34
Version 1.4 May 09
Geosense configure and supply equipment manufactured by both Campbell Scientific
Ltd and DataTaker Ltd. These are the most commonly adopted third party
manufacturers of data loggers that can be used with Vibrating Wire Instruments.
Specific configuration and programming advice can be obtained from Geosense and
or the manufacturers documentation.
6.2 Data
Reduction
Overview
Readings from a Vibrating Wire Piezometer are typically in a form that is a function of
frequency rather than in units of pressure. Commonly the units would be either
Frequency
- Hertz,
Linear
- Hz
2
/1000 or Hz
2
/1000000 or
Period
- Time - (Seconds
x10
-2
or x10
-7
).
To convert the readings to units of pressure, calibration factors must be applied to the
recorded values. For most Vibrating Wire sensors, these factors are unique and are
detailed on the sensor calibration sheet. A unique calibration sheet is supplied with
all Geosense Vibrating Wire Piezometers.
If the readout display is in Period units ( e.g. 0.03612 or 3612 - depending upon the
readout used ) the first step to producing an engineering value is to convert the
reading to Linear Digits ( Hz
2
/1000 ) . Two examples of this calculation can be seen
below. The first (1) where the readout includes a decimal point and displays the
Period in
Seconds
–2
and the second (2) where the readout displays the Period in
Seconds
-7
(1)
Readout
Display =
0.03612
Linear
Digits
(Hz
2
/1000) =
( 1 / 0.03612
–2
)
2
/ 1000
=
7664.8
(2)
Readout
Display =
3612
Linear
Digits
(Hz
2
/1000) =
( 1 / 3612
–7
)
2
/ 1000
=
7664.8
If the readout displays ‘Frequency’ values, ( e.g. 2768.5 Hz ) only a simple calculation
is required to convert the readings to Linear Digits.
Linear
Digits
(Hz
2
/1000) =
(
2768.5
)
2
/ 1000
=
7664.6
Certain data loggers store their Vibrating Wire data in Linear Digits but further divided
by 1000. In this case the data would have to be multiplied by a further 1000 to
maintain the standard Linear Digits (Hz
2
/1000) format for standard calculations.
There are many ways to achieve the conversion from recorded data to useful
engineering values. The following are included as a guide only and as a basis for
alternative approaches.
(Continued from page 33)
(Continued on page 35)
