I've built one of these http://en.wikipedia.org/wiki/Level_sensor#Air_bubbler Example picture at http://beta.globalspec.com/ImageRepository/LearnMore/20123/Air%20bubbler13fc76c9695f432ca0e111880003ddb3.png The goal is to measure absolute level and see wave action (DC-1.5Hz). I'm using a $10 aquarium air pump with a Freescale MPX5010 pressure sensor. This works well except that the signal contains a 5-10Hz cyclic signal because of the reciprocating action of the compressor diaphragm/piston assembly. This unwanted peak to peak signal is about a 50 times larger than the signal created by wave action. The compressor frequency depends on the voltage applied to the DC motor (major) and the back pressure (minor). In commercial systems it would appear that they use a large air cylinder with baffles to dampen the pressure variations. This is perhaps 200+ times the volume of a single piston stroke.
Does anyone have pointers to writing a digital low pass digital filter with a steep slope or a band block filter that can automatically detect and subtract the unwanted compressor frequency? I'm assuming that this would be similar in concept to noise cancelling headphones that subtract cyclic background sound like engines etc. from speach.