Sec. 1 -2] 



.1 / EOHA XICA L INPUT TRAXSD UCE RS 



33 



Two different forms of wire strain gauges are employed, the 

 unbonded and the bonded strain gauge. 



unbonded strain gauge. An example of an unbonded strain 

 gauge is shown in Fig. (1-2)14. Four sets of strain sensitive wires, 

 electrically connected to form a Wheatstone bridge, are mounted 

 under stress between a frame F and a movable member M . If the 

 element M moves through an angle around the pivot point P (dotted 

 line), the wires B and C will be elongated and the wires A and D will 



Fig. (1-2)14. Unbonded strain gauge. 



be shortened. The changes of resistance will cause an unbalance of 

 the bridge and give rise to an output signal. 



The range of movement of practical unbonded resistance strain 

 gauges is in the order of ^0.0015 in. A typical resistance strain 

 gauge will require a force of about 0.15 oz (5 g, or 5 x 10 3 dynes). The 

 resistance of unbonded strain gauges ranges from about 60 to 5,000 

 ohms; special strain gauges have been built with resistances in the 

 vicinity of 50,000 ohms. The resistance variation caused by the 

 maximum practical displacement is about 1 per cent. The magnitude 

 of the output signal depends, of course, upon the applied voltage; 

 for an input voltage of 10 volts, i.e., near the maximum safe limit, 

 one can expect an output of the order of 10 to 100 mV. 



Increase of the applied voltage leads to an overheating and break- 

 down of the wire. Forced-air cooling is not effective and causes 

 uneven distribution of temperature. Cooling in a helium atmosphere 

 at reduced pressure has been recommended and permits a fourfold 

 increase in current. 



The deviation from linearity of unbonded strain gauges is less than 

 1 per cent; the accuracy can be better than 0. 1 per cent. About 1 min 

 is usually required, after excitation has been applied, to reach ther- 

 mal equilibrium. The zero point drifts, in general, by about 0.05 to 



