Spider DSA User’s Manual
225
Modulus of Elasticity:
The Modulus of Elasticity (or Young’s Modulus) is a number that measures a
substance's resistance to being deformed elastically (i.e. non-permanently)
when a force is applied to it. The modulus of elasticity is defined as the initial
slope of the material’s stress/strain curve; it has the units of pressure. Make
your entry in metric pressure units, giga-Pascals (GPa). Most steels have a
Young’s Modulus of about 200 GPa (29 x 10
6
PSI).
Excitation Voltage:
Specifies the maximum magnitude of the voltage, V
in
, used to excite the strain
gage bridge. The supported voltages are ±2.5, ±5 and ±10 volts.
Shunt Resistance:
Spider-80SG provides a high-precision 100 kΩ shunt for calibration; this is the
default value. Enter a different value if you plan to use an external shunt.
Lead Resistance:
Enter the nominal resistance, R
Wire
, of each wire connecting the strain gage
bridge to the Spider-80SG. The default is zero ohms.
Offset Nulling and Shunt Calibration
Before any measurement (including a calibration measurement) is made, the
bridge circuit must be nulled. This should be done with the structure in a
relaxed or unstressed condition. Whenever the bridge is nulled, the output is set
to 0 microstrain.
It is always good practice to run a shunt calibration before a course of
measurements. This calibration simulates a strain input by placing a high
precision shunt resistor in parallel with one of the four resistors in the bridge.
The normal procedure to verify the output of a strain gauge measurement
system relative to some predetermined mechanical input or strain is called
shunt calibration. Shunt calibration involves simulating the input of strain by
changing the resistance of an arm in the bridge by some known amount. This is
accomplished by shunting, or connecting, a large resistor of known value across
one arm of the bridge, creating a known resistance. The output of the bridge can
then be measured and compared to the expected voltage value. The results can
then be used to correct the output.
Spider-80SG provides a 100 kΩ calibration shunt resistor. When applied to a
350 Ω gage or completion resistor, this causes a 1.22-ohm reduction in
resistance corresponding to a strain of -0.00349/GF. When a 120 Ω gage or
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