Key Strain Gage Terms and Definitions

BONDING ADHESIVES

Micro-Measurements offers a host of adhesives which are specially formulated to meet the highest performance standards under a wide range of environmental conditions. These adhesives and are packaged to ensure reliable and repeatable mixing and application results. Each package contains specific instructions for its proper handling – storage, mixing, application, curing and if appropriate, post-curing.

Consideration for the adhesives are:

1. Temperature range
2. Elongation
3. Cure temperature
4. Duration of test / installation
5. Clamping pressure
6. Humidity

ENVIRONMENTAL PROTECTION

Strain gage sensor performance is easily degraded by the effects of moisture, chemical attack or mechanical damage. As a result, strain gage sensors require varying degrees of protection according to the severity of the environment in which they must operate. While it is often practical in laboratory applications to operate fully encapsulated strain sensors without additional protection, open-faced strain sensors should always be covered with a suitable coating as soon as possible after installation.

Consideration for the Environmental Protection are:

1. Temperature range Environmental Protection

2. Contaminants encountered (water, steam, oil, grease, corrosive gases, etc.)

3. Longevity of the installation

4. Reinforcement

GAGE LENGTH

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. Gage length is often a very important factor in determining the gage performance under a given set of circumstances.

Consideration for gage length are:

1. Strain gradients
2. Area of maximum strain
3. Accuracy required
4. Static strain stability
5. Maximum elongation
6. Cyclic endurance
7. Heat dissipation
8. Space for installation
9. Ease of installation

GAGE PATTERN

The gage pattern refers cumulatively to the shape of the grid, the number and orientation of the grids in a multiple-grid gage, the solder tab configuration, and various construction features which are standard for a particular pattern. All details of the grid and solder tab configurations are illustrated in the “Gage Pattern” columns of our strain gage data book. The wide variety of patterns in the list is designed to satisfy the full range of normal gage installation and strain measurement requirements.

Consideration for gage pattern are:

1. Strain gradients (in-plane Gage Pattern and normal to surface)
2. Biaxiality of stress
3. Heat dissipation
4. Space for installation
5. Ease of installation
6. Gage resistance availability

OPTIONS

Micro-Measurements offers a selection of optional features for its strain gages and special sensors. The addition of options to the basic gage construction usually increases the cost, but this is generally offset by the benefits. Examples are:

• Significant reduction of installation time and costs
• Reduction of the skill level necessary to make dependable installations
• Increased reliability of applications
• Simplified installation of sensors in difficult locations on components or in the field
• Increased protection, both in handling during installation and shielding from the test environment
• Achievement of special performance characteristics
•  

Consideration for the options are:

1. Type of measurement (static, Options dynamic, post-yield, etc.)
2. Installation environment — laboratory or field.
3. Stability requirements
4. Soldering sensitivity of Substrate (plastic, bone, etc.)
5. Space available for installation time constraints]

RESISTANCE

120 Ω vs. 350 Ω vs. > 1 kΩ In the past, it was more common to use 120 Ω (instrumentation stability driven), but modern instrumentation has eliminated the need to use 120 Ω gages for stability. When a choice exists, the higher resistance strain sensor (gage) is preferable in that it reduces the heat generation rate by a significant factor. Higher gage resistance, such as 350 Ω and 1 kΩ, also have the advantage of decreasing leadwire effects such as gage factor desensitization due to leadwire resistance in series with the active gage. If the strain gage circuit is in an electrically noisy environment, the signal-to-noise ratio can be improved with higher resistance strain gages due to the ability to use a higher excitation without grid self-heating (zero instability).

Consideration for gage resistance selection are:

1. Heat dissipation
2. Leadwire desensitization
3. Signal-to-noise ratio

SERIES

The strain-sensing alloy and backing material are not subject to completely independent selection and arbitrary combination. Instead, a selection must be made from among the available gage systems, or series, where each series generally incorporates special design or construction features, as well as a specific combination of alloy and backing material.

Consideration for gage series are:

1. Type of strain measurement Gage Series application (static, dynamic, post-yield, etc.)
2. Operating temperature
3. Test duration
4. Cyclic endurance
5. Accuracy required 6. Ease of installation

STC

What is the STC number in the gage part number? STC stands for Self-Temperature-Compensation. The number itself relates to the material coefficient of thermal expansion (CTE) that the gage is intended to be used on. For example, STC 06 is for low-carbon steel, which has a CTE of approximately 6.7 ppm/°F (12 ppm/°C). Self-Temperature-Compensation is a technique used when manufacturing the foil strain gage sensor to provide a gage response that minimizes the free expansion of a test part subjected to a change in temperature. You can learn more about STC by reading Tech Note TN-504 .Self-temperature-compensated strain gages are designed to produce minimum thermal output (temperature induced apparent strain) over the temperature range from about –50° to +400°F (–45° to +200°C).

Consideration for STC are:

1. Test specimen material
2. Operating temperature range
3. Accuracy required

STRAIN-SENSING ALLOYS

The principal component which determines the operating characteristics of a strain gage is the strain-sensitive alloy used in the foil grid. However, the alloy is not in every case an independently selectable parameter. This is because each Micro-Measurements strain gage series (identified by the first two, or three, letters in the alphanumeric gage designation) is designed as a complete system. That system is comprised of a particular foil and backing combination, and usually incorporates additional gage construction features (such as encapsulation, integral leadwires, or solder dots) specific to the series in question. Micro-Measurements supplies a variety of strain gage alloys as follows (with their respective letter designations)

BONDING ADHESIVES

Micro-Measurements offers a host of adhesives, specially formulated to meet the highest performance standards under a wide range of environmental conditions. These adhesives are packaged to ensure reliable and repeatable mixing and application results. Each package contains specific instructions for its proper handling – storage, mixing, application, curing and if appropriate, post-curing. Consideration for the adhesives are:

1. Temperature range
2. Elongation
3. Cure temperature
4. Duration of test / installation
5. Clamping pressure
6. Humidity

ENVIRONMENTAL PROTECTION

Strain gage sensor performance is easily degraded by the effects of moisture, chemical attack or mechanical damage. As a result, strain gage sensors require varying degrees of protection according to the severity of the environment in which they must operate. While it is often practical in laboratory applications to operate fully encapsulated strain sensors without additional protection, open-faced strain sensors should always be covered with a suitable coating as soon as possible after installation.

Consideration for the Environmental Protection are:

1. Temperature range Environmental Protection
2. Contaminants encountered (water, steam, oil, grease, corrosive gases, etc.)
3. Longevity of the installation
4. Reinforcement

GAGE PATTERN

The gage pattern refers cumulatively to the shape of the grid, the number and orientation of the grids in a multiple-grid gage, the solder tab configuration, and various construction features which are standard for a particular pattern. All details of the grid and solder tab configurations are illustrated in the “Gage Pattern” columns of our strain gage data book. The wide variety of patterns in the list is designed to satisfy the full range of normal gage installation and strain measurement requirements.

Consideration for gage pattern are:

1. Strain gradients (in-plane Gage Pattern and normal to surface)
2. Biaxiality of stress
3. Heat dissipation
4. Space for installation
5. Ease of installation
6. Gage resistance availability