TYPES AND USES OF COMPASS

CIVIL_ENGINEERING

A compass is a navigational instrument for determining direction relative to Earth’s magnetic poles. It consists of magnetized pointer that is free to be aligned to magnetic North or true north or sometimes to an arbitrary direction based on the location of the celestial bodies. Magnetic North refers to the pole of Earth’s magnetic field. It  is the direction of the north tip of the Earth’s magnetic field and  true north  refers to the geographic north pole. The compass has been used extensive since ancient times for direction setting and for navigating across the oceans.

What is the history of the compass?

The compass was first used in India, around 1800 BC, for Navigational purposes and was known as “Matsya yantra” (which roughly translates to fish machine) because of the placement of a metallic fish in a cup of oil. The use of the magnetic compass started from 4th century in china, where a type of magnetite known as “lodestone” was used as a tool in a kind of divining magic, Geomancy. The Chinese were the first ones to have mastered the use of magnetic iron for navigation, which then rapidly spread to Europe and beyond. In the 13th century, Arabs started using the magnetic compass in navigation. The prismatic compass played a major role in the World War 1.

What are the various types of compass and their uses?

Magnetic compass: The most common type of compass is the magnetic compass, which is used to determine the direction of magnetic north. A magnetic compass is made by placing a bit of magnetized iron or steel which is set in a low friction so that it is allowed to move freely. In most compasses, the north end of the metal piece is marked with red paint so that all directions may be determined.

Gyro compass: It is a special type of compass developed in 19th century which determines the true north. A gyro compass is basically a very fast spinning wheel or ball which uses the law of conservation of angular momentum and the spinning of earth’s axis to point towards the true north. The gyro compass is commonly used in large ships and in other circumstances where the accurate prediction of true north is needed.

Astrocompass: This is another type of compass which can predict true north rather than magnetic north. This compass relies on the direction of celestial bodies to find true north, which is used in many circumstances, mainly in the far north and south poles where magnetic compass would become erratic and gyro compass stops working. The use of astro compass in determining the exact direction of true north requires the accurate information of time, date, longitudinal and latitudinal location.

Solid state compass: In this modern digital world, solid state compass are becoming more popular. These use many of the electronic magnetic sensors which would calculate the accurate direction the compass is pointing.

GPS compass: GPS compasses are rapidly replacing the use of other traditional compasses. However, most military and ships use the gyro compass or magnetic compass if GPS compass could not pick up enough satellite. GPS compasses make use of satellites in a geo synchronous orbit over the earth to distinguish the bearer’s exact location and direction they are heading. Many hikers and drivers like this compass due to its relative reliability.

Base plate compass: It is the most affordable compass. The liquid filled compass rise on the rectangular base made of clear plastic. This compass includes a magnifying lens for map reading, luminous components for low light conditions and different scales for world wide use. This compass is fine for plotting purposes.

Card compass: Card compass or marine compass is commonly used in ships and boats. It uses a fixed needle, depending on the moving compass card for directional readings since the moving card absorbs much of the motion of the boat.

Thumb compass: This compass attaches to the user’s thumb allowing the user to hold both the map and the compass in one hand while traveling at speed by bike or by canoe. It is also known as competition compass.

Prismatic compass: It is a sophisticated device designed for highly accurate navigation. The prism sighting arrangement allows the user to read the compass bearings while seeing distant objects.

TYPES AND USES:

TM 9-243TYPES AND USES – ContinuedCOMPASS  SAWThe compass saw is slightly larger than the keyholesaw. The teeth are so arranged that the blade can easilybe turned for cutting curves or holes. As with the keyholesaw, the compass saw will vary in size depending on thedesign and purpose.

COMPASS  SAW

The hacksaw is designed to cut almost any size orshape of metal object. The hacksaw uses two types ofblades, hard and flexible. The type of blade useddepends  on  the  nature  of  the  task.  The  blade  is  held  tothe saw frame by pins that fit into small holes at each endof the blade. Blade tension is adjusted by a screw andwingnut assembly at either the nose or the handle end ofthe  frame.  The  hacksaw  comes  in  various  designs,  de-pending on the purpose.

