TYPES OF STRAIN GAUGES?

What is strain?

Strain is defined as the ratio of the change in length of a material to the original, unaffected length.

 How do you measure strain?

•  strain Gage's electrical resistance varies in proportion to the amount of strain in the device. 
• The most widely used strain Gage is the bonded metallic strain Gage. The metallic strain Gage consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction. 
• the strain experienced by the test specimen is transferred directly to the strain Gage, which responds with a linear change in electrical resistance.

• A fundamental parameter of the strain Gage is its sensitivity to strain, expressed quantitatively as the Gage factor (GF). GF is the ratio of the fractional change in electrical resistance to the fractional change in length, or strain:

Semiconductor Strain Gauges

. Semiconductor strain gauges exhibit following key features:

• Unlike other strain gauges, semiconductor strain gages are based upon the piezoresistive effects of silicon or germanium and measure the change in resistance with stress as opposed to strain.
• The semiconductor bonded strain Gage is a wafer with the resistance element diffused into a substrate of silicon.
• No backing is provided for the wafer element and bonding it to the strained surface needs extra care since only a thin layer of epoxy is used to attach it.
• Size of a semiconductor strain gauge is much smaller and the cost much lower than for a metallic foil sensor.
• Advantages include higher unit resistance and sensitivity.
• Disadvantages: Greater sensitivity to temperature variations and tendency to drift as compared to metallic foil sensors. Also the resistance-to-strain relationship is nonlinear, varying 10-20% from a straight-line equation. However, by means of computer-controlled instrumentation, these limitations can be overcome via software compensation.

Thin-film Strain Gauges

Thin-film strain Gage is more advanced form of strain gauge as it doesn’t necessitate adhesive bonding. A thin film strain gauge is constructed by first depositing an electrical insulation, usually a ceramic onto the stressed metal surface, and then depositing the strain Gage onto this insulation layer. Techniques used to bond the materials molecular are:

• Vacuum deposition
• Sputtering method

Advantages

1. Since the thin-film gage is molecularly bonded to the specimen, the installation is very stable and the resistance values experience less drift.
2. The stressed force detector can be a metallic diaphragm or beam with a deposited layer of ceramic insulation

Diffused Semiconductor Strain Gauges

A further improvement in strain gage technology was introduced with the advent of diffused semiconductor strain gages since they purge the need for bonding agents. Main features are listed below:

1. By eliminating bonding agents, errors due to creep and hysteresis also are eliminated.
2. The diffused semiconductor strain gage employs photolithography masking techniques and solid-state diffusion of boron to molecularly bond the resistance elements.
3. Diffused semiconductors are frequently used as sensing elements in pressure transducers.
4. Limitations include sensitivity to ambient temperature variations, which can be compensated by intelligent transmitter designs.

Advantages

• Small size
• Inexpensive
• Accurate and repeatable
• Available wide pressure range
• Generate a strong output signal

Bonded Resistance Gauges

Following are the chief characteristics of bonded resistance strain gauges:

• They are reasonably inexpensive.
• They can pull off overall accuracy of better than +/-0.10%.
• They are available in a short gauge length and have small physical size.
• These strain gauges are only moderately affected by temperature changes.
• They are extremely sensitive and have low mass.
• Bonded resistance strain gages can be employed to measure both static and dynamic strain.
• These types of strain gauges are appropriate for a wide variety of environmental conditions. They can measure strain in jet engine turbines operating at very high temperatures and in cryogenic fluid applications at temperatures as low as -452*F (-269*C).

Selection of a Proper Gauge 
Three primary considerations in strain gauge selection are mentioned below:

1. Operating temperature
2. Nature of the strain to be detected
3. Stability requirements