This document discusses strain gauges used in industrial instrumentation. It describes the different types of strain gauges including electrical, mechanical, and optical gauges. Within electrical gauges, it focuses on wire, foil, semiconductor, and thin film gauges. It explains the theory of operation of resistance strain gauges and factors that affect strain gauges such as temperature, creep, linearity, and bonding techniques. Temperature compensation methods are also summarized.

1. Industrial Instrumentation
Chapter 2
Strain Gauges

2. Strain gauges
• If a metal conductor is stretched or compressed, its
resistance change on account of the fact that both
length and diameter of the conductor change.
• Also, there is a change in resistivity of the conductor.
• When it is strained and this property is called
piezoresistive effect.

3. Theory of operation of resistance strain
gauges
• If a strip of elastic
material is subjected to
tension (in other words
positively strained) its
longitude dimension
will increase, while
there will reduction in
the lateral dimension.

4. • Let us consider a strain gauge made of circular wire
the wire has a dimension: length L, Area = A,
diameter =D, before being strained.
• Resistance of unstrained gauge,
• Let us tensile stress s be applied to the wire. This
produce positive strain causing length to increase and
area to decrease.

5. • Let, = change in length, = change in area, =
change in diameter and = change in resistance.
• In order to find how depends upon the material
physical quantities, the expression for R is
differentiated with respect to stress s,
• Thus we get,
• …..(A)

6. • Dividing equation (a) by
….(b)
• It is evident from b,
• …(c)

7. Equation b can be written as,
….(d)
• ..(e)

8. • For small variations, the above relationship
can be written as :
• The gauge factor is defined as ratio of per unit
change in resistance to per unit change in
length..

9. • The gauge factor can be written as,

10. • If the change in the value of resistivity of a
material when strained is neglected, the gauge
factor is,

11. Introduction
• In the design and construction of machines and
construction the strength and of the material plays a
very important role.
• The theatrical knowledge of this property is essential
to estimate whether the mechanical components can
carry the loads demanded of them, without excessive
deformation or failure.

12. • These load carrying abilities are normally
characterized in terms of stress, which defined as the
force experienced per area.
• The mechanical deformation formed due to stress is
measured with strain gauge elements.

13. Deformation of structure

14. Factors affecting the strain gauge
• By measuring displacement corresponding to ∆X in
each direction and dividing it by original length X,
the strain can be determined.
• The strain are likely to vary point to point.
• Direct measurement of such small displacements over
the entire surface of the body is also difficult.

15. • This is overcome by measuring one displacement
component over a small portion of the body, along a
short line segment.
• It is better to reduce the size of the strain gauge to
improve accuracy of the measurements, but a size is
very much reduced, dimensional tolerance becomes
very critical.
• The basic characteristics of a strain gauge that one
looks for the gauge length gauge width, gauge
sensitivity, range of instruments, accuracy, frequency
response and ambient environment condition it can be
withstand.

16. Types of strain gauges
• Strain gauges can be classified as mechanical, optical
or electrical depending upon the principle of
operation and their constructional features.
• Out of these electrical strain gauge is most common.

17. Types of strain gauges
• Mechanical gauges:
In mechanicals gauges,
the change in ∆l is
magnified mechanically
using levers or gears.
• Example: Bourdon tube
Bourdon Tube

18. • Mechanical strain gauges are comparably large in size
and as such are suitable only in case where sufficient
area is available on the test specimen.
Optical gauges:
Optical strain gauges are very similar to mechanical
strain gauges except that the magnification is
achieved with multiple reflectors using mirrors or
prisms..
Example: Optical Pyrometer

20. Electrical strain gauge
• The principle of an electrical strain gauge is based upon
the measurement of the change in resistance, capacitance
or inductance.
• The most versatile device for experimental determination
of strain for the purpose of stress analysis is the bonded
resistance type of strain gauge.
• The basic concept is that the resistance of the iron or
copper wire is changed when it is subjected to tension.

21. • Another class of strain gauge which is of a recent
origin is the semiconductor type, piezoresistive strain
gauge. This gauge has the advantages of the high
sensitivity, small size and measure for both static and
dynamic measurement.

22. Types of electrical strain gauges
• Single length of wire can be used as sensing element
in a strain gauge.
• The gauges are classified into number of categories
depending upon the method of fabrication.
• But, the two major types are the wire Gauges and foil
gauges. Two other types which are of very recent
origin are the semiconductor and thin film gauges.

23. Wire gauges
• Bonded and unbounded type..
(depending upon the method of fabrication)
• In bonded type the strain gauges bonded directly to
the surface of the specimen being tested with a thin
layer of adhesive cement which serve to transmit the
strain from the specimen to the gauge wires and at the
same time serve as an electrical insulator.

24. • Wire gauges are fabricated into four basics verities.
• Flat grid, wrap around, single wire and woven..

25. Flat grid type
• In this type the wire is wound in flat grid type.
• This grid structure is bonded to a backing material
such as paper or epoxy with an adhesive that can hold
the wire element to the base firmly.

