experimental data. Statistical techniques are used for analyzing data to determine expected errors and deviations from the true measurements.

Additional specifications could be resolution, sensitivity, dynamic performance, and the environment where the instrument is operating.

To summarize instrument choice is a compromise between performance characteristics, ruggedness and durability, maintenance requirements and purchase cost.

To carry out such an evaluation properly, the instrument engineer must have a wide knowledge of the range of instruments available for measuring particular physical quantities, and he/she must also have a deep understanding of how instrument characteristics are affected by particular measurement situations and operating conditions.

10

1.6 GENERALIZED CONFIGURATIONS AND FUNCTIONAL DESCRIPTIONS OFMEASURING INSTRUMENTS

Instruments are built up of functional elements which may be described in general as

a) Transducer b) Signal conditioner c) Recorder or Indicator

Each functional element may have one or more components in the same instrument.

A transducer is a primary sensing element which receives energy from the measured medium and produces an output. Usually the input physical effect is transformed into another physical output,

11

in most cases to electrical signals.The intermediate stage involved in signal conditioning

includes variable conversion and variable manipulation elements. The output signal from the transducer is converted into a more suitable variable followed by amplification, filtering and other manipulation processes. The orders may also be different.

As an example, displacement-measuring strain gauge has an output in the form of a varying resistance. The resistance change cannot be easily measured and so it is

converted to a change in voltage by a bridge circuit, which is a typical example of a variable conversion element.

12

The final terminating stage will include data-transmission element for storage/playback or for presentation as an indication on an instrument or recording in a graphical presentation.fig_chp1\fig1.2.pptx

Example of piston pressure gage: fig_chp1\fig1.3.pptxPiston: Primary sensing element and variable conversion

element.Piston rod: data transmission elementSpring: Variable conversion elementLinkage: Variable manipulation elementPointer and Scale: Data presentation element

13

Example of pressure thermometer fig_chp1\fig1.4.pptx

Bulb: Primary sensing element and variable conversion element

Tubing : Data transmissionBourdon Tube: Variable conversion elementLinkage and gear: Variable manipuation elementScale and Pointer: Data presentation

1.6.1 Active and Passive TransducersThis is another generalization of instruments

concerned with energy considerations. 14

A component whose output energy is supplied entirely or almost entirely by its input signal is commonly called a passive transducer. Input and output signals may involve same or different form of energy. All the instruments seen before are passive transducers.

If the major part of the output energy from the instrument is due to auxiliary source, then the transducer is called active transducer. Typical example is a petrol fuel tank, where the input is from the fuel level and the output is a voltage from an auxiliary source.

Examples of the two types are shown in the next figures.

15

Passive pressure gauge16

Petrol-tank level indicator (active)17

1.6.2 Analog and Digital Modes of OperationAnalog gives an output that varies continuously as the

input changes. The output can have an infinite number of values within the range of the instrument.

A digital instrument has an output that varies in discrete steps and so can have a finite number of values. The revolution counter is a typical instrument. The switching operations are counted by an electronic counter. As computers are used often in modern control system, interfacing of analog measurements will require AD converters.

18

Revolution counter19

1.6.3 Null and Deflection MethodsThis is another classification of instruments. In a

deflection type device, the measured quantity produces some physical effect that triggers a similar but opposing effect in some part of the instrument. The opposing effect increases until a balance is achieved and then the deflection (indicator of the measured parameter) is measured. In a pressure dial gage the pressure triggers a balancing force from the spring.

20

In contrast a null type device attempts to maintain deflection at zero by suitable application of an effect opposing that generated by the measured quantity.

fig_chp 1\fig1.5.pptx shows pressure measurement.The accuracy attainable by the null method is of a

higher level (good for calibration) than that by the deflection method. Considering the pressure dial gage and the deadweight pressure gage, accuracy depends on the spring (linearity of spring and calibration-standards)in the former case while the weights do not need calibration in the latter case.

Disadvantage of null type in dynamic measurement-no time for balancing. 21

1.7 INPUT-OUTPUT CONFIGURATION OF MEASURING INSTRUMENTS AND INSTRUMENT SYSTEMS

There are three categories of inputs: desired input, interfering inputs, and modifying inputs.

Desired inputs: quantities that the instrument is specifically intended to measure.

Interfering inputs: quantities to which the instrument is unintentionally sensitive.

Modifying inputs: quantities that cause a change in the input-output relations for the desired and interfering inputs. eg temperature (ambient) and gravitational force (location). fig_chp1\fig1.6.pptx22

Examples of interfering inputs on a manometer are shown in fig_chp1\fig1.7.pptx

1.7.1 Methods of Correction for Interfering andModifying Inputs

Method of inherent insensitivity: If temperature is an interfering and modifying, it will be ideal if the instrument is inherently sensitive to only the desired inputs. As an example a strain gage material that exhibits an extremely low temperature coefficient of resistance while retaining its sensitivity strain stress.

Method of high-gain feedback: By suitable arrangements of the component (that introduce interfering/modifying inputs)gains in a 23

feedback control system, the output can be made to be dependent on the feedback gain only.

Method of calculated output corrections: This requires measurement or estimation of the magnitudes of the interfering and/or modifying inputs, effect on the output and finally determine the corrections to be made on the output.

Method of signal filtering: If a filter is on the path of the interfering/modifying input, the effect can be reduced. Examples of mechanical filters: mass spring system for interfering vibration, mounting scheme of a manometer for interfering tilt-angle, thermocouple insulation for interfering ambient

24

temperature, a restriction on the flow passage which will attenuate the high frequency outputs fig_chp1\fig1.8.pptx , fig_chp1\fig1.8b.pptx, fig_chp1\fig1.8c.pptx .

Method of opposing inputs: Consists of intentionally introducing into the instrument interfering and /or modifying inputs that tend to cancel the bad effects of the unavoidable spurious inputs. A manometer for measuring mass flow rate will be affected by change in temperature of the gas. Decreased temperature increases the density thus tending to increase the mass flow rate. The low temperature contracts the bellows thus decreasing the flow area and negating the tendency of mass flow increase. fig_chp1\fig1.9.pptx 25

1.8 STANDARDSThere are four fundamental quantities of the

international measuring system. These are length, time, mass and temperature. Standards for this system have been defined to establish consistency of measurements in different parts of the world.

Meter is defined as the length of path travelled by light in an interval of 1/299 792 458 seconds.

The second is defined as the time corresponding to 9,192,631,770 cycles of the atomic resonant frequency of cesium 133.

The kilogram is defined as a certain platinum-iridium mass maintained in France. 26

The above are primary standards( fig_chp1\table1.docx ) Secondary standards of length and mass are maintained at the National Bureau of Standards of countries for calibration purposes.

Temperature is defined in terms of the absolute temperature scale. For temperature standard, international temperature scale has been adopted and the primary points are shown in fig_chp1\table2.1.pptx. Secondary fixed points are also shown in fig_chp1\table2.2.pptx .

Interpolation procedures for the above scale are shown in fig_chp1\table2.3.pptx .

Latest fixed points for international scale are given in fig_chp1\table2.4.pptx . 27

All other units are derived from the above four basic measurement systems.

Typical is force which arises from Newtons 2nd law asF = ma

which gives rise to the definition of a Newton force (N) as1N=(1kg)(1m/s2)=1 kg m/s2

For more refer to fig_chp1\table1.docx

28

Slide Number 1Slide Number 2Slide Number 3Slide Number 4Slide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide Number 21Slide Number 22Slide Number 23Slide Number 24Slide Number 25Slide Number 26Slide Number 27Slide Number 28