Chapter 2
Configuration and Installation
©
National Instruments Corporation
2-29
Assuming a quarter-bridge strain-gauge configuration with a gauge factor
of
GF
= 2, the equivalent strain change introduced by the
R
SCAL
shunting
resistor is –199 µ
ε
. Determine the change as follows:
1.
Determine the change caused by the shunting resistor using the
following formula:
2.
Using the appropriate strain-gauge strain formula, and assuming that
you have no static voltage, determine the equivalent strain that the
R
SCAL
should produce. For example,
R
SCAL
= 301 k
Ω
and a
quarter-bridge 120
Ω
strain gauge with a gauge factor of
GF
= 2 and
V
exc
= 3.333 V and
R
= 120
Ω
produces the following result:
V
change
= 0.3321 mV
Replacing the strained voltage with
V
change
in the quarter-bridge strain
equation produces an equivalent –199 µ
ε
of change.
Terminal Block Temperature Sensor
To accommodate thermocouples with the SCXI-1121, the terminal block
has an onboard temperature sensor for cold-junction compensation. You
can connect this temperature sensor in two ways:
•
You can connect the temperature sensor to the MTEMP pin (C4) on the
module front connector and multiplex the sensor at the output
multiplexer along with the amplifier outputs. This is the Multiplexed
Temperature Sensor (MTS) mode. Refer to the
section in Chapter 4,
, for further details.
•
You can connect the temperature sensor to a separate data acquisition
channel via MCH4± (pins 11 and 12 on the module rear signal
connector). This is the Direct Temperature Sensor (DTS) mode.
Note
Use an average of a large number of samples to obtain the most accurate reading.
Noisy environments require more samples for greater accuracy.
The SCXI-1320 and SCXI-1321 temperature sensors output 10 mV/°C and
have an accuracy of ±1 °C over the 0 to 55 °C temperature range. To
determine the temperature, use the following formulas:
V
change
V
exc
R
d
R
SCAL
R
g
+
(
)
R
SCAL
R
d
R
SCAL
R
g
+
(
)
+
-----------------------------------------------------------
V
exc
2
----------
–
=
T
°
C
( )
100
V
TEMPOUT
(
)
=