When a 10A74-4C is connected to full-bridge strain gage transducers, “TWO-POINT
(DEADWEIGHT)” calibration may be applied, in addition to or as an alternative to
“EMM-CALCULATED” and “SIMULATED (SHUNT)” calibration (in this case, as before,
calibration via the
MVV command is not recommended).
NOTE: If you are using a Model 10CJB-4 for bridge completion of a 10A74-4C chan-
nel and,
prior to initial calibration, observe a significantly nonzero reading when no
load is placed on the gage(s), you may impose a
nominal ±1 mV/V offset by means of
solder pads in the 10CJB-4 box. See Section 3.d for complete instructions.
EMM-CALCULATED CALIBRATION
To calibrate a 10A74-4C-based Channel No. “x,” turn ON the system EEPROM SWITCH
and then apply a
SCALING FACTOR (EMM) command of
EMM x = m [CR]
where “m” is the scaling factor corresponding to the channel’s full-scale range in
microstrain, as given in Table 4. In calculating the numeric value of “m,” note that “N”
is the number of active bridge arms (“1” for a 1/4-bridge configuration; “2” for a half-
bridge configuration; and “4” for a full-bridge configuration); and “G” is the
gage factor
supplied by the strain gage manufacturer.
Table 4 Model 10A74-4C Scaling Factor (“m”) Values
Full-Scale Range
(microstrain) “m”
±7500 15,000 / (N x G)
±15000 30,000 / (N x G)
±30000 60,000 / (N x G)
TWO-POINT (DEADWEIGHT) CALIBRATION
PLEASE NOTE: AS MENTIONED ABOVE, THIS CALIBRATION TECHNIQUE APPLIES
TO A MODEL 10A74-4C CHANNEL ONLY WHEN THAT CHANNEL IS CONNECTED TO
A
FULL-BRIDGE STRAIN GAGE TRANSDUCER, AS ARE THE FOUR CHANNELS
SHOWN IN FIG. 4.
Using the standard ZERO (ZRO) and FORCE (FRC) commands, this conventional
“zero and span” method can be applied to a “full-bridge” 10A74-4C channel if the final
measurement accuracy provided by EMM-CALCULATED CALIBRATION does not
meet the requirements of the measurement application. The mainframe’s
EEPROM
Write Protect Switch
must be ON for the ZRO and FRC commands to be effective.
See Manual Section 1.G.5 for a general discussion of this calibration technique.
SIMULATED (SHUNT) CALIBRATION
This is a convenient “shunt resistor” method, where the second (“span”) input is not
produced by loading the source transducer, but by “simulating” a particular up-scale
value of mechanical input. This known EQUIVALENT INPUT then serves to determine
the SCALING FACTOR for the channel.*
For a 10A74-4C data channel, the equivalent input is produced by shunting a resistor
of known magnitude across one arm of the strain gage bridge, thereby simulating a
known value of input for either a
positive or negative up-scale reading. If the transduc-
SETUP AND/OR OPERATING CONSIDERATIONS 10A74-4C.11
QUAD DC STRAIN GAGE TRACK-HOLD CARD 10A74-4C
*Shunt calibration is easier though generally less accurate than two-point (deadweight) calibra-
tion. It is good for an accuracy of about 0.2% (depending, of course, on the accuracy of the
specified equivalent input, and on the resistor/bridge tolerance and temperature).
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