1. Measurement of strainandtemperature
DEPARTMENT OF MECHANICAL ENGINEERING, BCE, SHRAVANABELAGOLA. 1
CHAPTER: 1
MEASUREMENT OF STRAIN AND TEPERATURE
INTRODUCTION
Temperature measurement is the most common and important measurements
in controlling any process. Temperature may be defined as an indication of intensity of
molecular kinetic energy within the system . It is difficult to define. Temperature can not
be measured using basic standards through direct comparison. It can only be determined
through some standardized calibrated device. Change in temperature of substance causes
variety of effects such as
i. Change in physical state,
ii. Change in chemical state,
iii. Change in physical dimensions,
iv. Change in electrical properties,
v. Change in radiating ability.
And any of these effects may be used to measure the temperature.
The change in physical and chemical states can not be used for direct temperature
measurement. However , temperature standards are based on changes in physical
state. A change in physical dimension due to temperature shift forms the bases of
operation for liquid in gas and bimetallic thermometers. Changes in electrical
properties such as change in electrical conductivity and thermoelectric effects which
produce electro motive force forms the basis for thermocouples . Another temperature
measuring method using the energy radiated from hot body forms the basis of
operation of optical, radiation and infrared pyro meters.
1.1. Theoryof Strain Gauge:
A strain gauge is a device used to measure strain on an object. As the object is
deformed, the foil is deformed, causing its electrical resistance to change. This resistance
change, usually measured using a Wheatstone bridge, is related to the strain by the
quantity known as the gauge factor.
The change in the value of resistance by straining the gauge may be partly
explained by the elastic behaviour of the material. It an elastic material is subjected to
tension or in other words positively strained, its longitudinal dimension will increase
while its lateral dimension decreases. And resistance of conductor given by

2. Measurement of strainandtemperature
DEPARTMENT OF MECHANICAL ENGINEERING, BCE, SHRAVANABELAGOLA. 2
R= ρ (L/A)
R = Resistance
ρ = Resistivity of the conductor material
L = Length of the electrical conductor
A = Cross sectional area of the conductor
The resistance of strain gauge increases because the resistance of conductor is
directly proportional to its length and inversely proportional to its cross sectional area.
With the positive strain, the increase in the value of resistance of a strained conductor is
more than that can be accounted for an increase in resistance due to
1. An increase in length
2. An increase in resistance due to reduction in cross sectional area.
1.2. Types of strain gauges:
Based on principle of working:
• Mechanical
• Electrical
• Piezoelectric
Based on mounting:
• Bonded strain gauge
• Unbounded strain gauge
Based on construction:
• Foil strain gauge
• Semiconductor strain gauge
• Photoelectric Strain gauge
1.3 Electricalstrain gauge:
If in the gauge, change in strain produces change in some electrical characteristic
such type of strain gauge is called as Electrical strain gauge.
Types of electrical strain gauge:
 Electrical capacitance gauges
 Electrical inductance gauges

3. Measurement of strainandtemperature
DEPARTMENT OF MECHANICAL ENGINEERING, BCE, SHRAVANABELAGOLA. 3
 Piezoelectric gauges
 Electrical resistance gauges
1.4 Electricalresistancestraingauges:
Electrical resistance strain gauges are very widely used for strain measurement. Its
operation based on the principle that the electrical resistance of conductor changes when
it is subjected to mechanical deformation. Typically, an electrical conductor is bonded to
specimen with insulating cement under no load conditions. A load is then applied, which
produces a deformation in both measurement of a change in resistance of an element.
R= ρ (L/A)
R = Resistance
ρ = Resistivity of the conductor material
L = Length of the electrical conductor
A = Cross sectional area of the conductor
In other words when the conductor is stretched, its length (L) will increase, cross
sectional area (A) will decrease and hence the resistance (R) will increase. Similarly,
when the conductor compressed resistance (R) will decrease.
1.4.1 Unbounded resistance strain gauges:
In unbounded type of strain gauge, the electrical resistance element (Grid) is
unsupported. A fine wire is stretched out between two or more points which may forms
shown in figure in below. Moments of the point A and B away from each other causes a
tensile strain in the resistance wire, and the change in its resistance is measured by a
suitable circuit, to give signal which is a function of strain.
Fig 1.1: Unbounded resistance strain gauges.

4. Measurement of strainandtemperature
DEPARTMENT OF MECHANICAL ENGINEERING, BCE, SHRAVANABELAGOLA. 4
Generally four separate resistance wires are connected electrically to form a
Wheatstone bridge and arranged mechanically so that the wires in adjacent bridge arms
are subjected to strain of opposite sign as shown in fig. and the resistance wires are
wrapped around electrically insulated pins. When movable platform is moved relative to
fixed base, tensions in the wires are either increase or decrease and the corresponding
resistance changes can be calibrated in term of strain. The assembly must provide a built
in the grids greater than the maximum compressive strain to be sensed.
1.4.2 Bonded resistance strain gauge:
A bonded resistance strain is suitably shaped piece of resistance metal which is
bounded close to the surface whose strain as to be measured. The exploded thin wire
shaped into a grid pattern, which cemented between thin sheets of insulating materials
such as a paper or plastic. The grid can also be made from thin metal foil. The assembled
gauge bonded top the surface with a thin layer of adhesive and finally waterproofed with
a layer of wax. The grid experiences the same strain as material to which it is bonded.
The gauge is most sensitive to the strain along the axial direction X-X while the
strain in the Y-Y direction occurs due to poisons ratio effect. This causes a change of
resistance, and may lead to an error of 2% in the measured strain when using the bonded
gauge. However, in the foil gauge the thickened ends reduced this cross-sensitivity effect
to virtually zero. If direction of principal strain is not known then the cluster of three or
more strain gauges are used which are called rosettes.
There are 2 types in bonded resistance strain-gauge
a) Wire type
b) Foil type resistance strain gauge