HACKSAW

TERMS USED IN LEVELLING

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Before studying the art of levelling, it is necessary to clearly understand the following terms used in levelling:

1. Level Surface: A surface parallel to the mean spheroid of the earth is called a level surface and the line drawn on the level surface is known as a level line. Hence all points lying on a level surface are equidistant from the centre of the earth. Figure shows a typical level surface.

 

 

 

A level Surface

2. Horizontal Surface: A surface tangential to level surface at a given point is called horizontal surface at that point. Hence a horizontal line is at right angles to the plumb line at that point [Ref. Fig. ].

3. Vertical Line: A vertical line at a point is the line connecting the point to the centre of the earth. It is the plumb line at that point. Vertical and horizontal lines at a point are at right angles to each other [Fig. ].

4. Datum: The level of a point or the surface with respect to which levels of other points or planes are calculated, is called a datum or datum surface.

5. Mean Sea Level (MSL): MSL is the average height of the sea for all stages of the tides. At any particular place MSL is established by finding the mean sea level (free of tides) after averaging tide heights over a long period of at least 19 years. In India MSL used is that established at Karachi, presently, in Pakistan. In all important surveys this is used as datum.

6. Reduced Levels (RL): The level of a point taken as height above the datum surface is known as RL of that point.

7. Benchmarks: A benchmark is a relatively permanent reference point, the elevation of which is known (assumed or known w.r.t. MSL). It is used as a starting point for levelling or as a point upon which to close for a check. The following are the different types of benchmarks used in surveying:

(a) GTS benchmarks (b) Permanent benchmarks

(c) Arbitrary benchmarks and (d) Temporary benchmarks.

(a) GTS Benchmark: The long form of GTS benchmark is Great Trigonometrical Survey benchmark. These benchmarks are established by national agency. In India, the department of Survey of India is entrusted with such works. GTS benchmarks are established all over the country with highest precision survey, the datum being mean sea level. A bronze plate provided on the top of a concrete pedastal with elevation engraved on it serves as benchmark. It is well protected with masonry structure built around it so that its position is not disturbed by animals or by any unauthorised person. The position of GTS benchmarks are shown in the topo sheets published.

 

b) Permanent Benchmark: These are the benchmarks established by state government agencies like PWD. They are established with reference to GTS benchmarks. They are usually on the corner of plinth of public buildings.

(c) Arbitrary Benchmark: In many engineering projects the difference in elevations of neighbouring points is more important than their reduced level with respect to mean sea level. In such cases a relatively permanent point, like plinth of a building or corner of a culvert, are taken as benchmarks, their level assumed arbitrarily such as 100.0 m, 300.0 m, etc.

(d) Temporary Benchmark: This type of benchmark is established at the end of the day’s work, so that the next day work may be continued from that point. Such point should be on a permanent object so that next day it is easily identified.

PRECISION MEASURING INSTRUMENTS

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Since modern production processes is concerned with interchangeable products, precise dimensional control is required in industry. Precision measurement instruments use different techniques and phenomena to measure distance with accuracy. We will discuss some of the precision measuring instruments in this section.

Vernier Calipers:

Vernier calipers are precision measuring instruments that give an accuracy of 0.1 mm to 0.01 mm. The main scale carries the fixed graduations, one of two measuring jaws, a vernier head having a vernier scale engraved on. The vernier head carries the other jaw and slides on main scale. The vernier head can be locked to the main scale by the knurled screw attached to its head. Enlarged diagram of the metric vernier scale is shown in Figure.

 

Vernier Caliper

 

To understand the working principle of a vernier caliper, let us consider that the vernier scale has got 20 divisions which equals to 19 divisions of the main scale. Thus, one smallest division of the vernier scale is slightly smaller than the smallest division of the main scale. This difference is called vernier constant for that particular vernier caliper and when it is multiplied with the smallest unit of the main scale gives the least count of that vernier.

