Chapter 8 Tutorial

Measurement Fundamentals

For more information on strain gages, refer to “Practical Strain Gage Measurements” (HP Application Note 290-1).This application note is also available on the HP Website in the Test and Measurement section (www.hp.com).

Strain Sensors The metal foil resistance strain gage is by far the most widely used strain measurement sensor. It consists of a thin metallic foil grid bonded to a thin insulating, adhesive backing. The resistance of the foil varies linearly with strain. Strain in the test body is simply the ratio of the foil’s strained to unstrained resistance: ε = R/R.

The Gage Factor (GF) indicates the sensitivity of a strain gage and is a measure of the fractional resistance change per strain: GF = ( R/R)/ε . Devices with a higher gage factor will exhibit a greater resistance change per strain applied.

Strain gages are available in many patterns with various numbers and configurations of elements. The most common element is the single gage form shown below. Multi-element strain gages, also called rosettes, are used to measure components of strain in different directions. Two-element (90°) and three-element (45° or 60°) configurations are the most common.

HI Source

HI Sense

LO Sense

LO Source

Common Uses for Strain Gages Strain gages are used to sense many types of physical parameters. Strain gages are primarily a force-sensing device. Force is measured indirectly by measuring the deformation of a test body to a known applied force thus producing a resistance change proportional to the applied force. Many other physical quantities can be measured through force measurements. Common applications of strain gages include weight, pressure, flow, and level measurements.

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