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Weldable Strain Gages & Temperature Sensors


Micro-Measurements Weldable Strain Gages and Temperature Sensors are specially designed for spot welding to structures and components. They are ideal for applications where test or environmental conditions preclude clamping and curing an adhesively bonded gage installation. These gages are equally advantageous when strain measurements must be made at an elevated temperature, but the nature of the test object does not permit the use of an elevated-temperature-curing adhesive.

Surface preparation requirements are minimal; only an appropriate solvent cleaning and abrasion of the test part surface with silicon-carbide paper or a small, hand-held grinder is needed. Spot welding is accomplished with a portable stored-energy hand-probe spot welder, such as the Vishay Measurements Group Model 700 . Environmental protection is as easily applied to a welded gage installation as to an adhesively bonded gage.

Refer to Micro-Measurements Instruction Bulletin B-131, Attachment Techniques for Weldable Strain Gages and Temperature Sensors and Catalog A-110 for further information on installation and protective coatings, and to Bulletin 302 for specifications on the Model 700 Welding/Soldering Unit.

General
All sensors are laboratory-prebonded, with a high-performance adhesive, to thin [0.005 in (0. 13 mm)] metal carriers. Sensor grids are fully encapsulated for protection against handling and installation damage. Weldable strain gages are offered in two series to meet differing performance requirements. Both series are available in either 06 or 09 self-temperature compensation. Strain gages with 06 S-T-C have Inconel carriers, while S-T-C 09 gages and temperature sensors are mounted on 300-series stainless steel.


CEA-Series Weldable Strain Gage
Polyimide-encapsulated constantan foil grid, with large, rugged, copper-coated tabs. In most cases, the carrier can be contoured to a radius as small as 1/2 in (13 mm). The CEA Series is ideal for direct leadwire attachment, before or after installation.

Strain range is + 5000 microstrain, and normal operating temperature range is -100 to +200 deg F (-75 to +95 deg C). Short-term maximum temperature is +300 deg F (+150 deg C).

250 Patterns
W250A Uniaxial; 120 to 350 ohms; Special Purpose ; CEA ; Super Stock
W250C Tee Rosette; 120 to 350 ohms; Special Purpose ; CEA


LWK-Series Weldable Strain Gage
Nickel-chromium alloy grid, encapsulated in fiberglass-reinforced epoxy-phenolic. The LWK gage is provided with a three-wire lead system with 10 in (250 mm) of Teflon-insulated leadwire.

This construction simplifies leadwire temperature compensation and provides for easy connection of the lead system to the instrumentation cable. Minimum installation radius is generally limited to 2 in (50 mm).

Strain range is + 5000 microstrain, and normal operating temperature range is -320 to +500 deg F (-195 to +260 deg C). Short-term maximum temperature is +550 deg F (+290 deg C).

250 Patterns
W250B Uniaxial; 350 ohms; Special Purpose
W250D Tee Rosette; 350 ohms; Special Purpose


WWT-Series Temperature Sensor
High-purity nickel foil grid encapsulated in fiberglass-reinforced epoxy-phenolic, and equipped with integral three-tab terminal to facilitate leadwire attachment. The temperature sensor is normally installed on a flat surface of the workpiece, but, in any case, should always be oriented with the gridlines in the direction of minimum strain to avoid strain-induced errors (see Vishay Measurements Group Tech Note TN-506, Bondable Resistance Temperature Sensors and Associated Circuitry ). With an appropriate LST Matching Network, the temperature-response characteristic of the nickel can be linearized and scaled for direct readout (in degrees) with any strain indicator.

200 Patterns
W200B 50 ohms; Special Purpose

MEASUREMENT CONSIDERATIONS

It is important to note that operating characteristics of weldable strain gages (gage factor, transverse sensitivity, and thermal output) are specified for the basic strain gage itself - without the metal carrier. Thus, the properties are measured by bonding a conventional strain gage directly to an appropriate calibration specimen, following standard methods specified for all Micro-Measurements strain gages. This procedure assures the most accurate results, independent of the variables introduced by welding.

In particular, the user should be aware that the gage factor specified on the engineering data sheet accompanying the gage applies only to the basic strain gage, without the shim. The effective gage factor of the weldable assembly (after welding to the test member) is commonly 5 to 10% lower than this, due primarily to the stiffness of the shim. The reduction in gage factor is not subject to quantitative generalization, because it depends on the cross-sectional properties of the test specimen, and on the mode of loading (e.g., bending versus direct stress). It has been demonstrated, however, that for a group of like specimens, loaded in the same manner, the weldable gages exhibit very good repeatability and uniformity of response. Therefore, when test requirements dictate greatest accuracy, the weldable gages should be calibrated on a specimen of the same material and cross section as the test part, and under the same mode of loading.