Now, 20 vernier scale divisions (VSD) = 19 main scale division (MSD):

 

Vernier constant (VC) = 1 MSD – 1 VSD

Now, if the smallest unit of the main scale be 1 mm, the least count of the vernier scale = VC  one smallest unit of the main scale

If the smallest unit in the main scale be 0.5 mm, the least count of the vernier scale is,

To read a measurement from a vernier caliper, first the main scale reading up to the zero of the vernier scale is noted down. It will give accuracy up to the smallest division of the main scale. Now, vernier number of vernier scale division from its zero, which coincides exactly with the main scale is noted. This number when multiplied with the vernier constant gives the vernier scale reading. The actual length is obtained when the vernier scale reading is added to the main scale reading.
The caliper is placed on the object to be measured and the fine adjustment screw is adjusted until the jaws tightly fit against the Workpiece. There are vernier calipers that incorporate arrangements for measurement of internal dimensions and depth. The vernier calipers are designed to measure both internal and external dimensions.  The lower jaws of a vernier scale are used for external measurement and the upper jaws for the measurement of internal dimensions. The rectangular rod carried by the movable jaw is used for the measurement of depth.

FIRM JOINT TYPE

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They work on the friction created at the junction of the legs. The two legs are identical in shape with the contact points equally distant from the fulcrum and are joined together by a rivet. The component parts of the calipers should be free from seams, cracks and must have smooth bright finish. The distance between the rivet centre and the extreme working ends of the legs is known as nominal size and these calipers are available in the nominal size of 100, 150, 200 and 300 mm.

Firm joint calipers are of following types :

(i) Outside caliper
(ii) Inside caliper
(iii) Transfer caliper
(iv) Hermaphrodite caliper

Outside Firm Joint Caliper:
Figure  shows the diagram of an outside firm joint caliper. Unlike spring type outside calipers, it does not have any spring. The construction is quite simple with two identical legs held firmly by the fulcrum. If direct reading is desired, a steel rule must be used in conjunction with them.

Outside Firm Joint Caliper

Inside Firm Joint Caliper:
Inside firm joint calipers are almost similar to inside firm joint caliper with the exception that it does not have any spring to hold the legs as shown in  Figure Micrometers generally make adjustment in them. Like spring type inside calipers, they are also used for comparing or measuring hole diameters, distances between shoulders, or other parallel surfaces of any inside dimensions.

Inside Firm Joint Caliper

Transfer Caliper:
These are used for measuring recessed areas from which the legs of calipers can not be removed directly but must be collapsed after the dimension has
been measured. Therefore, an auxiliary arm is provided with two legs so that it can preserve the original setting after the legs are collapsed. The nut N in Figure is first locked and the caliper opened or closed against the work. The nut is then loosened and the leg is swung to clear the obstruction leaving the auxiliary arm in position. The leg can be moved back to the auxiliary leg, where it will show the size previously measured.

Transfer Caliper

Hermaphrodite Caliper
It is also known as odd leg caliper consisting of one divider and one caliper leg. It is used for layout work like scribing lines parallel to the edge of the work and for finding the centre of a cylindrical work. It can be with two types of legs, viz. notched leg or curved legs.

Hermaphrodite Caliper

MICROMETERS

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Micrometer is one of the most widely used precision instruments. It is primarily used to measure external dimensions like diameters of shafts, thickness of parts etc. to an accuracy of 0.01 mm. The essential parts of the instruments shown in Figure consist of

(a) Frame
(b) Anvil and spindle
(c) Screwed spindle
(d) Graduated sleeve or barrel
(e) Thimble
(f) Ratchet or friction stop
(g) Spindle clamp

Micrometer

 

 

The frame is made of steel, malleable cast iron or light alloy. The anvil shall protrude from the frame for a distance of at least 3-mm in order to permit the attachment of measuring wire support. The spindle does the actual measuring and possesses the threads of 0.5 mm pitch. The barrel has datum and fixed graduations Thimble is tubular cover fastened with the spindle. The beveled edge of the spindle is divided into 50 equal parts, every fifth being numbered. The ratchet is a small extension to the thimble. It slips when the pressure on the screw exceeds a certain amount. It produces uniform reading and prevents damage or distortion of the instruments. The spindle clamp is used to lock the instrument at any desired setting.

