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Bonded and Unbonded Strain Gauge: Structure, Principle, Types, And Attributes

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What is strain gauge?

The strain gauge is a type of passive transducer, on which the resistance changes with the change of applied pressure (Strain). A strain gauge is basically a thin wire of 25µm diameter, on which stress (strain) is applied to change its length and cross-sectional area, thereby changing its resistance.

When a positive strain is applied to the strain gauge, the length of the strain gauge increases and its cross-section decreases.

Since the resistance of a conductor is proportional to its length and inversely proportional to its cross-section, the resistance of the conductor increases when a positive strain is applied. The magnitude of the change in resistance is much greater than the change in size of the conductor due to the application of strain. The reason for this greater change is the result of changes in resistivity. This property is the piezoresistive effect.

Hold on,
Length of thin wire = L (m)

Area of cross section of wire = A ( m ^ 2 )

Weight resistivity (specific resistance) = ρ (Ω-m)

The resulting resistance, R=frac{rho L}{A} 

Types of strain gauges

The strain gauge can be divided into two parts based on its placement on the base board, viz-

  1. Bonded type strain gauge
  2. Unbonded type strain gauge

The bonded type strain gauge can be further divided into four parts, viz-

  1. Linear type strain gauge
  2. Torque type strain gauge
  3. Helical type strain gauge
  4. Rosette type strain gauge

In addition, the strain gauge can be divided into three parts, viz-

  1. Wire and wound type
  2. Foil type
  3. Semiconductor type

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Structure and Working Principle of Bonded type resistance Strain Gauge

Structure of Bonded type Strain Gauge

Below is the construction diagram of a bonded type train gauge. When a strain wire is mounted on the gauge in a bonded form by gum or adhesive, it is called a bonded type strain gauge. In this case, a piece of resistive material less than 25µm in diameter is gummed onto a base board or carrier. The base board is made of thin paper seat teplon material.

Structure and Working Principle of Bonded type resistance Strain Gauge​

After applying the strain, two connecting leads are connected at both ends of the wire to measure the changed resistance. The lead is made of two conductive materials and must be stretched so that it does not bend even when applied to it.

Working Principle of Bonded type Strain Gauge

It acts like a passive transducer. If it is used as the arm of a white stone bridge, it is possible to obtain an electrical output from the pressure force applied to its connecting lead. This is because applying pressure on it changes the length (L) and cross-sectional area (A) of the strain wire and changes the resistance. As a result, the bridge becomes unbalanced and produces an electrical output. Therefore, it can be used to convert pressure or ball into electrical signals and indirectly measure it.

Structure and Working Principle of Unbonded type resistance Strain Gauge

An unbonded metal strain gauge is shown in the figure below. In this a wire is drawn between two points in any non-conducting medium. The wire is made of copper, nickel, chrome nickel or nickel iron alloy. Wire diameter is typically 0.003 mm, gauge factor is 2 to 4 and can apply 2mN force. The length of the wire is 25mm or less.

Structure and Working Principle of Unbonded type resistance Strain Gauge

The fracture element is connected to the diaphragm via a rod, which acts as a pressure sensing element. When compressive force is applied, tensile force is applied to prevent the cable from getting stuck.

Unbonded metal wire gauges are used as arms of a Wheatstone bridge, as shown in Fig. In the initial state, the strain and resistance of the four arms are equal, so that the output voltage is zero. The wire has a maximum deflection of 0.004m when stress is applied. As a result the resistance of two arms increases and the resistance of the other two arms decreases. In this the bridge is unbalanced and an output voltage is obtained, which is proportional to the displacement resulting from the applied pressure. Resistance per arm 120Ω to 1000Ω, input voltage 5V to 10V and output voltage 20mV to 50mV

Attributes of resistance strain gauge

A resistance strain gauge should have the following properties to obtain accurate measurements:

  • Its resistance must be of high quality.
  • The frequency response of the strain gauge must be good.
  • The gauge factor of the strain gauge must be of high quality.
  • It will not have any hysteresis effect.
  • Lead material must have low resistivity and temperature coefficient.
  • Its temperature coefficient must be of low quality.
  • The change in resistance with strain must be linear.

In order to obtain the above properties, strain gauge wire cannot be made of a single material, so it has to be made as an alloy of several materials.

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