Surface Preparation and bonding techniques

Surface preparation and bonding techniques have been discussed under the following three topics namely:

1. Backing, base or carrier material.
2. Bonding material or cement.
3. Surface preparation and mounting of strain gauges.

Backing, Base or Carrier Material.

The purpose of providing the carrier/backing material ina strain gauge arrangement has been listed as follows;

1. The backing material provides support to the resistance wire (grid) of the strain gauge arrangement.
2. The backing material provides protection to the sensing resistance wire of the strain gauge arrangement. It also provides dimensional stability for the resistance wire of the strain gauge arrangement.

Characteristics Required for Backing Materials

1. The backing material should be an insulator of electricity.
2. The backing material should not absorb humidity, that is should be non-hygroscopic.
3. The backing material should be very thin.
4. It should go along with the adhesive material used to fix (bond) it on the structure under study.
5. It should not be affected by temperature changes.
6. It should be strong enough to transmit the force from the structure under study to the sensing resistance wire.

Bonding Materials or Cements (Adhesive)

The strain gauge has to be fixed (bonded) on the structure under study using an adhesive or paste. These adhesive are called as bonding material or cements.
The different adhesive, their composition and the temperature for which they can be used are shown in following table.

Adhesive, that is, Bonding Material       Composition                 For Temperature
Thermo-plastic cement                               Celluloid dissolved in acetone    Upto 75’C
Thermo-setting Cement                              Phenol resin             From 150’C to 210’C
Special Ceramic  – cement                                 -                                  Above 175’C

Characteristics Required

1. The characteristics required for a bonding material are listed.
2. The bonding material should be an insulator of electricity.
3. The bonding material should not absorb humidity, that is, it should be non-hygroscopic.
4. It should go along with the backing material so that the backing material is fixed (bonded) rigidly on the structure under study.
5. It should not be affected by temperature changes.
6. It should have good shear strength to transmit the force from the structure under study to the sensing resistive wire.
7. It should be easy to apply and should spread easily and should provide good bonding adhesion.
8. The bonding material should have a high creep resistance.

Surface preparation and mounting of strain gauges

The steps involved in preparing a surface to mount a strain gauge are listed:

1. The structure under study is made even and free from dust and dirt by rubbing with an emery sheet or by sand blasting.
2. The even surface is then cleaned by a volatile solution (acetone) using a cloth to remove oil/grease.
3. The bottom side of the backing (gauge carrier) is also cleaned by a solvent using a cloth.

Bonded Strain Gauges

These gauges are directly bonded (that is pasted) on the surface of the structure under study. Hence they are termed as bonded strain gauges. The three types of bonded strain gauges are

1. Fine wire strain gauge
2. Metal foil strain gauge
3. Semi-conductor gauge

Fine wire strain gauge

This is the first type of Bonded Strain Gauges.

Description

The arrangement consists of following parts,

A fine resistance wire diameter 0.025 mm which is bent again and again as shown in diagram. This is done to increase the length of the wire so that it permits a uniform distribution of stress. This resistance wire is placed between the two carrier bases (paper, Bakelite or Teflon) which are cemented to each other. The carrier base protects the gauge from damages. Leads are provided for electrically connecting the strain gauge to a measuring instrument (Wheatstone bridge).

Operation

With the help of an adhesive material, the strain gauge is pasted/bonded on the structure under study. Now the structure is subjected to a force (tensile or compressive). Due to the force, the structure will change the dimension. As the strain gauge is bonded to the structure, the stain gauge will also undergo change in both in length and cross-section (that is, it strained). This strain (change in dimension) changes the resistance of the strain gauge which can be measured using a wheat stone bridge. This change in resistance of the strain gauge becomes a measure of the extent to which the structure is strained and a measure of the applied force when calibrated.

Fine Wire strain gauge Materials

Material Composition
Nichrome Ni - 80% ; Cr – 20%
Constantan Ni – 45%; Cu – 55%
Nickel ----
Platinum ----
Isoelastic Ni – 36%; Cr – 8%; Mo – 0.5%

Advantages of Fine Wire Strain Gauge

The range of this gauge is +/- 0.3% of strain.
This gauge has a high accuracy.
Has a linearity of +/- 1%.

Limitation of Fine Wire strain gauge

• These gauges cannot be detached and used again (because the gauges are bonded to the structure).
• These gauges are costly.

Metal Foil Strain Gauge

Description of Metal Foil Strain Gauge

The arrangement consists of the following;

The metal foil of 0.02mm thick is produced using the printed circuit technique. This metal foil is produced on one side of the plastic backing. Leads are soldered to the metal foil for electrically connecting the strain gauge to a measuring instrument (wheat stone bridge).