Procedure for Reading in a Micrometer:
The graduation on the barrel is in two parts divided by a line along the axis of the barrel called the reference line. The graduation above the reference is graduated in 1 mm intervals. The first and every fifth are long and numbered 0, 5, 10, 15, etc. The lower graduations are marked in 1 mm intervals but each graduation shall be placed at the middle of the two successive upper graduations to be read 0.5 mm. The thimble advances a distance of 0.5 mm in one complete rotation. It is called the pitch of the micrometer. The thimble has a scale of 50 divisions around its circumference. Thus, one smallest division of the circular scale is equivalent to
longitudinal movement of 0.5  1/50 mm = 0.01mm. It is the least count of the micrometer.

 

The job is measured between the end of the spindle and the anvil that is fitted to the frame. When the micrometer is closed, the line marked zero on the thimble coincides with the line marked zero on the barrel. If the zero graduation does not coincide, the micrometer requires adjustment. To take a reading from the micrometer,  (1) the number of main divisions in millimeters above the reference line, (2) the number of sub-divisions below the reference line exceeding only the upper graduation, and (3) the number of divisions in the thimble have to be noted down. For example if a micrometer shows a reading of 8.78 mm when

 

 

 

 

The various important terms used in connection with micrometers are given below.

REMOTE SENSING AND GEOGRAPHIC INFORMATION SYSTEM

CIVIL_ENGINEERING

INTRODUCTION:

Now-a-days the field of Remote Sensing and  GIS has become exciting and glamorous with rapidly expanding opportunities.  Many organizations spend large amounts of money on these fields. Here the question arises why these fields are so important in recent years.  Two main reasons are there behind this. 

1) Now-a-days scientists, researchers, students, and even common people are showing great interest for better understanding of our environment.  By environment we mean the geographic space of their study area and the events that take place there.  In other words, we have come to realise that geographic space along with the data describing it, is part of our everyday world; almost every decision we take is influenced or dictated by some fact of geography.

2) Advancement in sophisticated space technology (which can provide large volume of spatial data), along with declining costs of computer hardware and software (which can handle these data) has made Remote Sensing and G.I.S. affordable to not only complex environmental / spatial situation but also affordable to an increasingly wider audience.

REMOTE SENSING:

Meaning:

Literally Remote Sensing means obtaining information about an object, area or phenomenonwithout coming in direct contact with it.  If we go by this meaning of Remote Sensing, then a number of things would be coming under Remote  Sensor, e.g. Seismographs, fathometer etc.   Without coming in direct contact with the focus of earthquake, seismograph can measure the intensity of earthquake.  Likewise without coming in contact with the ocean floor, fathometer can measure its depth. However, modern Remote Sensing means acquiring information about earth’s land and water surfaces by using reflected or emitted electromagnetic energy.

 

From the following definitions, we can have a better understanding about Remote Sensing: According to White (1977), Remote Sensing includes all methods of obtaining pictures or other forms of electromagnetic records of Earth’s surface from a distance, and the treatment and processing of the picture data… Remote Sensing  then in the widest sense is concerned with detecting and recording electromagnetic radiation from the target areas in the field of view of the sensor instrument.  This radiation may have originated directly from separate components of the target area, it may be solar energy reflected  from them; or it may be reflections of energy transmitted to the target area from the sensor itself.

According to American Society of Photogrammetry, Remote Sensing imagery is acquired with a sensor other than (or in addition to) a conventional camera through which a scene is recorded, such as electronic scanning, using radiations outside the normal visual range of the film and camera- microwave, radar, thermal, infra-red, ultraviolet, as well as multispectral, special techniques are applied to process and interpret remote sensing imagery for the purpose of producing conventional maps, thematic maps, resource surveys, etc.

