Strain Gage Selection 101

We need to know the type of material to which you plan to bond the sensor to in order to select the proper thermal compensation or modulus compensation.

One of the first steps in selecting a strain gage requires knowing something about the material to which the gage will be bonded. Most strain gage types are available in a variety of STC (self-temperature compensation) values which correspond to the coefficient of thermal expansion (CTE) of the substrate material in PPM/F°.  For example, normal alloy steel has a TCE of approximately 6.7 ppm/F°. When bonding gages to this material a strain gage STC number of 06 is selected to minimize thermally induced changes in gage resistance. Other materials require different STC numbers as shown in the chart below:

 

 

Available STC numbers in Constantan and Karma alloys are 00, 03, 05, 06, 09, 13, 15. Also available in Constantan alloy are 30, 40 and 50. These high STC numbers are typically selected for use on plastics.

 

Foil strain gage designations always show the STC number, if available, as the second two characters of the part number. For example, EA-06-125AD-120 has a 06 STC number and is designed to be mounted on mild or tool steel alloys.

Some foil  strain gage alloys cannot be Self-Temperature Compensated so instead of an STC number in the gage designation, code letters take their place:

Isoelastic alloy lists DY in the place of STC number. DY stands for dynamic (for dynamic measurements where STC is not required)

Platinum Tungsten alloy lists NC in place of the STC number. NC stands for non-compensated.

Modulus compensating Karma alloy lists MC in the place of STC number. As you can guess, MC stands for modulus compensation. When modulus compensation gages are ordered, four different gage factor slopes are available, depending on the spring element material requirements, and are specified by adding the appropriate M option to the end of the gage designation:

 

OPTION M1

Gage factor slope is –1.50% per 100°F [–2.70% per 100°C].

Provides span-versus-temperature compensation for many

stainless steels.

OPTION M2

Gage factor slope is –2.35% per 100°F [–4.23% per 100°C].

Provides span-versus-temperature compensation for most

aluminum alloys.

OPTION M3

Gage factor slope is –1.25% per 100°F [–2.25% per 100°C].

Provides span-versus-temperature compensation for many

tool steels.

OPTION M4

Gage factor slope is –1.35% per 100°F [–2.43% per 100°C].

Provides “mid-range” compensation between M1 and M3

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Yuval Hernik

StrainBlog Editor in Chief