| 摘要 |
A vibrating probe for detecting a threshold level within a vessel of fluent material, particularly a liquid, includes: an ultrasonic acoustic transducer assembly comprising a transducer element having a natural resonant frequency, and loading elements effective to shift that frequency to an effective resonant frequency of the assembly, the assembly being supported within the vessel and including a coupling member to couple the transducer acoustically to liquid reaching the threshold level, a transmitter generating pulses applied to the transducer element for repeatedly exciting the transducer element to cause the transducer assembly to resonate at its effective resonant frequency, a circuit for measuring the effective resonant frequency of the transducer over a ringdown period following cessation of each pulse, and a circuit for comparing the effective resonant frequency measured during the ringdown period following each pulse with a predetermined threshold level. The threshold level is selected so as to distinguish between the effective resonant frequency of the assembly in free air and a lower effective resonant frequency when coupled to fluent material through the coupling member. The damping effect of the fluent material, particularly if a viscous liquid, may accelerate the ringdown to an extent such that vibration is effectively suppressed before the end of the frequency measurement, thus further depressing the apparent measured frequency.
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