 

in the fields of agriculture, archaeology, forestry, geography, geology and others.    According to the United Nations (95 th  Plenary meeting, 3 rd  December, 1986), Remote Sensing means sensing of earth’s surface from space by making use of the properties of electromagnetic wave emitted, reflected or diffracted by the sensed objects, for the purpose of improving natural resource management, land use and the protection of the environment.

 

According to James B.Campell (1996), Remote Sensing is the practice of deriving information about the earth’s land and water surfaces using images acquired from an overhead perspective, using electromagnetic radiation  in one or more regions of the electromagnetic spectrum, reflected or emitted from the earth’s surface.

So the stages of Remote Sensing include :  

 

•  A source of electromagnetic radiation or EMR (Sun)
• Transmission of energy from the source to the surface of the earth, through  atmosphere                    
•  Interaction of EMR with earth’s surface.
• Transmission of energy from surface to Remote Sensor mounted on a platform, through atmosphere
•  Detection of energy by the sensor.
• Transmission pf sensor data to ground station  

•   Processing and analysis of the sensor data  
•    Final data output for various types of application

 

NON-PRECISION MEASURING INSTRUMENTS

CIVIL_ENGINEERING

Introduction:

Non-precision instruments are limited to the measurement of parts to a visible line graduation on the instrument used. There are several non-precision measuring devices. They are used where high measurement accuracy is not required. This section describes some of the non-precision measuring devices.

Steel Rule:

It is the simplest and most common measuring instruments in inspection. The principle behind steel rule is of comparing an unknown length to the one previously calibrated. The rule must be graduated uniformly throughout its length. Rules are made in 150, 300 500 and 1000 mm length. There are rules that have got some attachment and special features with them to make their use more versatile. They may be made in folded form so that they can be kept in pockets. The degree of accuracy when measurements are made by a steel rule depends upon the quality of the rule, and the skill of the user in estimating part of a millimeter.

Calipers:

Calipers are used for measurement of the parts, which cannot be measured directly withthe scale. Thus, they are accessories to scales. The calipers consist of two legs hinged at top, and the ends of legs span part to be inspected. This span is maintained and transferred to the scale. Calipers are of two types : spring type and firm joint type.  

Spring Type:
As the name explains, the two legs are attached with spring in this type of calipers. The working ends of each leg of a spring calipers should be identical in shape and have contact points equally distant from the fulcrum. The cross-section of the legs is either rectangular or circular in shape. The calipers are adjusted to set dimensions by means of either a knurled solid nut or a knurled quick action release nut operating in a finely threaded adjusting screw. The top portion of the legs are located in a flanged fulcrum roller and held in position by a spring in order to maintain the alignment of the working ends. The spring provides sufficient tension to hold the legs rigid at all points of the adjustment. A separate washer under the nut minimizes the friction between the adjusting nut and the leg.

Spring type calipers are of following types :

Outside Spring Calipers
These are designed to measure outside dimensions. The accuracy in caliper measurement depends upon the inspectors’ sense of feel. The legs are held firmly against the end of the proper dimensions by adjusting nut with the thumb and forefinger. For accurate settings, the distance between the outside calipers may be set by slip gauges or by micrometer anvils. Figure shows the diagrams of Outside spring calipers. A steel rule must be used in conjunction with them if a direct reading is desired.

Outside Spring Caliper

 

Inside Spring Calipers:
They are designed to measure the inside dimensions. An inside spring caliper is exactly similar to an outside caliper with its legs bent outward as shown in Figure . Adjustment in them is generally made by knurled solid nut. They are used for comparing or measuring hole diameters, distances between shoulders, or other parallel surfaces of any inside dimensions. To obtain a specific reading, steel scale must be used as with the outside calipers.

Inside Spring Caliper

USE OF A PLANIMETER

CIVIL_ENGINEERING
A planimeter will give correct results for “any” scale factor whatsoever, at least within the tolerance imposed by the operator and a small mechanical uncertainty.

