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Old October 2nd, 2007, 12:55 PM   #1
TimD
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Pulse Count - not exactly what flowmeter says it should be (Logix 500)

Hello All.

I have a flowmeter with a pulse module. The flowmeter generates one pulse per gallon. I am reading approx. 5GPM. The pulse width is approx 2-sec.

I am sending 24v through the meter's pulse module, every pulse is 'seen' by the controller and through a one-shot, recorded. I am using an ADD block because it's count resolution is 9,999,999

I created a counter (see above) with help from this thread yesterday that works perfectly. I set my totalizer to match the meter and for 10 minutes the count was absolutely perfect, however, I let the controller count away overnight, when I showed up this morning, my gallon reading on the flowmeter was off from my plc version by almost 5% (flowmeter was ahead).

My initial thoughts were perhaps I need to execute the pulse read code more often than the rest of my ladder code? The manufacturer (GWF) swears that the mechanics and pulse module are dead-on.

My program is about 90 lines of ladder, no subroutines, two PID loops (0.30 sec rate).

If anyone can shed light on my dilemma, please do! Any help appreciated.
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Old October 2nd, 2007, 01:20 PM   #2
TimD
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Quick update, I studied the flowmeter intensely, it appears that the wheel which is marked: 0 1 2 3 4 5 6 7 8 9 is 'slipping'. Approx every 5 revolutions, I notice it sticking for a fraction of a second. I think this might be the issue. Funny, both flowmeters are doing this to a degree.

Anyone with similar experience, please chime in, I'd like some backup to tell my manager!!!

Thanks
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Old October 2nd, 2007, 01:23 PM   #3
averytc
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Tim,

What card is this terminated on, and is the flow relativley constant?

With a 2-second pulse width the flow would be missed if it exceeded 30 gal/min.
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Old October 2nd, 2007, 01:25 PM   #4
averytc
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Is this a roto-meter? We used to use those for water, but replaced them with mag tubes because of problems like that.
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Old October 2nd, 2007, 01:33 PM   #5
TimD
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averytc,

The flowmeter is terminated into channel 0 (DI) of a MicroLogix 1500 controller. I have two meters hooked up (0, 1) with two more (2, 3) planned.

Yes, the flow is constant and the pulse module's max pulse train is 22GPM (22 pulses per-minute). I'm pretty sure this does not require a high-speed pulse card, besides, my pulse maximum is around 5 per-minute.

I'm worried about the slipping of the pulse count wheel.

I'm wondering if this is mechanics, or because I have 24v applied.

Tim
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Old October 2nd, 2007, 01:44 PM   #6
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One question I would have:
Does the pulse get shorter in duration as flow rate increases.


If I understand your observation about the skipping or slipping You would need to determine if this could cause the flow meter register to still peg and NOT generate a pulse. That doesn't seem likely but I doubt that the meter could be "perfectly acurate" with this "slippling" or "skipping" going on.

Stationmaster
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Old October 2nd, 2007, 01:47 PM   #7
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I agree with Stationmaster. I'm not sure how the 24v power would hamper anything. It realy sounds like a meter error.
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Old October 2nd, 2007, 02:14 PM   #8
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I have seen A/C voltage damage REED type switches by demagnetizing them over time, I'm hoping you are using 24vdc.

Contact "bounce" can cause problems with accuracy, it is actually quite a common phenomenon. BUT that would cause EXTRA pulses and therefore your PLC would read HIGHER than your meter.

If your meter has an electro-magnetic pick-up, perhaps it is causing magnetic drag on the actuator. Does the meter info include instructions on maximum excitation voltage?

Stationmaster
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Old October 2nd, 2007, 05:06 PM   #9
TimD
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Hello All,

Thanks for all the suggestions.

I am using 24vdc, I'm glad that is acceptable.

What I have found in recent research is that roto flowmeters are less accurate at the lower end of the pulser's flow rate scale. My meter measures 0-22GPM, but I am on the 5GPM scale. Lesson learned!

Indeed, the pulse wheel part of the meter, mechanically, is operating incorrectly.


Thanks again.

Tim
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Old October 2nd, 2007, 09:55 PM   #10
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That rate of pulses shouldn't be an issue with the scan time of a 1500. However, this IS AB we're talking about! It looks like that at your max of 22gpm, that's 22 ppm, or about .33 pps. So, you should get a pulse every 2 and some change seconds at max flow.

I still wouldn't trust it!

Look into setting up your counter routine in a STI section, using an interval of 500 ms or less. Maybe use IIM. You should never miss a pulse.

I try to keep time-specific stuff clocking separately from program scan times; yes, scan is fast, but you will miss things when dealing with frequency type inputs.

ETA - that pulse width of 2 seconds doesn't allow for much trailing edge time before the next pulse at full flow. Is it adjustable?
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