1.3.3 Advantages and Disadvantages of Foil Strain Gauges
An advantage of the foil strain gauge is that it is mass-produced making it cheap, reliable, and relatively small. Also, the workings of the foil strain gauge are simple yet accurate. The resolution of a foil strain gauge is 0.1 μstrain with a maximum strain of 200,000 μstrain. The inherent temperature compensation of the Wheatstone bridge adds to the advantages of the foil strain gauge system. The foil strain gauge also has very little transverse sensitivity compared to wire strain gauges, because the large amounts of area at the turns on the foil gauge prevent perpendicular strains from affecting the performance.
A disadvantage of foil strain gauges is their inability to be multiplexed. Each sensor is attached to its own conditioning equipment and in order to get data from multiple spots, a dedicated system is needed for each one. Also, because the strain gauges are electrical they are susceptible to electromagnetic interference.
Mounting the sensors is also a problem. The test piece must be sanded and washed with great attention to detail as no to contaminate the epoxy. For accurate measurements the gauge must be fixed to the test piece very securely and as close as possible, which means using a clamp to get a very thin and uniform amount of epoxy. Finally the lead wires must be soldered in place, cleaned of excess flux, and a protective coating applied. With some practice these tasks can be mastered for a simple laboratory test piece, however if the test piece is not readily accessible or removable some of the tasks can be nearly impossible.
The epoxy itself can also be a problem because of curing temperatures and times. Also the epoxy is not a perfectly elastic material. The sensors can undergo creep, which is a gradual lowering of the strain that acts on the gauge. Creep can occur due to a bad bond, or from steady or highly repetitive strain. Higher operating temperatures can also weaken the epoxy and cause creep if special high temperature epoxy is not used.
For highly precise measurements hysteresis is another disadvantage of foil strain gauges. Hysteresis is when a high amount of strain is applied and then released from the strain gauge. The gauge will seem deformed and will have a higher resistance value at zero strain than initially. A similar effect will happen in the opposite direction and setting up a hysteresis loop. The loop can be narrowed to negligible values by repeatedly stressing the gauge in opposite directions. However, A typical value of error is 0.1 percent and usually can be ignored for measurements less than 1500 microstrain. A plot of a hysterisis loop is shown in Figure 1.5.