Single-Frequency-Based Dry Rubber Content Determination Technique for In-Field Measurement Application

In this paper, it is shown that extensive characterization and modeling of dielectric permittivity of rubber latex enables dry rubber content (DRC) to be accurately determined using a measurement at a single microwave frequency. The model is demonstrated in a low-cost sensing system based on a six-port reflectometer that is suitable for application in the field. To develop an accurate model, a wide range of samples (with the DRC in the range 28%–61%) is first measured using the Agilent Technologies 85070E dielectric probe system for a range of frequencies and at temperatures from 15
$^{\circ}{\rm C}$
to 39
$^{\circ}{\rm C}$
. This data is then used to determine the parameters of a dielectric mixture model, which is based on the assumption that rubber latex is a diphase liquid consisting of the water and solid rubber. To take the effect of temperature and signal frequency on the molecular polarization behavior into account, the dielectric permittivity of the water is described using a Debye-type relaxation spectral function; and the frequency-dependant polarization parameter of neighboring inclusions is applied to the dielectric mixture model. After determining all the model parameters, it is then shown that the measurement of the sample's temperature and dielectric permittivity at a single frequency is sufficient to determine the DRC with a high degree of accuracy. Compared with the measurements made with the standard oven-drying method, using a single 2.5 GHz measurement the technique exhibits 0.36% DRC of mean absolute error with 0.28% DRC standard deviation, over the temperature range 15
$^{\circ}{\rm C}$
–39
$^{\circ}{\rm C}$
. Finally, the technique is implemented with a six-port reflectometer. The measurement system employing the model is applied and tested with several fresh rubber latex samples. A wide range of measured results confirm the validity of the technique.