85th AMS Annual Meeting

Tuesday, 11 January 2005
Measurement errors using a Geonor weighing gauge with a Campbell Scientific datalogger
Harry H. Lamb, MSC, Downsview, ON, Canada; and J. Swenson
Poster PDF (118.8 kB)
With the introduction of the very sensitive Geonor weighing gauge into the Environment Canada surface weather/RCS network, careful analysis of the measurement errors must be made in order to take full advantage of the measurement capabilities of the gauge.

The collection bucket rests in a cradle supported by three flexible members, of which, one must be a vibrating wire transducer and the other two may be chains. If there is only one transducer, gauge leveling will cause significant errors. If there are three transducers, gauge leveling errors are greatly reduced.

The ends of the wire are nodes and the vibration mode is fundamental. The frequency will be determined soley by the length of the wire, the tension exerted by the weight and the mass per unit length of the wire. The density of the wire will have a small temperature coefficient which may be very nearly zero, depending on the alloy used.

The frequency of the vibrating wire may be measured in the Campbell Scientific datalogger in two ways, the Period Average (P27) instruction and the Pulse (P3) instruction. For the range of frequencies being measured and the execution interval of the datalogger program, both methods give similar results.

For Period Average, the error is established by the CPU clock accuracy, the resolution and the error of a small correction term. For Pulse (P3), the error is determined by the accuracy at which the execution interval is repeated.

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