Band pass/block filter for a bubbler pressure level sensor

Other Hardware Sensors
1

I've built one of these

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.

Mike

2

Have seen a few posts on a topic, can't find in bookmarks. Search returns recent post:
http://arduino.cc/forum/index.php/topic,115139.0.html
and one more: Google Code Archive - Long-term storage for Google Code Project Hosting.
Personally, I'd run FFT, especially when interference isn't strictly defined, than check output and decide on pass-edge, stop edge, filter order , FIR/IIR, do I need a filter or continue using fft ?
Click on my signature, it's linked to arduino FFT code.

3

The goal is to measure absolute level and see wave action

I know you want a band pass filter but until you make one, here are a few more things you can try:

  1. Run the pump at a very slow speed. Your goal does not require a high flow rate.
  2. Pinch the tube or use an orifice to restrict the air flow to a bare minimum.
  3. Put a large volume between the pump and the pressure sensor. It doesn't need to be absolutely airtight. Anything will do from a plastic bag to a bottle or can. An empty bong would work great if you have one.
  4. For the vertical portion of the tube, use as large a diameter as feasible.
  5. Soft silicone tubing would dampen vibrations better than hard PVC tubing.
  6. A pump-up garden sprayer would work great if you have one. No unwanted pulsations.
  7. Depending on the depth of the water, you might be able to use a hair dryer for the air pump.
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