Chapter 4
Theory of Operation
©
National Instruments Corporation
4-5
To simulate the effect on strain of applying a shunt resistor across R
3
, use
the following equation:
Notes
To minimize temperature drift errors, the strain gauge must have a
self-temperature-compensation (STC) number that corresponds to the thermal expansion
coefficient of the material under test. STC gauges have a temperature sensitivity that
counteracts the thermal expansion coefficient of the test specimen. The STC number
approximately equals the thermally induced change in strain with change in temperature
and is expressed in units of microstrain per degree Fahrenheit. For example, if the test
specimen is aluminum, use a gauge with an STC number of 13.0. If the test specimen is
steel, use a gauge with an STC number of 6.0.
You can neglect lead resistance (
R
L
) of the wiring if shunt calibration is performed or if lead
length is very short (
∼
<10 ft), depending on the wire gauge. For example 10 ft of 24-AWG
copper wire has a lead resistance of 0.25
Ω
.
SCXI-1521/B Theory of Operation
This section includes a brief overview and a detailed discussion of the
circuit features of the module. Refer to Figure 4-4 while reading this
section.
ε
s
4
U
–
GF
1 4
U
+
(
)
------------------------------
=