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mckeand13

Pulse input from flowmeter - How to?

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In looking at various paddle wheel type flow meters I notice a lot of them have a pulse type output. Literature generally says this pulse output can be brought directly into a PLC. This is new territory for me so I'm just trying to get up to speed by learning the basics. - What type of input card would be needed (micrologix platform)? High speed? - How would this type of input compare with an analog input (4-20) in price? I'm not looking for hard numbers just an idea. Most of the flow meter manufacturers have an additional unit that will change the pulse into a 4-20mA. - What is a K factor, and how is it used in programming? If anybody has any links that I could read up on that would be great also. Thanks!

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Do you want to measure flow (rate) or volume? What frequency of pulses do you anticipate? If you can use either a normal or high-speed input on the micrologix it will be cheaper than adding analogue. Pulses will give you accurate volume data from which you can estimate flow. Analogue will give you accurate flow data from which you can estimate volume. K factor would possibly be volume/pulse.

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I want to measure flow rate. That could be done with pulses by counting pulses over a given time, dividing by the time, and then continuously moving that number to another location? Why would one be more accurate than another? I don't know what the frequency of the pulses is. I'm sure it varies based mainly on ID of the flowmeter & various other manufacturing specs. I'm just looking for somebody who's done this before to describe the methodology.

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An analogue signal will give you instantaneous flow rate - if coming directly from the flow transmitter. If the analogue is derived from pulses, then it is an average.

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Never used a pulse output meter with a ML, but I've done lots of applications with SLC's and PLC-5's. Generally speaking for these platforms on a per point cost basis, pulse inputs are about double the cost of an analog input. I primarily use the 1771-CFM or 1746sc-CTR4 cards for pulse meters. Pulse counting is the only way to ge accurate volume measurment, these cards also measure the frequency of the pulses, this can be used for real time flow rates, and in my opinion is just as accurate as the flow rate through an analog input. I know the ML has a high speed counter, but I suspect there isn't a frequency register, so for your application analog might be best. K factor, usually the meter K factor is used in scaling the pulse counter. Generally speaking the counter value is divided by the K factor. So if you were looking for gallons and the meter pulse represented 0.01 gals/pulse, the K factor would be 0.01, so if you had 100 counts, divided by the K factor, would be 1gallon of flow. Some high end meters allow you to adjust the K factor in the meter, the higher end meter input cards also have K factor adjustments.

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Hi, We have several of these pulse type meters in use. We use a PLC-5. All of ours also have the additional control unit from the manufacturer. The add on controller counts the pulses and sends a discrete singal to the PLC when a set point has been reached. We don't use them for flow speed readings. We just use them for volume. Our brand is Accurate Meter. Some of them will put out 12 pulses per rev of the wheel. Others put out less. The "K" factor is a factory calibration variable that will be different for each meter. The "K" factor basicly lets the user know how much volume has passeed thru the meter for a given (usually 1) amount of pulses. On a different machine we have some "F/I" converters that are cheap (<$150). These units take in a frequency signal from a proximity sensor that is sensing gear teeth and convert the rate into a 4-20ma signal. They sound like a good fit for your application. I can't say what the cost of a factory matched control unit costs, even though we have replaced several, I haven't seen the paperwork. I would guess that they are >$1000. There is a lot of functionality you get for the price though. Different modes of operation, 8 digit readout etc. All of which can probably be replaced by a micrologix if so desired. Flow measuring is a huge field. The options are many. As accuracy / reliabality goes up so does the price. GoodLuck BD

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I use a ML1200 with the input filter set at 1ms. We normally have around 30-60 pulse/sec on most of our units. I have done some testing with a frequency generator and was able to read upto 9000 pulses/sec using an Event Input Interupt and a input filter of 25us. Edited by mvandyke

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