Vibrating Wire Units of Measurement

1. Introduction

This article outlines the units of measurement used by Soil Instruments. These include hertz, linear and period readings and what the units are actually showing

2. The various units of measurement

The measurement of vibrating wire instruments is the frequency of oscillation of the wire.
This is measured by the Data Recorder as the number of oscillations per second or hertz (Hz)
This unit of measurement is not linear in relation to the change in strain of the wire, so the Hz reading is often linearised by squaring and dividing by 1000 to give the unit F²/1000 also called linear or digits.
Another unit for measuring the frequency is Period, this is the length of time taken for one oscillation to occur and is measured in seconds.
Typically for vibrating wire instruments this value is very small, so it is multiplied by 10^7 to give a four or 5 digit number.
Hz and Period require more complex formulae in order to convert the frequency readings into engineering units.
Soil Instruments recommends using the F²/1000 units displayed and stored by the Data Recorder for all readings and conversion into engineering units.