Operations of Metal foil Strain gauge

With the help of an adhesive material, the strain gauge is pasted/bonded on the structure under study. Now the structure is subjected to a force (tensile or compressive). Due to the force, the structure will change the dimension. As the strain gauge is bonded to the structure, the stain gauge will also undergo change in both in length and cross-section (that is, it strained). This strain (change in dimension) changes the resistance of the strain gauge which can be measured using a wheat stone bridge. This change in resistance of the strain gauge becomes a measure of the extent to which the structure is strained and a measure of the applied force when calibrated. Same as Fine Wire strain gauge operation.

Advantages of Metal foil Strain gauge

• These strain gauges can be manufactured in any shape.
• Perfect bonding of the strain gauge is possible with structure under study.
• The backing can be peeled off and the metal foil with leads can be used directly on the structure under study. In such cases, a ceramic adhesive is to be used.
• These gauges have a better fatigue life.
• Has good sensitivity and have stability even at high temperatures.

Semi – conductor or Piezo Resistive Strain Gauge

Description of Piezo Resistive Strain Gauge.

The arrangement of a semi-conductor strain gauge is as follows:

The sensing element is rectangular filament made as a wafer from silicon or geranium crystals. To these crystals, boron is added to get some desired properties and this process is called doping and the crystals are called doped crystals. This sensing element is attched to a plastics or stainless steel backing. Leads made of gold are drawn out from the sensing element for electrically connecting the strain gauge to a measuring instrument (wheat stone bridge).

There are two types of sensing element namely:

• Negative or n-type (resistance decrease with respect to tensile strain).
• Positive or P-type ( resistance increase with respect to tensile strain).

Operation

With the help of an adhesive material, the strain gauge is pasted/bonded on the structure under study. Now the structure is subjected to a force (tensile or compressive). Due to the force, the structure will change the dimension. As the strain gauge is bonded to the structure, the stain gauge will also undergo change in both in length and cross-section (that is, it strained). When the sensing element (crystal) of the semiconductor strain gauge is strained, its resistivity changes contributing to a change in the resistance of the strain gauge. The change in the resistance of the strain gauge is measured using a wheat stone bridge. . This change in resistance of the strain gauge becomes a measure of the extent to which the structure is strained and a measure of the applied force when calibrated.

Advantages of semi-conductor Strain gauges

• These gauges have high gauge factor and hence they can measure very small strains.
• They can be manufactured to very small sizes.
• They have an accuracy of 2.3%
• They have excellent hysteresis characteristics.
• They have a good frequency of response.
• They have good fatigue life.

Limitation of semi-conductor Strain gauges

• These gauges are brittle and hence they cannot be used for measuring large strain.
• The gauge factor is not constant.
• These gauges have poor linearity.
• These gauges are very costly and are difficult to be bonded onto the structure under study.
• These gauges are sensitive to change in temperature.

Unbonded Strain Gauges

These strain gauges are not directly bonded (that is, pasted) onto the surface of the structure under study. Hence they are termed as unbounded strain gauges.


Description of the Unbonded Strain gauges:
     The arrangement of an unbonded strain gauges consists of the following. Two frames P and Q carrying rigidly fixed insulated pins as shown in diagram. these two frames can move relative with respect to each other and they are held together by a spring loaded mechanism. A fine wire resistance strain gauge is stretched around the insulated pins. The strain gauge is connected to a wheat stone bridge.

Operation of Unbonded strain gauges:

    When a force is applied on the structure under study (frames P & Q), frames P moves relative to frame Q, and due to this strain gauge will change in length and cross section. That is, the strain gauge is strained. This strain changes the resistance of the strain gauge and this change in resistance of the strain gauge is measured using a wheat stone bridge. This change in resistance when calibrated becomes a measure of the applied force and change in dimensions of the structure under study.

Application of Unbonded strain gauge:

     Unbonded strain gauge is usedin places where the gauge is to be detached and used again and again.
unbonded strain gauges are used in force, pressure and acceleration measurement.

Advantages of Unbonded strain gauge:

• The range of this gauge is +/- 0.15% strain.
• This gauge has a very high accuracy.

Limitation of unbonded strain gauges

It occupies more space.

Load cell and Load cell Types

If need to read by post on Load Cell, if you are new to my site and read about strain gauge load cell to have a better idea about load cells

There are two types of Load Cells, they are

1. Hydraulic load cells
2. Pneumatic load cells

Hydraulic Load Cell

Basic Priniple of Hydraulic Load cell

When a force is applied on a liquid medium contained in a confined space, the pressure of the liquid increases. This increase in pressure of the liquid is proportional to the appilied force. Hence a measure of the increase in pressure of the liquid becomes a measure of the appilied force when calibrated.

Description of Hydraulic Load Cell

The main parts of a hydraulic load cell are as follows

A dirphragm
A piston with a loading platform (as shown in figure) placed on top of the diaphragm.
A liquid medium which is under a pre-loaded pressure is on the other side of the diaphragm.
A pressure gauge (bourdon tube type) connected to the liquid medium.

