Readings can be collected from the instrument either through a portable readout or a logger. You can find more information on the readout/loggers’ section, however a quick instruction is provided below for the VW2106 readout.
- Connect signal cable from the sensor to the readout following the wiring colour code.
- Conductor colours may vary depending upon the extension cable used. Commonly these are:
- Switch on the unit and, where necessary, select range B
- The readout displays the Vibrating Wire reading (in Hz2/1000 - Linear Digits) and a temperature reading in degrees °C.
Sweep Range is the frequency range over which VW piezometers operate and is 2200 – 3500 Hz. The appropriate sweep range should be selected on any readout.
Calibration Factor - each VW piezometer has its own calibration certificate. Calibration factors must be applied to the recorded values in order to obtain the readings in engineering units.
Barometric pressure - In some locations, barometric variations are small except when there are storms. In other locations, normal weather may bring barometric pressure changes as large as 34 mb (0.5 psi) during a day, and 68 mb (1 psi) during a year.
If a piezometer is sealed into a borehole to measure pore-water pressure, the only pressure acting on the piezometer’s diaphragm is the water pressure at that depth, so a barometric correction does not need be applied since the influence will be negligible.
If the transducer is measuring the water level in a standpipe or well that is open to atmosphere, the pressure measured by the piezometer is the combined pressure of water and the air above the surface of the water. If the barometric pressure drops, the piezometer will return a decreased pressure, even if the water level remains unchanged. To eliminate the measurement uncertainty introduced by changes in barometric pressure, a correction can be applied. Either a special barometric transducer or an additional low pressure piezometer can be used to measure atmospheric pressure.
Barometric Pressure Correction
The following is an example of how to carry out the correction using data from a special barometric transducer or other weather station information:
Where a low pressure piezometer is used to measure the barometric pressure, the atmospheric pressure data can be recorded in the same units as the other piezometers, making compensation a simple addition of the difference (positive or negative).
Obtain barometric pressure readings on site at the time of reading the piezometer. Ideally the barometer should provide the actual pressure of the atmosphere at the location of the monitoring site. Off-site reports from weather stations can also be adequate for this purpose since it is only the relative change in pressure.
|Atmospheric Pressure when Zero recorded (mb):
The same source of pressure that was noted when the piezometer zero values recorded must be used for all subsequent readings.
|Zero Reading in Linear Digits (B units):
|Current Reading in Linear Digits (B units):
|Current Atmospheric Pressure (mb):
Calculated water pressure in mH2O
|Calculated water pressure in mH2O (m)
Subtract the barometer reading obtained when the site zero value was recorded, from current barometer reading in mb. This is the barometric pressure correction value.
|Change in Atmospheric Pressure (mb)
Convert the barometric correction value to the engineering units being used for the piezometer data, by multiplying it by the applicable factor
(for example:- 0.1 for kPa, 0.014504 for psi or 0.0101972 for mH2O)
Convert mb change to mH2O
= 15 x 0.0101972
Add the barometric correction, in engineering units, to the pressure reading:
Compensate for pressure change
Where a piezometer is installed in a zone where its temperature is likely to fluctuate significantly, records of both pressure and temperature data should be recorded. They can then be used to assess any temperature effects on the pressure readings.
Where the piezometer is sealed in a borehole or buried in fill, there is usually little variation in temperature, so temperature effects will be small and corrections will not be necessary. However, if a low range piezometer is suspended in a shallow standpipe or well, it may be affected by day to day changes and seasonal changes in temperature. In this instance, temperature corrections could become more important. Thermal influences on Piezometer readings are complex. Therefore, in order to correct for temperature it is first necessary to establish the effects of temperature changes on a particular piezometer and the medium in which it is installed. (It must be remembered that the density of water also changes with temperature). To establish the true effects of temperature changes, it is necessary to accurately verify the head acting on a particular piezometer using an alternative means. It is necessary to record both the water pressure from the piezometer, the piezometer temperature and the true water depth over the piezometer diaphragm. Over a full annual cycle, both the seasonal and daily thermal affects can be computed from these data. Corrections would be carried out using a similar principle as for Barometric Pressure.