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Convert Pulse Signal to 4-20mA Signal

Convert Pulse Signal to 4-20mA Signal

Convert Pulse Signal to 4-20mA Signal


bear with me as i'm a gearhead . . .

there is a fuel meter that measures gas flow (acf-actual cubic feet) and with an internal p & T sensor, which converts acf to scf (standard cubic feet).  the meter output signal has 3 FULLY PROGRAMMABLE PULSE OUTPUTS
Corrected, Uncorrected, and Alarm/Fault:
Loop Voltage: 5-15 Vdc
Loop Current: 10 mA maximum
Pulse Width: 62.5, 125, 187, or 312 ms
On/Off Resistance: <10_/> 10MΩ
Isolation: 2,500 Vdc

for a user configurable setting of 1-pulse output signal per 10 scf (1, 10, 100, & 1,000-scf are the choices) gas flow, pulse width of 125-ms, and max gas flow of 500-scfm, this output signal needs to be converted to a 4-20mA signal - input signal to AB Control Logix PLC.

an API 4380 (http://www.api-usa.com/pdf/api/apd4380.pdf) is being considered to convert the signal.  the concern is with the pulse output signal, it will result in a fluctuating or pulsating 4-20mA signal which MAY result in an erratic display.

an option being considered is an input card for the PLC, but this MAY be a little more expensive.

please advise on other options to consider or how best to convert the pulse output signal to a steady 4-20mA input signal to PLC.  the meter OEM does not provide such a device.


RE: Convert Pulse Signal to 4-20mA Signal

The PLC digital input cards can scan pulses up to a certain rate. I'm not sure what that rate is for the ControlLogix but it won't be all that high. Above that rate you'll need a high speed counter card.

If we learn from our mistakes I'm getting a great education!

RE: Convert Pulse Signal to 4-20mA Signal


The device you are considering does not meet your needs. It will convert an analog signal of one form into an analog signal of another form. It does not convert a digital signal pulse width into an analog signal. I do not know of a device that will; perhaps someone else does. I would do the calculation in the PLC.

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RE: Convert Pulse Signal to 4-20mA Signal


That was my first thought too, but the pulse origin is a meter which gives a pulse per unit flow: there's no guaranteed minimum pulse rate. This is a totaliser application.

If we learn from our mistakes I'm getting a great education!

RE: Convert Pulse Signal to 4-20mA Signal

Since the pulse is used in a totaliser function, I think you will need to use an output that will pulse slow enough to be accurately recognised by the PLC.  If your maximum gas flow is 500scfm, then the 100scf pulse output will give you a pulse every 12 seconds maximum.  You could then use an algorithm in the PLC to decode the gas rate by looking at the elapsed time between pulses.  If you use the 10scf output, then your PLC needs to detect a pulse every 1.2 secs which may also be feasible.

RE: Convert Pulse Signal to 4-20mA Signal


thanks for the postings!

i'm evaluating the phoenix product suggested by SteveWag.

i am aware of that the PLC input card is the preferred method and the PLC input card and the 1140a Phoenix product costs will be compared (if evaluation reveals product to be acceptable).

this meter will be used for current engine flow consumption and as a totalizer.

there is another means available to obtain the 4-20mA signal; however, this means additional cabling (cable between bldgs) and another device (Fisher 809 ROC which serves as a different function.  like to keep unit functions separate from plant functions in processors.).

the 10scf setting will most likely remain as there are transient flow conditions that need to be continuously monitored.

i shall post again as to what will be done.


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