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V I S H AY M I C R O - M E A S U R E M E N T S

Strain Gages and Instruments

Tech Note TN-502

Optimizing Strain Gage Excitation Levels
Introduction
A common request in strain gage work is to obtain the recommended value of bridge excitation voltage for a particular size and type of gage. A simple, definitive answer to this question is not possible, unfortunately, because factors other than gage type are involved. The problem is particularly difficult when the maximum excitation level is desired. This Tech Note is intended to outline the most significant considerations that apply, and to suggest specif ic approaches to optimizing excitation levels for various strain gage applications. It is important to realize that strain gages are seldom damaged by excitation voltages considerably in excess of proper values. The usual result is performance degradation, rather than gage failure; and the problem therefore becomes one of meeting the total requirements of each particular installation. 3. Zero (no-load) stability is strongly affected by excessive excitation. This is particularly true in strain gages with high thermal output characteristics, and when inheren
t half-bridge or full-bridge compensation is relied upon to meet a low zero-shift vs. temperature specification. The zero-shift occurs because of variation in heat-sink conditions between gages in the bridge circuit. Another point should be emphasized. Any tendency for localized areas of the grid to operate at higher temperatures than the rest of the grid will restrict the allowable excitation levels. Creep and instability are particularly susceptible to these "hot-spot" effects, which are usually due to voids or bubbles in the glueline or discontinuities in the substrate. Imperfections in the gage itself can cause hot spots to develop, and only gages of the highest quality should be considered for high-excitation applications. When other factors are constant, the power-dissipation capability of a strain gage varies approximately with the area of the grid (active gage length x active grid width). The amount or type of waterproofing compound or encapsulant is relatively unimportant. Open-face gages mounted on
metal show only 10 to 15% less power-handling capacity than fully encapsulated gages with the same grid area. Note, however, that proper waterproofing materials must always be applied to open-face gages to prevent loss of performance through grid corrosion. It is sometimes stated that gage adhesives of high thermal conductivity can considerably improve the power-handling capability of strain gage installations. Generally, this is not correct. These adhesives incorporate high-conductivity fillers such as aluminum oxide and metal powders. This produces an adhesive of high viscosity, resulting in excessively thick gluelines and a longer thermal path from gage to substrate. Any net gain in thermal conductivity is more than offset by the performance degradation due to thicker gluelines. It is much better, for high gage excitation as well as normal gage applications, to use high-functionality adhesives that permit thin, void-free gluelines. On smooth mounting surfaces, ideal glueline thicknesses range from 0.0001
to 0.0003 in [0.0025 to 0.0075 mm].

Thermal Considerations
The voltage applied to a strain gage bridge creates a power loss in each arm, all of which must be dissipated in the form of heat. Only a negligible fraction of the power input is available in the output circuit. This causes the sensing grid of every strain gage to operate at a higher temperature than the substrate to which it is bonded. With exceptions, which are discussed later, it can be considered that the heat generated within a strain gage must be transferred by conduction to the mounting surface. The heat flow through the specimen causes a temperature rise in the substrate, which is a function of its heat-sink capacity and the gage power level. Consequently, both sensing grid and substrate operate at temperatures higher than ambient. When the temperature rise is excessive, gage performance will be affected as follows: 1. A loss of self-temperature-compensation (S-T-C) occurs when the grid temperature is considerably above the specimen temperature. All manufacturers' data on S-T-C are necessarily obtai
ned at low excitation levels. 2. Hysteresis and creep effects are magnified, since these are dependent on backing and glueline temperatures. A gage backing normally rated at +250 F [+120 C] in transducer service might have to be derated by 20 to 50 F [10 to 30 C] under high-excitation conditions.
Revision 16-Jul-07

TECH NOTE

Factors Affecting Optimum Excitation
Fol low i ng are factors of pr i mar y i mpor tanc e i n determining the optimum excitation level for any strain gage application:
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