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Use an average of a large number of samples to obtain the most accurate reading.
Note
Noisy environments require more samples for greater accuracy.
© National Instruments Corporation
Assuming a quarter-bridge strain-gauge configuration with a gauge factor
of GF = 2, the equivalent strain change introduced by the R
resistor is –199 µε. Determine the change as follows:
1.
Determine the change caused by the shunting resistor using the
following formula:
V
change
2.
Using the appropriate strain-gauge strain formula, and assuming that
you have no static voltage, determine the equivalent strain that the
R
should produce. For example, R
SCAL
quarter-bridge 120 Ω strain gauge with a gauge factor of GF = 2 and
= 3.333 V and R = 120 Ω produces the following result:
V
exc
Replacing the strained voltage with V
equation produces an equivalent –199 µε 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,
•
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.
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:
Chapter 2
(
V
R
R
exc
d
SCAL
=
---------------------------------------------------------- -
(
R
R
R
+
SCAL
d
SCAL
SCAL
V
= 0.3321 mV
change
in the quarter-bridge strain
change
Register
Descriptions, for further details.
T °C
(
)
(
100 V
=
TEMPOUT
2-29
Configuration and Installation
shunting
SCAL
)
R
V
+
g
exc
–
--------- -
)
R
2
+
g
= 301 kΩ and a
Configuration Register
)
SCXI-1121 User Manual
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