TBX-1329 Installation Guide
4
ni.com
National Instruments is
not
liable for any damages or injuries resulting from inadequate
safety earth-ground connections.
Do
not
loosen or re-orient the safety-ground lug hardware when connecting the
safety-ground wire; to do so reduces the safety isolation between the high voltage and
safety ground.
Clean the module and accessories by brushing off light dust with a soft non-metallic brush.
Remove other contaminants with a stiff non-metallic brush. The unit
must
be completely
dry and free from contaminants before returning it to service.
The terminal block
must
be used with a UL-listed SCXI chassis.
Strain-Gauge Voltage Drops When Using the SCXI-1121
When you use the SCXI-1121 with the TBX-1329 to measure strain,
a small amount of voltage drop develops across the excitation wires in
the SH32-32-A cable. This voltage drop is due to the wire resistance and
the current flow in these leads when a strain gauge is connected at the
TBX-1329 across the excitation outputs provided on the SCXI-1121.
To reduce errors in your output voltage, first you need to calculate the
voltage drop across the SH32-32-A cable. This voltage drop, which results
in an error in your measurement, depends on the cable length and on the
strain gauge value and configuration. The SH32-32-A cable has a
resistance of 0.21
Ω
/m.
To determine the amount of error introduced by the cable, do the following:
1.
Calculate the total resistance (RTL) of the cable based on the cable
length.
2.
Refer to your strain-gauge data sheet for the bridge resistance of the
strain gauge (RSG) connected at the TBX-1329.
3.
Use the following formula to determine the total voltage drop (
V
drop
)
in the SH32-32-A cable excitation leads:
V
drop
= [
R
TL
/(
R
TL
+
R
SG
)] x
V
ex
where
V
ex
= excitation voltage.
For example, if you have a 1 m SH32-32-A cable, 120
Ω
full-bridge strain
gauge, and
V
ex
= 3.333 V, your calculations from Steps 1 through 3 would
be:
1.
R
TL
= 2 x 0.21
Ω
/m x 1 m = 0.42
Ω
; you multiply the cable length by
two to take into consideration both the V
ex+
and V
ex-
lead resistances.
2.
R
SG
= 120
Ω
, is the total equivalent bridge resistance as seen from the
V
ex
±
terminals of the TBX-1329.
