Campbell Scientific CDM-VW300 Series Dynamic Vibrating-Wire Analyzer System - Manual (Page 83)

Appendix D. Digits Conversion
Vibrating-wire sensors are typically supplied with a calibration report from the
manufacturer. Use the calibration data to convert the frequency output of the
CDM-VW300 to engineering units. While basic measurement of CDM-
VW300 analyzers is frequency, which is expressed in hertz (Hz), some
calibration reports express the base measurement in terms of 'digits'. If this is
the case, first convert the frequency output of the CDM-VW300 to a digits
equivalent using the following formula,
digits = Hz 2 / 1000.
Use gage factor and polynomial fit data from the calibration report to then
convert digits to engineering units. Following is an example that converts the
frequency output of the CDM-VW300 first to digits, then to engineering units
of millimeters of displacement.
D.1 Example: Frequency to Digits to Displacement
Measuring a Geokon vibrating-wire displacement sensor, the CDM-VW300
analyzer outputs a value of 2400 Hz. FIGURE D-1 shows the calibration
report for the sensor. The relationship of sensor output to displacement, s, is
expressed as,
s = (3.598e–9) • digits 2 + (1.202e–3) • digits + (–3.1682)
First, 2400 Hz is converted to digits using the following formula:
digits = (2400 Hz) 2 / 1000 = 5760
Next, the digits value is placed in the polynomial equation:
s = (9.139E–08) • (5760) 2 + (0.03054) • 5760 + (–80.471)
s = 98.53 mm
D-1