# Low Frequency Input

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I am trying to use a pulse flow meter to measure the drain flow of a hydroponic garden system. We are wanting to use this to alarm if we have low flow in certain parts of the system. This is a low budget project so I am having to work with what I have. My V130 plc only has two high speed counter inputs and I would like to measure the pulse flow in four different areas of the system. So my question is, is there a way to read the low freq pulse of a regular digital input? The freq of this input would be no more that 20 pulses per second. I have been able to use a counter to count the pulses but I am not sure how to divide this number by a period of time.

By the way I am new to programming. This is more or less a project for me to get some hands on practice.

Thanks,

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• MVP 2023

Hi and welcome,

20 Hz is well within the capabilities of a standard DI.  I wouldn't even bother with using the high speed inputs.  How you go about the count depends on what time period you want to use.  If you work on one second, base things around using a positive transition of SB3 as your one second timer.  Count the number of pulses of each input by using a positive transition of the relevant input using "+ increment" into a "master" MI for each input. When SB3 recycles/turns on, store the number in that master MI into another MI, and then reset the master MI to 0.  The master MI then counts up again, and you can do maths on the copied MIs to get your result.  This count is based on the number of pulses per second so it lets you easily work out the flow rate per second/minute etc.

But given that the max input frequency is going to be 20Hz, perhaps the time period should be higher for greater accuracy.  Maybe 5, or 10 seconds.  Only you will know what suits your setup the best.

cheers,

Aus

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• MVP 2023

A V130 will probably always scan in less than 10 ms, so 100 Hz is safe assumption for any digital input.

If you use Immediate Inputs in a 1.25 ms Interrupt subroutine you can get 400 Hz response on a V130, with the caveat that it only works on local I/O (not on expansion modules).

I've done this with water flow meters myself.  It works.

Joe T.

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