reading current pulse from turbine meter in PLC


Thread Starter

Shishir Gupta

How to read current pulse from turbine flow meter in PLC. 4 ma corresponds to Low level & 20 ma corresponds to High level.
Shishir Gupta

Rafael Becerra

I faced a similar problem last year. Unfortunely I didnt found any device to convert the 4-20 mA pulses to 0-15 VDC pulses, so I had to design a comparator circuit to do the conversion using OPAMPs, using resistors to produce a voltaje proportional to the current, and using this voltaje as input for the operational amplifier.

The circuit have 1 year working without problems. The circuit layout could be found in any National Op Amps book, Boylestad, etc...
Perhaps there is some confusion. 4 and 20 mA sounds like the pulse output from the turbine pickup to an analog 4-20 mA signal. I.e. 4-20 mA no longer has anything to do with pulse. You need an analogue 4-20 mA input.

If the output from the turbine indeed is a pulse, then I guess there is no 4 or 20 mA. To accept the pulse your PLC should use a frequency input module. If this is a regular metering application the frequency input module is simple. If it is a custody transfer application you typically have two out of phase signals and need a more complex frequency module with fidelity checking as per ISO 6551.

Jonas Berge
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Hi all,
Jonas put it correctly. If the turbine meter gives an analog output then the 4-20 mA is directly indicating flow. otherwise the 4 and 20 mA signal levels indicate the mark and space of the pulse train. Drop it across a resistor to get voltage pulses and read it via a counter module if the pulses are too fast or if accuracy is not so critical and the pulse train is slow you could even do with a digital input hooked up to it. The second approach might have associated problems with the voltage that appears due to the 4 mA (low)signal.