5. Measurement of strainandtemperature
DEPARTMENT OF MECHANICAL ENGINEERING, BCE, SHRAVANABELAGOLA. 5
Fig 1.2: Bonded resistance strain gauge
a) Wire type:-
It consist of a very fine wire of diameter 0.25mm and the grid is cemented
between 2 pieces of thin paper with plastic or ceramic backing. This done in order that
the grid of wire may be handled. This is securely bonded with a suitable cement to the
surface of the member in which strain has to be measured.
Fig 1.3: Wire type
b) Foil type resistance strain gauges:-
These gauges usually employ a foil less than 0.005mm thick. The
common form of foil type gauge consists of a metal foil grid element on a thin epoxy

6. Measurement of strainandtemperature
DEPARTMENT OF MECHANICAL ENGINEERING, BCE, SHRAVANABELAGOLA. 6
support. Epoxy filled with fiber glass is used for high temperatures these foil type gauges
are manufactured by printing on a thin sheet of metal alloy with an acid-resistant ink, and
then the unprinted portion is etched away, foil gauges have the advantages of improved
hysteresis, better fatigue life and lateral strain sensitivity. It is thinner and more flexible,
thus permitting it to be applied to fillets and sharply curved surfaces. The common wire
or foil gauges are called metallic gauges.
Fig 1.4: Foil type resistance strain gauge
1.4.3. Semiconductor or piezo resistive strain gauge:
Semiconductor gauges are cut from single crystals of silicon or
germanium in which are combined exact amount of impurities such as boron which
impart certain desirable characteristics. The same type of backing, bonding materials, and
mounting techniques as those used for metallic gauges can be used for semiconductor
gauges. When the gauge is bonded to a member which is strained, causes changes of
current in the semiconductor material.
The advantages of semiconductor gauges are their high strain sensitivity which
allows very small strains to be measured accurately. A gauge whose electrical resistance
increases in response to tensile strain is known as positive or p-type semiconductor
gauge. On the other hand when the resistance decreases in response to tensile strain then
it is known as Negative or type N-type semiconductor gauge.

7. Measurement of strainandtemperature
DEPARTMENT OF MECHANICAL ENGINEERING, BCE, SHRAVANABELAGOLA. 7
Fig 1.5: Piezo resistive strain gauge
The semiconductor gauge consists of a rectangular filament of about 0.05mm
thick by 0.2mm wide and gauge length varies from 1.5 to 12mm as shown in fig. 8.21).
They are made as thin as possible because as the breaking stress of the material raises the
cross sectional area deceases. And also the gauge can be bent to much smaller radius of
curvature without fracture. In addition these gauges have very high temperature
coefficient of resistance. The disadvantages of semiconductor gauges are:
1) The output of the gauge is nonlinear with strain.
2) Strain- sensitivity is dependent on temperature.
3) It is more fragile than the corresponding wire or foil element.
4) More expensive then the ordinary metallic gauges.
1.5. Preparationand mounting of strain gauge:
Following steps should be followed for proper mounting of strain gauges.
 The surface on which strain gauge has to be mounted must be properly
cleaned by an emery cloth and bare base material must be exposed
 Various traces of grease and oil must be removed by using solvent like
acetone
 The surface of the strain gauge coming in contact with test item should also
be free from grease etc.…
 Sufficient quantity of cement is applied to the cleaned surface and the
cleaned gauge is simply placed on it. Care should be taken to see that there
should not be any air bubble in between the gauge and surface. The
pressure applied should not be heavy so that the cement may puncture the
paper and short the grid.

8. Measurement of strainandtemperature
DEPARTMENT OF MECHANICAL ENGINEERING, BCE, SHRAVANABELAGOLA. 8
 The gauges are then allowed to set for at least 8 or 10 hours before using it.
If possible a slight weight may be placed by keeping a sponge or rubber on
the gauge.
 After the cement is fully cured, the electrical continuity of the grid must be
checked by ohm meter and the electrical leads may be welded.
1.6. Gauge factor:
The electrical resistance of a metallic wire conductor varies directly with respect
to it resistivity and length while inversely to its cross sectional area i,e.,
R= ρ (L/A)
The metallic crystal lattice forms a regular atomic pattern and particular form of
bounding between the atoms-metallic bounding-involves mutual shearing of all the
valance electrons by all the atoms in the metal. Thus, current passes along a conductor in
the form of directional motion of the electrons, called electron flow. The increased length
and decreased area of the gauge under tensile strain accounts for parts of the increase in
resistance as the metallic lattice will suffer distortion but these changes alone do not fully
account for the total resistance change, thus other changes in the metal lattice must also
occur bringing about a change in the resistivity of the metal. This latter effect is an
important consideration as it is well known that the resistivity of metals also changes with
temperature.
GF= (dR/R)/(dL/L)
Where R is the nominal resistance of the gauge
The gauge factor is supplied by the manufacturer and may range from 1.7 to 4
depending length of the gauge. Thus, provided a means is available for measuring
resistance, strain can be determined and hence stress. As increase in gauge factor increase
in sensitivity, but it is limited in metal gauges, largely because of the relatively low
resistivity of metals, a limitation which have gauge factor of the order of +_ 100 or more.

9. Measurement of strainandtemperature
DEPARTMENT OF MECHANICAL ENGINEERING, BCE, SHRAVANABELAGOLA. 9
REFERENCES
Text book
[1]. Dr. T. Chandrashekar (2011). “Mechanicalmeasurementsand
metrology” Lakshmi mudranalaya , 275-282.