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.
TYPES OF PROBE
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.
The Impulse Method
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.
The Tuned Oscillator Method
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 Digitrans Digital Method
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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