Sinking 12V through a Reed switch into a discrete input module (NI 9425)

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Joined Mar 14, 2016
We have successfully had a system fielded for about a year now, and one of its several functions is to measure product flow through its channels. Given the application, chemical compatibility, cost, size and overall flexibility, I chose a Reed switch actuated flow meter and am sinking it into a National Instruments 9425 discrete input module, which is basically an I/O module for a PAC system called a cRIO 9068. I am using period-based calculations on the FPGA to derive a tick count and go from there, utilizing a known K-factor to calculate rate. The system is pretty accurate and has measured within manufacturer specified tolerance when compared to a Weights & Measures calibrated positive displacement flow meter that is far better.

Over time, especially recently, we have seen the flow meters get returned to us with tags describing two particular issues:

1.) Massive GPM rate (well beyond the range of the meter)

2.) No function

I recently tested a flow meter tagged "Broken, massive GPM" and replaced the side of the shell containing the Reed switch with a new one from the manufacturer. It worked correctly thereafter.

My question is... am I protecting my Reed switch well enough? Do I need some kind of debouncing circuit on the physical side to protect triggering and inherently filter it in the process? If so, I am not sure what those values for R1,R2 and C1 would be.

I don't drive inductive loads. I don't heat lamps. The length of cable (containing 22AWG 2 conductor) is, at most, 20 feet each way to the meter. This obviously isn't an A/C circuit. The 9425 has an input impedance of 30kOhms +/- 5% and I additionally supply a 1kOhm in series for good measure. This is a 12V power supply with a common ground. The flow meter manufacturer is very poor on communication, but has more or less stated this is fine. I question their tech support.

I assume the 10V (ON state requirement) / 30kOhm input impedance = how the minimum input current is derived? (330uA)

See attached image (apologies for the quality.)

Thank you all,