Powerware Horsepower Computer System ST-2400S - Manual (Page 14)

dynamometer. The calibration torque produced is equal to the sum of the torque
produced by the Torque Test Bar itself plus the torque produced by the test
weight(s) – which is equal to the total weight of the test weight(s) times the
horizontal distance from the center line of the dyno to the position of the test
weights. For example, assume the torque of the torque test bar itself is 17 Ft.-
Lbs., the total weight is 100 Lbs. and the horizontal distance is 4.00 Ft. The
calibration torque value produced by this setup is equal to 417 Ft.-Lbs. ( = 4.00
Ft. * 100 Lbs. + 17 Ft.-Lbs.)
2.) Shunt Resistor: The 2400S horsepower computer has an internal shunt resistor
which can be used to check the torque span calibration. The 2400S can
(optionally) be equipped with a precision shunt resistor which can be used to
perform a NIST (National Institute of Standards and Time) traceable torque
calibration. The “torque” introduced by the shunt resistor depends on the
resistance value of the shunt resistor as well as the properties of the load cell. For
the NIST traceable torque calibration to be accurate, both the shunt resistor value
and the load cell must have NIST traceable calibrations which should be checked
periodically. A 2400S horsepower computer without the precision shunt resistor
is equipped with a 1% precision 45.3 K Ohm shunt resistor (not adequate for
NIST traceable calibrations) which can be used for a quick and fairly accurate
check on the torque calibration and the torque sensor circuits of the 2400S. After
performing the calibration of the unit using the dead weight test fixture calibration
method, remove the dead weight test fixture and set the Shunt Resistor Switch to
ON. The shunt resistor will produce a torque reading that is (roughly) 2/3 of the
full scale torque of the unit (i.e. about 3300 Ft.-Lbs. on a 5000 Ft.-Lbs. full scale
2400S). More precise values are shown in the table below. The reading should
be within +/- 2 or 3 % of the appropriate value from the table below. The
tolerance derives from the +/- 1% tolerance of the 45.3 K Ohm shunt resistor plus
the tolerance of the load cell (likely +/- 1 or 2 %). If the reading is not this close,
you should double check the calibration and your procedure as a likely problem is
indicated. If the reading is within the above limits, record its value. It can be
used later for a quick check on your unit’s calibration as it should be stable over
time and temperature to within +/- 0.25% or better.
Full scale Torque (Ft.-Lbs.)
45.3 K Ohm Shunt Resistor Torque
100 64.14
200 128.3
500 320.7
1,000 641.4
2,000 1283
5,000 3207
10,000 6414
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