Campbell CDM-VW300 Series Manuel télécharger pdf (Page 83)

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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

/ 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

+ (1.202e–3) • digits + (–3.1682)

First, 2400 Hz is converted to digits using the following formula:

digits = (2400 Hz)

/ 1000 = 5760

Next, the digits value is placed in the polynomial equation:

s = (9.139E–08) • (5760)

+ (0.03054) • 5760 + (–80.471)

s = 98.53 mm

D-1

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Campbell CDM-VW300 Series Manuel Page 83