| Abstract | Vibrating-wire earth pressure cells are often used to measure soil pressure in fills and embankments or contact pressure between soil and buried structures. The cell consists of two metal plates welded on their periphery and filled with oil. Changes in applied pressure result in variations in fluid pressure in the cell, which is detected by a transducer, one type of which is the vibrating wire. The vibrating wire pressure cell is commonly used due to its long-term reliability. Geotechnical instrumentation companies provide each cell with a formula to calculate pressure based on frequency and temperature readings. Small diameter (76 mm) pressure cells installed in the field, around and in the vicinity of a concrete culvert, were responding in a manner that seemed to be strongly affected by temperature variations. Laboratory work was subsequently done to confirm the behaviour of the cells. Temperature calibration work was carried out for a series of 76 mm diameter cells and several 228 mm cells. The calibration was run with temperatures ranging from -10°C to 30°C with and without the effect of applied pressure on the cells. From this work, and from data gathered from different field sites, it was found that temperature calibration factors, given on the manufacturer's calibration sheet specific to each cell, largely underestimated the temperature effect. It was also found that the correction factors were dependent not only on temperature, but also on the pressure applied to the cell. The results showed that the temperature calibration curve, given as a linear correction on the calibration datasheet, became parabolic as pressure was applied to the cell. Recommendations are provided for minimising the temperature effect on sensitive pressure cells as well as for more accurately correcting temperature effects on cell readings. In addition, suggestions for improvement in the manufacture and calibration of pressure cells before they are installed in the field are presented. |
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