Carrier frequency sensing of fluids in vessels

Abstract

A carrier frequency based system employs a capacitive probe which is located at a predetermined desired fluid detection level in a fluid containing vessel. The probe forms one plate of a capacitor where the other plate is formed by the fluid in which the probe is immersed. A carrier frequency is injected into the fluid by means of a suitable oscillator and at a frequency which is close to the system resonant frequency. When the probe is immersed in fluid, the probe will receive a predetermined carrier frequency of an amplitude determined by the system. The output of the probe is coupled to suitable sensing circuits which monitor the amplitude of the signal frequency coupled to the probe. When the fluid level recedes and therefore a portion of the probe or the total probe is out of fluid, the capacitance of the system changes. A different signal from the injected carrier frequency is now applied to the probe indicating to the sensing circuit that the probe is not immersed and therefore indicating an alarm condition. The output of the sensing circuit is monitored by control circuitry which can operate an alarm indicating to the user that the probe is not fluid immersed and that preventive measures should be taken. The fluid sensing probe utilizing the carrier injected frequency can discriminate between fluid and foam, 'probe fouling' and probe shorting and is relatively fail proof.