 

In practice, an operator will trace an area on a plan with a known scale factor, and then multiply the “raw” planimeter reading by a constant, Ca, to get a corrected reading.

 

The constant Ca can be computed using an equation given in the instructions that come with each instrument. It is Ca = u*Sc^2 . “u” is the number of square inches per planimeter count, unique for each instrument and dependent on the arm length of that instrument. It is provided with each instrument shipped. Sc is the scale factor of the drawing to be measured.

 

For example, suppose an Operator is using a planimeter with its arm length set to medium length. He or she should refer to the calibration record that comes with the instrument and find that u is 0.017324 for a medium arm. The operator sees that the scale factor for his or her drawing is 1:400, meaning 1 unit = 400 units, where a unit can be an inch, a foot, or whatever. For this example the operator wants his answer to be in square feet so he must convert his 1:400 scale to the number of inches per foot. Since 1inch = 400 inches, it also equals 400 divided 12, or 33 1/3 feet. The hard part over, he or she uses the equation and finds that Ca equals 0.017324*(33 1/3)^2 , or 19.249.

 

If the operator has a digital readout, he simply keys in his scale factor of 19.249 and begins measuring. His answer will be in terms of square feet for each measurement. If the Operator has a mechanical planimeter, he or she must manually multiply each planimeter reading by

19.249 to get the number of square feet. If he or she stores the 19.249 in a calculator memory, this task becomes easier.

 

Some planimeters also give the value of u for metric measurements, that is, u is the number of square centimeters per planimeter count. In this case, the operator might have to convert the scale factor to give the number of centimeters per meter or whatever before computing Ca.

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Construction:

 

There are several kinds of planimeters, but all operate in a similar way. The precise way in which they are constructed varies, with the main types of mechanical planimeter being polar, linear and Prytz or “hatchet” planimeters. The Swiss mathematician Jakob Amsler-Laffon built the first modern planimeter in 1854, the concept having been pioneered by Johann Martin Hermann in 1814. Many developments followed Amsler’s famous planimeter, including electronic versions.

A linear planimeter on scrolls for the determination of stretched shapes:

They consist of a linkage with a pointer on one end, used to trace around the boundary of the shape. The other end of the linkage is fixed for a polar planimeter and restricted to a line for a linear planimeter. Tracing around the perimeter of a surface induces a movement in another part of the instrument and a reading of this is used to establish the area of the shape. The planimeter contains a measuring wheel that rolls along the drawing as the operator traces the contour. When the planimeter’s measuring wheel moves perpendicular to its axis, it rolls, and this movement is recorded. When the measuring wheel moves parallel to its axis, the

wheel skids without rolling, so this movement is ignored. That means the planimeter measures the distance that its measuring wheel travels, projected perpendicularly to the measuring wheel’s axis of rotation. The area of the shape is proportional to the number of turns through which the measuring wheel rotates when the planimeter is traced along the complete perimeter of the shape.

Developments of the planimeter can establish the position of the first moment of area (center of mass), and even the second moment of area.

 

The pictures show a linear and a polar planimeter. The pointer M at one end of the planimeter follows the contour C of the surface S to be measured. For the linear planimeter the movement of the “elbow” E is restricted to the y-axis. For the polar planimeter the “elbow” is connected to an arm with fixed other endpoint O. Connected to the arm ME is the measuring wheel with its axis of rotation parallel to ME. A movement of the arm ME can be decomposed into a movement perpendicular to ME, causing the wheel to rotate, and a movement parallel to ME, causing the wheel to skid, with no contribution to its reading.

CALCULATIONS OF BEARINGS AND INCLINED ANGLES

CIVIL_ENGINEERING
Calculation of Included Angles:

Having conducted the compass survey as described in Section , next step in plotting the survey results on maps is to calculate the included angle between two consecutive survey lines of the traverse.

