Seametrics FT450 pulse output for flow, having issues

defcon.klaxon

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Feb 2015
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Hi all,

To make a long and annoying story short, a client has two flowmeters in their water plant and the flowmeter that controls the CL2 dosage pumps stopped working. They have a second flowmeter that is local readout only, so they asked me to make the system run on that one instead. Unfortunately the Seametrics FT450 does not have 4-20mA outputs, only pulses.

First off, their system of K-factor and Set P for setting up the pulse output is utterly confusing, I don't see how they relate and their tech support was not exactly helpful. If anyone has experience with these, please let me know.

Second, I'm amazed that the pulse output is maxed at 6.5Hz; not kHz, Hz. Unfortunately the PLC I'm working with is an ICLinks Everest and it does have a spare digital input with high speed counting, but it's meant for *high speed* and the Rate measurement tag is an integer. So my concern is that the resolution is going to be absolute garbage. I'm working with ICLinks tech support but I don't have a good feeling about making this work, from what I've read thus far.

That all being said, is there some sort of device that'll take such a low max freq pulse and create a 4-20mA signal out of it? Unfortunately the client is a tiny, tiny rural system and they have very little money; the notion of replacing the transmitter unit with a 4-20mA capable unit is likely going to make them cringe, even though it's clearly the easiest and most straightforward solution.

Thanks guys!
 
Last edited:
6.5 Hz gives you a pulse every 153 milliseconds. If the PLC scan time is fast enough you might be able to get away with measuring the time between pulses and calculating your flow rate from that.

Or else, rather than varying the speed of the dosing pump to maintain a continuous flow of CL2, could you trigger a dose of a fixed volume every time you accumulate some number of pulses.
 
The "K" factor is the number of pulses per what ever so for example if in litres the senor will be usually marked as something like 54.57 pulses per litre although you can only read whole pulses given a dose of 100 litres this would equate to 5,470 pulses so pretty accurate. normally the pulse output would be litres or gallons etc. we used similar systems, however, I removed the controller and used the actual flow sensor directly into a high speed input so effectively got 50 odd pulses per litre we used the PLC rather than the unit & wrote a calibration routine so rather than rely on the pulses being as stated on the sensor ("K" factor) we would dose a known amount & then enter the value & it would calculate the K factor.
Using pulses for flow rate is not ideal.
 
Seametrics makes an interface that can give you a 4 to 20mA output. I think it was the FT420, but google tells me that they have upgraded to FT430. I have used the pulse output from their insertion magmeters directly into a Micrologix with good results, and some of their meters can deliver much faster frequency outputs. I think the FT450 you have is battery powered which might be one reason why the pulse frequency is limited.

To get a 4 to 20mA signal from the transmitter, I bet you are looking at around $400 and if you don't need a display, you might do just as well for less money with a generic signal conditioner.

What is the part number of the actual flowmeter? You might get a much higher pulse rate right out of it by skipping (or wire in parallel) with the FT450.
 
To get a 4 to 20mA signal from the transmitter, I bet you are looking at around $400 and if you don't need a display, you might do just as well for less money with a generic signal conditioner.


I've had great success using an Acromag TT239 for frequency to 4-20mA conversion. Looks like they offer a TT339 model as well. Very easy to configure with their free software, but you'll need a Mini (NOT Micro) USB cable to connect to the unit.
 
Thanks for the replies, all. Long story short, the client shockingly suggested to ME that they want to get a loop powered transmitter. SO much easier, I'm thrilled.
 

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