is caused to move longitudinally by an external force, two of the fila- 

 ments will elongate and the other two will shorten. The resistances 

 of the elongated filaments increase as the resistances of the shortened 

 filaments decrease. The changes in resistances of the filaments will 

 be proportional to their changes in length. The resistance changes 

 of the filaments alter the electrical balance of the bridge to produce 

 an electric signal in the output circuit. 



Specifications of a commercial strain gauge are as follows: 



(1) Ranges: to 50 to to 1,000 pounds compression. The 

 full-scale displacement is approximately 0.002 inch. 



(2) Load limits: two times range in compression 



(3) Transduction: resistive, complete balanced bridge 



(4) Nominal bridge resistance: 350 ohms 



(5) Excitation: 14 volts d.c. or a.c. (rms), including carrier 

 frequencies 



(6) Output: 40 millivolts full scale open circuit at 14 volts 

 excitation 



(7) Non-linearity and hysteresis: not more than ±0.5 percent 

 of full scale 



(8) Ambient temperature limits: -100° to 275°F 



(9) Thermal coefficient of sensitivity: approximately 0.01 

 percent per °F from -65° to 250°F. Temperature compensation can 

 be incorporated. 



(10) Thermal zero shift: approximately 0.01 percent of full 

 scale per °F from -65° to 250°F. 



Strain gauges are used in bottom pressure measuring equipment 

 but, the Hydrographic Office has only one depth gauge which has a 

 strain gauge element. This device has been used to monitor the depth 

 of a towed magnetometer. Prior to its use in the field, the gauge and 

 associated recorder were calibrated on a dead weight scale. However, 

 since depth is not critical in the application, the calibration was super- 

 ficial. 



V-5 