(a) If the whole circle bearings of two lines at a station where these lines intersect are recorded, then the included angle between these lines 50 would be equal to the difference between the whole circle bearings of two lines. If the difference is less than 180o  the included angle would be interior angle and if it is more that 180o it will be the exterior angle between the two lines forming the traverse .
In Figure , it is given that back bearing (BB) of line AB, i.e. (α) = 240 o and fore bearing (FB) of line BC, (β) = 120^o . Then the included angle ABC, θ = α – β = 240 o  – 120 ^o  = 120 ^o

. Therefore, it can be said that if both the bearings are measured from a common point (B) then included angle can be obtained by subtracting FB of next line (BC) from the BB of previous line (AB).

Included Angle from WCB

(b) If the WCB at point of intersection of survey lines AB and BC (i.e. at station B) are not given but rather fore bearing of line AB (i.e. WCB of line AB at A) and back bearing of line BC (i.e. WCB of line BC at C) are known, then the included angle at station B between survey lines AB and BC (Figure ) can be obtained as follows.

 

 

 

 

WCB of AB at B = Back bearing of line AB at B = 150^o  180 ^o  = 330^o. Back bearing of line BC at C = 220 ^o .

WCB of BC at B = Fore bearing of line BC at B = 220o – 180^o = 40^o.  Compass Surveying
Included angle θ1 = 340^o – 40^o = 290^o = Exterior angle.
Hence, Interior angle θ = 360^o – θ1 = 360o – 290^o = 70^o.

ELECTRICAL MEASURING DEVICES

CIVIL_ENGINEERING

Electrical measuring devices give the most precise value of measurement among all the instruments discussed above. They use electrical transducers that transform a variety of physical quantities and phenomena into electrical signals. We will discuss some of the widely used electric devices in linear measurement in the following sections.

 

Strain Gauge:
The most widely used pressure and force sensitive transducer is the strain gauge. The principle of the strain gauge is based on the resistive properties of electrical conductors. Electrical conductor possesses resistance based on the relationship

where R is the resistance, P is the resistivity, L is the length and A is the area of cross-section. When a metal conductor is stretched or compressed, its resistance changes because of the fact that both length and diameter of the conductor change. These effects, called piezoresistive effect, can be used for measurement of several variables like strain and associated stress in experimental stress analysis, and small dimensional changes. Figure shows the influence of forces.

Metrology and Instrumentation:

 

 

At the top of the figure, the conductor is unstressed. At the bottom of the figure, the conductor is in tension, increasing its length and reducing its area. The resistance of the strain gauge changes in proportion to its change in dimensions.

 

The gauge factor, G, of a strain gauge is the ratio of relative change in resistance to the relative change in length.

There are two primary constructions used in making strain gauges : bonded and unbonded. These are shown in Figure 5.22. In the unbonded strain gauge, the wire
resistance element is stretched between two flexible supports. The wire stretches in accordance with the force applied to the diaphragm. The resistance of the wire changes due to these forces.

(a) Unbonded; and (b) Bonded

In a bonded strain gauge, a wire metal foil is placed in a thin metal diaphragm. When the diaphragm is flexed, the element deforms and change in resistance occurs. Generally, bonded strain gauge is more durable than unbonded.
There are three types of strain gauges :
(a) Metallic resistance strain gauge made of metallic wires such as constantan (Cu-Ni alloy) Nichrome V or Platinum alloy.

(b) Foil strain gauge consists of a thin, 8-to 15  nitro-cellulose impregnated paper on which photo etched metal alloy filaments are attached as resistance material. For higher temperature, an epoxy backing is used instead of paper. The active length of the gauge is along the transverse axis. The gauge should be mounted with its transverse axis in the same direction as the direction of application of force or strain. Thus, the elongation of the gauge
reduces the length and consequently the resistance.

(c) The third type is the semiconductor gauge. It depends on the piezoresistive properties of silicon and germanium. They have high sensitivities with gauge factor from 50 to 200. Their chief defects are fluctuations due to temperature and non-linear output. The p-type gauges increase resistance with applied tensile strain while n-type gauge resistance decreases. The gauge is generally bonded to the structure by epoxy adhesive or ceramic cement.