I have been trying to detect a very low frequency, very low strain signal with different types of piezos (hobby type, hobby type enclosed in plastic casing, shielded contact mic type, and flexible piezo film type). The signal is generated from a small stream of water passing through a pipe 2" in diameter.
If I put these sensors on the pipe, and measure the AC signal directly with my oscilloscope, I cannot detect a signal. Although I have not been able to do a frequency analysis of the water pipe, I would guess that the water passing through the pipe at the frequency I'm trying to detect to be <10Hz. Additionally, the water passing through the pipe would create what I'd call "very low strain."
If I turn the water up to a "high" amount, the signal is easily detectable -- the frequency of the water is what I'd estimate 100 - 200 Hz and there is more strain on the pipe.
I have also created voltage amplifier to see if I can amplify this (undetectable) low strain signal I mentioned at the top of this post. The amp seems to only amplify noise or interference.
In short, I think that the piezos I have cannot detect a low strain <5Hz signal.
Can anyone recommend a way to detect <5Hz low strain signal? It appears ultrasonic sensors could be a solution for this problem.