How to connect flow/rpm paddlewheel sensor to BRX

[PLCwannabe]

 I have a George Fisher Signet 515 paddlewheel sensor that I'm using for measuring water flow. It is self-powered, and has a sinusoidal wave output, where the hz and amplitude of the signal is directly proportional to flow. I have had this connected directly to a digital input, and it works well enough if the flow is high, but at lower flow rates the amplitude of the signal generated is not high enough to trigger the digital input. Any ideas how I could make this work? I also have analog inputs available.

[rlp122]

You might be able to run it through a diode to clip the negative half of the wave, then use a pull up resistor to try to get it into a usable range.

[PLCwannabe]

I connected the sensor to an analog input configured as +/- 10v, and triggering at 0.5 &-0.5volts, which works great at the current flow rate of 100 gpm. Getting about 20 pulses per gallon, which is exactly double what I got with the digital input. I'm thinking this is going to be generating too many pulses at about 400 gpm, so i might have to block the negative pulses. What I don't understand is how this sensor worked at all before, as the amplitude of the pulses is never more than 3vdc, according to my osciloscope.

[Controls Guy]

What a weird signal format.  You'd expect a nominal square wave (with a bit of slope and maybe a bit of ringing)

[rlp122]

What a weird signal format.  You'd expect a nominal square wave (with a bit of slope and maybe a bit of ringing)

It is pretty much the standard for paddlewheel sensors.  No idea why, but it's been this way for the over 35 years I've been dealing with them.

[Controls Guy]

It is pretty much the standard for paddlewheel sensors.  No idea why, but it's been this way for the over 35 years I've been dealing with them.

The only ones I have experience with are the little tiny Equiflo high-purity turbines (optical, like maybe 1/2" tubing size), bronze utility types (gear maybe?), and a little bit with GEMS paddlewheels.   As far as I can remember the Equiflo and GEMS were specified as square wave, but I guess it could be that they're putting out something closer to a sine wave, but at full supply voltage peak-peak, which was probably always 24VDC.  I never had any trouble, but if that's what they're doing, you could probably still count pulses without realizing they're sine-ish.