Validation of a new sensor to measure the concentration of polyethylene glycol for unsaturated soil testing

Accurate measurement of PEG concentration is primordial when using the osmotic method to control soil suction. Because PEG is an inert molecule, common chemistry methods are not adequate to measure its concentration and researchers resort to the measurement of Brix Index. Such measurement can be manual, which is not very accurate and time consuming but cheap. On the other hand, more advanced sensors do exist but they tend to be quite expensive. This paper presents a new sensor for the indirect measurement of PEG concentration based on a tuneable diode laser and fibre optics. The laser signal is shone through the PEG and the reduction in signal voltage is correlated to the PEG concentration. The system has been assessed for temperature stability and accuracy. Such sensor offers the possibility of multipoint measurement, which significantly reduces the capital cost of the equipment.