27. Wrap around type
• This type of gauge is
wound on a flattened
tube of paper or
alternatively on a thin
strip or a card.

28. Single wire gauge
• Single wire is stretched.
• Instead of loop formed
by the same wires, thick
copper wire are welded
at the end to reduce the
cross sensitivity.
• This gauge is not very
much popular.

29. Woven type
• It is used for
measurement of large
strain.
• A silk insulated eureka
wire is wound as the weft
on a rayon wrap to form
a woven type gauges.
• Which are useful for tests
on fabrics and leather.

30. Unbonded strain gauges
• A Unbonded strain gauge device is a free filament
sensing element where strain is transferred to the
element without any backing.
• Platinum tungsten alloy is wound between insulated
pins. One of them is attached to a stationary frame
and other is attached to a movable frame. so that the
winding experiences an increase and decrease of
stress for a given input.

33. Foil gauge
• It is a basically a extension of a wire strain gauge.
• In foil gauges the required grid pattern with a very
thin foil of the same material as that used for wire
gauges.
• On account of higher surface area to area of cross
section ratio, it has a higher heat dissipation
capability hence better thermal stability.

35. Semiconductor strain gauges
• Semiconductor strain gauges employ the
piezoresistive property of doped silicon and
germanium.
• A unique feature of the device is that the change in
resistance due to strain is 40 to 100 times more than
that of the conventional metal alloy types.
• Other advantages are the freedom of hysteresis and
creep, good fatigue life, and low cross sensitivity.

36. • The semiconductor gauge exhibits the a strong
piezoresistive effect, resulting in a high gauge factor
of the 100 to 140.
• The common material for general purpose gauges is
the p type silicon doped with a resistivity of 2*10-4
ohm.m.

38. Creep
• Creep is the tendency of a solid material to move
slowly or deform permanently under the influence of
mechanical stresses.

39. Bridge Measurement Circuit

40. Temperature Compensation using Dummy
Gauge
• By using a “dummy” strain gauge in place of R2, so
that both elements of the rheostat arm will change
resistance in the same proportion when temperature
changes, thus canceling the effects of temperature
change.
• Resistors R1 and R3 are of equal resistance value, and
the strain gauges are identical to one another. With no
applied force, the bridge should be in a perfectly
balanced condition and the voltmeter should register
0 volts.

41. • Both gauges are bonded to the same test specimen,
but only one is placed in a position and orientation so
as to be exposed to physical strain (the active gauge).
• The other gauge is isolated from all mechanical
stress, and acts merely as a temperature compensation
device (the “dummy” gauge).
• If the temperature changes, both gauge resistances
will change by the same percentage, and the bridge’s
state of balance will remain unaffected.

43. Thank You
43

45. (b) Linearity
• The sensitivity of the gauge is depend strongly on
strain level, thereby exhibiting non-linearity in
resistance variation with strain…
• The relative resistance change at constant temperature
is given by,

47. (c) Temperature effect
• Environmental temperature changes affects the
resistance and sensitivity of the gauge depending
upon the kind and level of doping of the material.
• The slope of this variation depends very much upon
the resistivity of the material.

48. (d) Temperature compensation
• Temperature compensation for resistance variations is
carried out either by self compensation or circuit
compensation.
• Self compensation is obtained by using n-type gauges
which have positive temperature coefficient of
resistivity.
• Circuit compensation is accomplished by the use of
different dummy gauges, p-n combination and
temperature sensitive resistors.

49. (e) Current carrying capacity
• The maximum permissible current in a bonded
semiconductor strain gauge is controlled by its heats
dissipation capacity and the thermal conduction to the
test specimen.

50. Wire Resistance Material
• Made of Advance and
constantan alloy
High resistivity
Constant gauge factor
over a wide range
Good stability

51. Backing Material
• Stress sensitive grid structure attach
• Provide Protection to the gauge
• Choice based on the temperature ranges

52. Gauge Bonding Cement
• Adhesive used to fix onto the specimen
• Transmitting the strain to the specimen
• Improper bonding may cause error
Hysteresis
Creep
Low insulation resistance e.t.c

53. Gauging Techniques and other Factors
• Installation
Gauge is bond correctly on the specimen
Chosen the cement for bonding the gauge
Surface is clean with solvent
Dilute acid also be helpful

54. Temperature Effect
• Change in resistance due to temperature
• Material go on expansion
• Proper selection of gauge material according to their
application

55. Humidity and moisture effect
• By absorb the moisture it changes the volume of the
material.
• Elongate or compress the material
• By bonding gauge in dry condition or apply heat to
the surface when bonding

56. Properties of the semiconductor gauge
Gauge sensitivity
• The variation of the gauge factor of p and n type as a
function of resistivity and crystal orientation.
• The sensitivity of a semiconductor gauges at low strain
limits at room temperature is computed from the
equation,

57. Application