Calculate flow from pulse flow meter

ThaDutch

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Mar 2013
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Denmark
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Hey

This might have been up several times, but i haven't found an answer that solves my problem.

I have some flow meters that measures with 10 liters per pulse. This setup transfers a lot of water and therefore there will be a lot of pulses per seconds. My RSLogix 5000/CompactLogix can't follow with.

I have tried with counters and timers, but i can't really make it works.

How do i set this up so i can atleast see 10 liters/second step by step?
I know i can multiply it up so i can se m^3/h and such.
 
It measures 10 pulses per second and i really can't make it work so i get something like liters per second.

I've tried setting up an environment with a counter and two timers and the accumulated is sent to another tag with the MOV-command. This tag is used for calculating the flow.
 
Generally you need some sort of a Pulse Input card. Something that has a buffer built in to count the pulses and report back the number of pulses since the last scan.

Can you adjust the pulse width from the flow transmitter?
 
Actually i really wants to know what you guys do?

The pulse width is fixed.

This pulse-card, how many flow transmitters can be connected to this?
 
CaseyRAS said:
Generally you need some sort of a Pulse Input card. Something that has a buffer built in to count the pulses and report back the number of pulses since the last scan.
Click to expand...

Most PLC manufacturers call these devices High Speed Counter Modules. Many discrete DC input modules would be capable of reading 10 pulses per Second, if the pulse width is sufficient. However, that doesn't guarantee that the PLC would be capable of seeing every input transition, due to the inherent lag in I/O updates -- especially if the I/O module is in a remote rack. Attempting to read this pulse with an AC input module would be futile. This application would probably be best addressed with a HSC module.
 
My matey said it was CompactLogix, but it is really a ControlLogix.

There is only:
1756-CFM 4 inputs (2 per channel) 2 outputs, current sourcing.

1756-HSC 2 counters, each with 3 inputs (A, B, Z for gate/reset) 4 outputs (2 points/group).

Which means that i'll need 6 of those cards, which would be all to expensive.
 
That could save me from trouble but heres the next trouble:
How to dynamic count this?
I operate with a flow between 0 and 4500 liters/hour which means the fastest puls i can get is 7,5 pulse/min. That means that i will need to count pulses over a longer time e.g. 30 seconds, then move the accumulated pulses to a new tag and let the old be overwritten. The moved accumulated pulses will then be used for calculations and then should show the actual flow for those 30 seconds? Is this the way to do it, or?
 
Your last idea should work as long as the pulse rate is slow enough for a regular input to follow. One way to do that is use a counter to count pulses then periodically move the count into a register and reset the counter. Depending on how many counts you accumulate, you may need to do some averaging to smooth out the data.
 
My plan is not to register a flow over time, but to show the current flow.

I just came up with this:
Pulse = 1 then flow := 10/timer.acc*100; IF actual_flow > 0 THEN timer_reset :=1 ; End_if;

Don't you think that this could show the actual flow?
 
OK, Finally mentioned the flowrate.
At 4500 l/h and 10 l/pulse, you get 450 pulse/h.
That's 1 pulse every 8 seconds. That's slow. You can count that with a normal input card.

If you want to totalize and +/- 10 to 20 liters is ok, you can make that work. That's the good news.

The bad news is it's going to make a poor flow rate reading. It will be very slow to update. The fastest it can update will be at 4500 l/h and that will be 8 seconds per update. That will degrade rapidly as the flow drops. You have to settle on a minimum flow or it will never update when the flow stops. Say you pick 450 l/h as the minimum, that's 80 seconds waiting to not see a pulse before you declare the flow stopped.
 
Bit_Bucket_07 said:
Are you reading the flow rates with analog inputs? If so, then why not simply do your own integration with PLC logic?
Click to expand...

Sorry. I misread your original post. It is the rate that you wish to infer. The time between pulses is certainly directly related to the flow rate. However, as noted by mellis, you'll suffer long update delays with lower flow rates and you would need to somewhat arbitrarily determine a time out for displaying a zero flow rate.
 

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