An RF Probe as used in the sugar industry, is an instrument which measures the concentration of sugar solutions by means of radio frequency impedance measurements.

If two electrodes are immersed in a solution the impedance presented to the circuit is a combination of resistance and capacitance depending on the conductivity and dielectric constant of the solution. The conductivity depends on the type and concentration of ionic salts while the dielectric constant is a function of the dielectric constants of the solvent and solute, in this case water and sugar. The dielectric constant of water is much greater than that of sugar so as the brix of the solution increases (quantity of water decreases), the dielectric constant of the solution decreases. From this it can be seen that the electrode impedance changes with change in brix therefore the impedance measurement can be used as a measurement of brix.

There are several ways of making the impedance measurement. Since the capacitance values are quite small the frequency used must be high so that the capacitive reactance is at least of the same order of magnitude as the resistance due to conductivity. This usually requires a frequency in the tens of Megahertz.