EMI – real site examples
Electromagnetic Interference (EMI) is a real phenomenon and can have an impact on data. All Geosense sensors are designed to mitigate EMI but there are times when external factors are to blame as demonstrated by the following examples.
Hydroelectric Dam – VW piezometers
Several vibrating wire piezometers were installed to monitor uplift pressures in an Hydroelectric dam. Spikes in data were observed which caused the Project Engineer to question the performance of the piezometers.
The data was reviewed, and it was soon established that the spikes occurred at the same time each night at midnight and therefore the question was asked “what happens at midnight?” The Project Engineer advised that the generators started at midnight. The EMI caused by the generators starting did affect the signal from the vibrating wire piezometers but it was not ongoing or detrimental to the sensor and these spikes were subsequently removed within the software, which is standard practice.
The spikes occurred at the same time each night at midnight and therefore the question was asked “what happens at midnight?”
Metro Site – In-Place Inclinometers
In-Place Inclinometers (IPIs) were installed within a ventilation shaft with the cable extending up to ground level and then connected into a data logger. The extension cable was a fully shielded twisted pair type.
Initially all the data was as expected but suddenly it started becoming erratic and the performance of the IPIs was brought into question. Our Senior Instrumentation Engineer attended site and found that the extension cable had been extended further but that a non-shielded cable had been used and it was laid alongside a heavy-duty cable that was powering the tower crane, lighting cables from generator and other cables with an unknown supply source.
Once the non-shielded extension cable was replaced including an EMC splice kit and the cable route changed the data returned to normal.
Diaphragm wall for deep foundation in Spain – Radio Burgos
During the installation of VW Strain Gauges within a diaphragm wall cage our Senior Instrumentation Engineer Chris Spalton (light blue overall) became aware of music and conversation. On asking the Spanish Engineer on his left where the music was coming from it became clear that the diaphragm wall was acting as a massive aerial demonstrating how radio waves can easily be part of EMI.
There was no direct consequence of the radio waves on the VW Strain Gauges or Inclinometers installed but just shows how EMI actually does occur around us all the time.
Packer testing – VW piezometers
A packer test was being carried out using two VW piezometers, one below and one above of the packer and connected into a Linx Logger.
A field laptop was used to configure the Linx software and to download the data during the packer test. The laptop was powered from the 12-volt battery on the water pump.
Initially the data from the logger was extremely erratic and as usual the prognosis was that the VW piezometers were faulty. It was then realised that the drain wire on the piezometer had not been connected into the Linx Logger and once this was done the readings stabilised. This clearly demonstrates that the shielding via the drain wire did its job by removing the EMI from the alternator on the water pump.
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