If I've got a soundwave... and then the same wave but with a harmonic on it.... is there any way of getting a sensor hooked up to a PLC to notice when the harmonic is present? I'm thinking maybe low pass filter, but this is going to have to operate in a steelworks, so there's going to be a lot of background noise going on. Will it still be effective? also, how powerful is a piezoelectric transducer when cranked to full voltage? the reason I ask... we need to engineer a way of automating the emptying of a waste tank from the blast furnace. when the tank reaches a certain level, we need to tell the PLC to switch off the first pump. So we need a way of sensing when the tank is full. Having considered systems using microwaves and radar and the like, we concluded they would be too expensive and might not even withstand the constant abuse that would be dealt from the slurry. capacitance was one thought, and then resonance frequencies was another (when the transducer is vibrating in air then we'll basically be getting the fundamental wave. When submerged in slurry, it noticably distorts the waveform with harmonic(s) ?) I don't think the rest of the system is relevant, but if anyone wants more information I'll gladly oblige.