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What is a Strain Gage?

Strain gages are fundamental sensing devices that function as the building blocks of many other types of transducers—including pressure, load, and torque sensors—used extensively in structural test and monitoring applications. An example of such a transducer is a load cell that converts a mechanical force to an electrical output signal.

 

In these designs, gages are connected as a Wheatstone bridge, resulting in an accurate and rugged transducer that can operate in extreme environments. To achieve accuracy, the Wheatstone bridge is adjusted for manufacturing initial tolerance and ambient and self-heating temperature effects. "Compensation" high precision resistors are then added to correct for bridge unbalance, and to adjust the output sensitivity. Other compensation resistors correct for the errors that result when the transducer is used over a widely changing temperature range.

The True Pioneer of Strain Gage Technology

While there are several ways of measuring strain, the most common one is using a bonded resistance strain gage, a device whose electrical resistance varies on proportion to the amount of strain in the device.

 

Today, the most widely used strain gage is the Advanced Sensors Technology bonded resistance strain gage. Micro-Measurements uses a precisely manufactured (in house) metallic foil to produce to resistive element providing for the best consistency and gage to gage matching available.  The metallic strain gage consist of metallic foil arranged in a grid pattern. The grid pattern maximizes the amount of metallic foil subject to strain in parallel directions.

The Advancement of the Strain Gage

The discovery of the principle upon which the foil resistance strain gage is based was made in 1856 by Lord Kelvin, who loaded iron and copper wires in tension and noted their resistance changes while applying strain in the wire. In his classical experiment Lord Kelvin introduced three important fundamentals which help to develop the foil strain gage:

The resistance of the wire changes as a function of the applied strain, Each material has a different sensitivity, using a Wheatstone bridge is vital to the measurement accuracy of the resistance change. Strain gages are resistive sensors whose resistance is a function of applied strain or force (unit deformation). 

 

Stress is calculated from the strain information. Typically a strain gage is attached to a structure and when such a structure is deformed (tension, compression, shear), the resistive strands in the strain gage follow the structure deformation which causes an electrical resistance change. The resistance change is then expressed in units of strain or stress. Strain gages are used in many types of transducers including pressure, load, and torque sensors—used extensively in structural test and monitoring applications. An example of such a transducer is a load cell that converts a mechanical force to an electrical output signal. In these designs, gages are connected as a Wheatstone bridge, resulting in an accurate and rugged transducer that can operate in extreme environments. To achieve accuracy, the Wheatstone bridge is adjusted for manufacturing initial tolerance and ambient and self-heating temperature effects. "Compensation" high precision resistors are then added to correct for bridge unbalance, and to adjust the output sensitivity. Other compensation resistors correct for the errors that result when the transducer is used over a widely changing temperature range.

 

Strain gages are also used for measuring strain in structures (stress analysis) such as spacecraft, airplanes, cars, machines, bridges and other structures. Performance specifications to consider when searching for strain gages include operating temperature, the state of the strain (including gradient, direction, magnitude, and time dependence), and the stability required by the application.

The construction of resistance strain gage involves bringing together the best electrical resistance material and backing in the optimal way from manufacturing, application and performance point of view.

 

What we expect from a  bonded resistance strain gage?

  •         Small size and mass.
  •         Agile development – ease of manufacturing in different resistance values, overall sizes and measurement configurations.
  •         Durable, with ease of handling and use.
  •         Excellent stability, repeatability and linearity over wide strain range.
  •         Practical sensitivity to strain
  •         Ability to control, effects of environmental variables in the measurement system such as temperature.
  •         Applicable for dynamic and static measurements and remote recording.

 

Stress Analysis 

The foil resistance strain gage sensor is the most frequently used sensor in stress analysis measurements through the world today.

Many factors, such as test duration, strain range required, and operating temperature, must be considered in selecting the best strain gage/adhesive combination for a given test profile.

 

Transducers

Since transducer behavior can be observed to a resolution better than one part in 20K, the advanced sensors technology strain gage system must be selected and installed with utmost care.

In contrast to the situation for stress analysis applications, the strain gages installed on a transducer can readily be calibrated against known physical standards – dead weights, for example , or previously calibrated transducer. The existence of precise standards and sensitive electronic instrumentation allow the constructor of a transducer to quantify its performance to a very high degree. It is possible, in fact, to observe transducer behavior to a resolution better than one part in 20000. This corresponds, in effect, to detecting a strain of 0.05 μϵ or less on the surface of the spring element. Clearly, the resolution of such small dimensional changes requires that the strain gage system be selected and installed with the utmost care.

 

The strain gage selection procedure for stress analysis and transducer applications is similar.  The preferred sequence is:

 

  •         Operating temperature range
  •         Gage length.
  •         S-T-C number.
  •         Pattern Geometry.
  •         Strain Gage Series.
  •         Grid Resistance.
  •         Creep compensation code (transducer only). 
  •         Optional features and custom strain gages. 

 

 

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