Digitrans Industrial Controls
Principle of the RF Probe
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 impedence 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.
All the methods of determining the capacitance and resistance of the electrode involve measuring the effect that the probe impedance has on a tuned circuit.
A tuned circuit coupled to the probe is excited by a short pulse and the resultant decaying oscillation is used to determine capacitance and resistance of the circuit.
A tuned circuit coupled to the probe is connected as part of an oscillator. The impedance of the probe causes the oscillator to change frequency and amplitude. A feedback circuit corrects the frequency and the ammount of correction is an indication of the capacitance while the amplitude is a measure of the resistance of the circuit.
The method used in the Digitrans probes also uses a tuned circuit of an oscillator coupled to the probe but no manipulation of the tuning is applied. The oscillator is allowed to shift due to the loading of the probe impedance and the frequency shift is measured. This measurement is done at two frequencies about 10 MHz apart and this gives enough information to calculate the capacitance and resistance. This method is patented by Digitrans Industrial Controls.
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