Thus, to calibrate an AA72-based Channel No. “x,” you need only
1. Turn ON the system EEPROM SWITCH and then apply the following
MV/V CALI-
BRATION (MVV)
command:
MVV x = i, u [CR]
For “i,” enter the manufacturer-supplied transducer sensitivity rating in “mV/V, full
scale.” For a “Type 70” channel (7.5/1.5/0.75 mV/V, full scale), you should enter an
“i” value greater than 0.02 and less than or equal to 1.00 (mV/V). For a “Type 71”
channel (15.0/3.0/1.50 mV/V, full scale), you should enter a value greater than 0.04
and less than or equal to 2.00 (mV/V). For a “Type 72” channel (30.0/6.0/3.00
mV/V, full scale), you should enter a value greater than 0.80 and less than or equal
to 4.00 (mV/V).
For “u,” enter the
transducer’s nominal full-scale rating in whatever engineering
units are desired for the channel’s data reading—but
only if the channel is set for
10-volt excitation
(Section 3.a). NOTE: If the channel being calibrated is set for
1-volt excitation, the “u” value to be entered should be 10 times the actual value of
the nominal full-scale rating. If it is set for
5-volt excitation, “u” should be 2 times
the actual value.
The
MVV command will only work if Channel No. x has been assigned the proper
“type” code (“70,” “71,” or “72”).
2. Zero the channel by commanding
ZRO x [CR]
Note that a channel calibrated by the MVV command will report measurement data to
a precision matching that of the entered “u” value. If, for example, you're measuring
“psi,” and enter a “u” of “500,” then all subsequent channel readings will be rounded to
the nearest psi. If the entry is “500.0,” then all readings will be rounded to the nearest
tenth of a psi.
TWO-POINT (DEADWEIGHT) CALIBRATION
Using the standard ZERO (ZRO) and FORCE (FRC) commands, this conventional
“zero and span” method can be applied to an AA72 channel if the full-scale “mV/V”
rating of the channel’s strain gage transducer is unknown, or if the final measurement
accuracy provided by 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
Suitable for all AA72 excitation levels, this method is similar to the conventional TWO-
POINT (DEADWEIGHT) procedure. The difference is that 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 an AA72 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-
er manufacturer has supplied the exact value of the transducer’s equivalent input, it
can be used as a reference point for calibrating the channel.
Equivalent input can be approximated from a knowledge of the
Shunt Calibration
Resistance (R)
, the transducer’s Bridge Resistance (B), and the transducer’s Full-
Scale Sensitivity (K, in mV/V full scale)
. To determine the EQUIVALENT INPUT (X) as
an approximate percentage of full-scale output, you may use the following equation:
AA72-2/4.10 SETUP AND/OR OPERATING CONSIDERATIONS
AA72-2 / AA72-4 DUAL / QUAD DC STRAIN GAGE CARD
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