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4.4.1 Scaling
In most cases, the system supplied is a scaled system. Scaling refers to the selection of system
component variables so that the natural binary output represents a discrete number of inches per
count, such as 0.002 inch, 0.001 inch, or 0.0005 inch per count. For BCD outputs, scaling means
that the output reads directly in inches (mm, etc.), and need not be corrected mathematically.
The system variables that are matched include the transducer stroke, the number of recirculations,
the null point, and the counter card crystal oscillator clock. The transducer, DPM, and counter
card are factory calibrated to provide the desired resolution. The counter card crystal frequency is
calculated based upon the transducer velocity gradient, which is unique to the transducer serial
number. The zero is factory set on the counter card, and is also matched to the transducer. To
maintain a scaled output, the user should use a transducer and counter card with the same serial
number. Substitution of DPMs with the same number of recirculations does not affect the output
reading.
An unscaled system is provided upon request. An unscaled system may require calibration to
mathematically correct the digital output to read in inches, millimeters, etc. The scale factor for
unscaled systems is not a discrete number of inches per count, but should be within 0.2% of the
specified resolution for 27-28 MHz systems. For example, an unscaled system specified for 0.001
inches per count may have a scale factor of 0.00002 inches per count (approximately). Similarly,
an unscaled BCD output may have a scale factor of 1.002 inches per inch of reading. This does
not effect resolution but will vary the counts at any particular point on the stroke. Interchanging
system components which do not have the same serial number will also result in an unscaled
output.
4.4.2 Spare Parts and Inventory Considerations
The zero can be set on the counter card using DIP switches. An externally interrogated DPM or a
counter card can be used as a common spare. However, a system calibration should be per-
formed after any system component change.
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