Gage Selection
Many factors, such as test-time duration, strain range required, and operating temperature, must be considered in selecting the best strain gage/adhesive combination for a given test profile. These factors and others are addressed in the Tech Note TN-505, Strain Gage Selection &#8211 Criteria, Procedures, Recommendations. .

Self-Temperature Compensation (S-T-C)
All gages with XX as the second code group in the gage designation are self-temperature-compensated for use on structural materials with specific thermal expansion coefficients. A short table of the S-T-C numbers and test specimen materials to which gages are thermally matched is available for use in gage selection. A graph of the thermal output curve for a particular grid alloy lot is included on the engineering data sheet provided with the gages.

When ordering, replace the XX code group with the desired S-T-C number, which is the approximate thermal expansion coefficient of the structural material in ppm/deg F. The Gage Designation System lists the available S-T-C numbers for specific grid alloys. The 06 and 13 values, available in A and K alloys, are most common and more likely to be in stock. When not otherwise specified, the 06 compensation is shipped.

Gage Resistance
Micro-Measurements strain gages are available in various resistance values that range from 30 to 5000 ohms.

Strain gages with resistances of 120 and 350 ohms are commonly used in experimental stress analysis testing. For the majority of applications, 120-ohm gages are usually suitable; 350-ohm gages would be preferred to reduce heat generation (for the same applied voltage across the gage), to decrease leadwire effects, or to improve signal-to-noise ratios in the gage circuit. Higher resistance gages are typically used in transducer applications and on composite materials.

Gage Factor
Gage Factor (GF) is the measure of sensitivity, or output, produced by a resistance strain gage. Gage factor is determined through calibration of the specific gage type, and is the ratio between and (strain), where is the initial unstrained resistance of the gage. It is affected somewhat by pattern size, geometry, S-T-C number, and temperature. Each gage package is supplied with the GF, as well as its tolerance and temperature sensitivity. Nominal gage factors for various alloys are:

A = 2.05
K = 2.1
D = 3.2
P = 2.00

Transverse Sensitivity
All gages are sensitive, to some degree, to strains transverse to the grid direction. The transverse sensitivity factor (Kt) is given on the engineering data sheet supplied with all gage types for which the data are relevant.

Strain Gage Adhesive Selection
When selecting a strain gage, it is most important to consider the adhesive that will be used to bond the gage, since the adhesive becomes part of the gage system and correspondingly affects the performance of the gage. However, when the interaction of test characteristics becomes too complex for selecting the gage/adhesive combination in a straightforward manner, contact our Applications Engineering Department for recommendations.

Gage Dimensions
Gage length is an important consideration in strain gage selection, and is usually the first parameter to be defined.

Dimensions listed for gage length (as measured inside the grid endloops) and grid width refer to active grid dimensions. Overall length and width refer to the actual foil pattern, not including alignment marks or backing.

The matrix size represents the approximate dimensions of the backing/matrix of the gage as shipped. Matrix dimensions are nominal, with a usual tolerance of + 0.015 in ( + 0.4 mm). If the gages are encapsulated, the matrix may be smaller by as much as 0.01 in (0.25 mm). Most patterns also include trim marks, and, for use in a restricted area, the backing/matrix may be field-trimmed on all sides to within 0.01 in (0.25 mm) of the foil pattern without affecting gage performance.