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These
are highly accurate DC instruments for use with load cells,
pressure sensors, and other strain gage transducers employing
a 4-arm bridge.
The
Model 3170 Strain Gage Conditioner is the basic "Form
1" instrument.
The
Model 3270 Strain Gage Conditioner / Indicator is the "Form
2" instrument, providing vivid front-panel digital
indication of measured values, scalable in desired engineering
units.
The
Model 3370 Strain Gage Conditioner / Indicator /
Controller is the "Form
3" instrument, and includes HI/LO limit detection
with control output. For use of a Model 3370 equipped with
internal solid-state relay ("S"
Option), see Relay-Controlled
Filling Operation. |
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Advanced
circuit design overcomes many of the errors traditionally afflicting the
strain gage measurement process, resulting in three high-level,
drift-free, noise-free analog outputs (see Specifications).
Nearly all mechanical measurement and control requirements are covered by
these three simultaneously available outputs.
Other
important features include
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remote
sensing and regulation of bridge excitation—eliminates errors
from temperature effects on cable resistance |
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seven-wire
calibration circuitry—applies the internal shunt calibration
resistor at the transducer terminals, thereby eliminating
significant calibration transfer error in long-cable installations |
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true
differential input, with better than 80 dB of common-mode
rejection—eliminates errors from common-mode pickup and possible
"ground-loop" coupling |
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input
impedance in excess of 100 megohms preserves the validity of
factory calibration, prevents conversion of common-mode to
normal-mode signals, and eliminates remaining errors attributable to
cable resistance. Allowable cable length has virtually no
practical limits. |
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elimination
of both short-term and long-term drift through an advanced
solid-state chopper stabilization technique, while preserving the
full frequency passband, free of chopper noise; the rated accuracy
is obtained without "warm-up" period or periodic
"tweaking" of controls |
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active
low-pass filtering smooths unwanted dynamic signal components
arising from vibration, power impulses, etc., that might prevent
stable digital conversion or control action |
For
a special "P" version of the Model 3370 dedicated to the capture
and hold of peak torque values, see the Model
TM3 Torque Monitor. For conditioning inputs from AC-excited
strain gage transducers, see the Models 3178,
3278, and 3378.
For typical
3170/3270/3370 operations, see the following applications:
3000
Series options applying to these instruments include
DC
Strain Gage Instrument Specifications [Top 
]
(Click
here for 3000 Series General Specifications)
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Input
Type |
Conventional
4-arm strain gage bridge, nominal 90 to 2000 Ω |
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Input
Range (full-scale) |
Nominal
sensitivity 1 to 8 mV/V, full scale* |
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Excitation
Supplied |
Regulated
5 or 10 VDC, user selectable** |
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Analog
Outputs |
Three
outputs, each ±5 VDC full-scale with 50% overrange, 5 mA maximum; low-pass corner
frequencies of 2 Hz, 200 Hz, and 2 kHz, respectively |
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Common-Mode
Rejection |
Greater
than 80 dB |
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Input
Impedance |
Greater
than 100 MΩ |
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Analog
Filtering |
Active
low-pass filters provide -60 dB per decade above cutoff frequency
("f"); full-scale slew time is 1.4/f sec |
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Output
Ripple and Noise |
0.15%
of full scale (rms) maximum for 200-Hz and 2-kHz outputs; 0.02% of full
scale (rms) maximum for 2-Hz output |
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Accuracy
(typical, following calibration) |
0.05%
of full scale |
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Display
Resolution (Models 3270 and 3370) |
0.02%
of full scale*** |
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*
Ten-turn coarse and fine front-panel controls will balance 1.5 mV/V
initial unbalance and allow span adjustment over the stated full-scale
sensitivity.
** Transducers with sensitivity from 4 to 8 mV/V, full scale, or with
bridge resistance of 120 Ω or less, must use 5-V excitation.
*** Includes the combined effects of nonlinearity, random noise,
line-voltage variation between 105 and 130 volts, ambient temperature
variation of ±20° F about starting value, and six-months drift of zero
and span. Errors attributable to the transducer are not included. |
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