Operation of Hydraulic Load Cell

The force to be measured is applied to the piston.
The appilied force moves the piston downwards and deflects the diaphragm and this deflection of the diaphragm increases the pressure in the liquid medium (oil).
This increase in pressure of the liquid medium is proportional to the applied force. The increase in pressure is measured by the pressure gauge which is connected to the liquid meduim.
The pressure is calibrated in force units and hence the indication in the pressure gauge becomes a measure of the force applied on the piston.

Note about Hydraulic Load cell:

As the hydraulic load cell is sensitive to pressure changes, the load cell should be adjusted to zero setting before using it to measure force.
This hydraulic load cell have an accuracy of the order of 0.1 percent of its scale and can measure loads upto upto 2.5*10^5 Kgf
The resolution is about 0.02 prcent.

Pneumatic Load Cell

Basic Principle of Pneumatic Load Cell

If a force is applied to one side of a diaphragm and an air pressure is applied to the other side, some particular value of pressure will be necessary to exactly balance the force. This pressure is proportional to the applied force.

Description of pneumatic Load cell

The main parts of a pneumatic load cell are as follows:

A corrugated diaphragm with its top surface attached with arrangements to apply force.
An air supply regulator, nozzle and a pressure gauge arranged as shown in figure.
A flapper arranged above the nozzle as shown in figure.

Operation of Pneumatic Load cell

The force to be measured is applied to the top side of the diaphragm. Due to this force, the diaphragm deflects and causes the flapper to shut-off the nozzle opening.Now an air supply is provided at the bottom of the diaphragm. As the flapper closes the nozzle opening, a back pressure results underneath the diagram. This back presssure acts on the diaphragm producing an upward force. Air pressure is regulated until the diaphragm returns to the pre-loaded position which is indicated by air which comes out of the nozzle. At this stage, the corresponding pressure indicated by the pressure gauge becomes a measure of the appilied force when calibrated.

Note:

The pneumatic load cell can measure loads upto 2.5*10^3 Kgf.
The accuracy of this system is 0.5 percent of the full scale.

Measurement of Strain

Strain gauges are devices used to measure the dimensional changes of components under test. Strain gauges are used in a number of applications, some of them have been listed below:

1. Strain gauges are used in force measuring devices such as strain gauge load cell.
2. Strain gauge are used in measurement of vibration / acceleration such as strain gauge accelerometer.
3. Strain gauges along with diaphragm are used in the measurement of pressure.

Some important terms have been explained below:

Strain

Strain is the relative change in dimensions, that is, change in length of given original length.

Strain = change in length/original length = mm/mm (dimensionless).

Strain Gauges 

When a metallic conductor is stretched or compressed, its resistance changes due to a change in the length and diameter (cross section) of the conductor.Hence a strain gauge is a measurement transducer used to measure strain (that is, relative changes in dimension). It is a transducer because it converts information about relative change in dimension to a change in resistance.

Positive Strain

When a strain gauge (metallic conductor) is subjected to tension, it is said to be positively strained. That is, when the strain gauge is subjected to positive strain (tension), its length increases and its area of cross section decreases. As the resistance of a conductor is proportional to its length and inversely proportional to its area of cross section, the resistance of the strain gauge increases with the positive strain.

Negative Strain.

When a strain gauge (metallic Conductor) is subjected to compression , it is said to negatively strained. that is, when the strain gauge is subjected to negative strain (compression), its length decreases and its area of cross section increases. As the resistance of the conductor is proportional to its length and inversely proportional to its area of cross section, the resistance of the strain gauge decreases with negative strain.

Piezoresistivity

There will be a change in resistivity of a conductor when it is strained and this property is called as piezoresistivity.

Poisson's Ratio

Poisson's Ratio = lateral strain, that is, the relative change in dimension in the cross section / Longitudinal strain, that is the relative change in dimension in the length.

                        = (dD/D)/(dL/L)

where, D=Diameter:    L=Length

Gauge Factor (Strain Sensitivity Factor)

The fractional change in resistance due to unit change in length (unit strain) is called as gauge factor.

Gauge Factor, F = (dR/R)/(dL/L)

Where, R = Resistance, L=Length

The magnitude of the strain gauge factor indicates the sensitivity of the strain gauge. the high gauge factor implies that there will be a large change in resistance for a given strain input.

Load cell - A Simple Device to Measure Weight

Load Cell

A strain gauge – elastic member combination arrangement used for the measurement of load or weight is called load cell. It utilizes an elastic member as primary transducer and strain gauge as a secondary transducer. A suitable plane area is placed on the eleastic member as a weight stand. Then, strain gauge is attached to the eleastic member.