I've a 4-20mA Pressure Transmitter which is already installed and connected to a PLC. I need a copy of its value to connect it to another PLC. I don't want to connect something like signal duplicator inbetween the sensor and the first PLC. I'm looking for a clamp mA sensor that measure the current and send the same value throw wires.
Get out your wallet. The Fluke 773 is a clamp-on 4-20mA sensor with a re-transmit 4-20mA output.
I can't afford one, so I'm not sure whether the re-transmit output is powered by the Fluke battery or whether it is loop powered. When I win the lottery, my first purchase will be a Fluke 773.
you can wire in series if you plc uses isolated 4-20 ma inputs, otherwise you'll need a signal isolator.
you can also retransmit the signal from the first plc to the second if it has 4-20 ma outputs.
You can use a HART loop converter like one of these:
This will enable you to tap into transmitter signal without disconnecting the transmitter from first PLC (if this is your only concern).
All you need is to connect the converter input leads in parallel with the transmitter, which is just like connecting a HART configurator on the transmitter. The loop will remain intact and signal will be duplicated by the converter.
Thanks, is it safe to put an isolator or duplicator in between the transmitter and the plc? since my signal is critical one? I don't have spare A/O to use.
If this is involved with a safety shutdown system, you need multiple sensors (redundant) and careful coordination with process safety devices.
At a minimum redundant sensors are required, but in extreme cases quintuple redundancy is mandated, with an equal number of PLCs's checking each others logic and voting.
Don't save a penny and blow the plant.
Don brings up a good point.
I'm assuming it's not part of an ESD system, and you just need to get a reading for some non vital reason. I suggested adding resistors in series with the potential measuring wires so there's no way you can load the original signal down.
>If this is involved with a safety shutdown system, you need
>multiple sensors (redundant) and careful coordination with
>process safety devices.
>At a minimum redundant sensors are required, but in extreme
>cases quintuple redundancy is mandated, with an equal number
>of PLCs's checking each others logic and voting.
>Don't save a penny and blow the plant.
> I don't want to connect something like signal duplicator in between the sensor and the first PLC.
Ok then, not "IN Between" but in parallel. Get a signal isolator with 1 - 5 Volt input and connect it across the first PLC input terminals. if you are worried about loading down the input stick something like a 1K resistor in each of the wires you add to the terminals. So now you are measuring the Voltage across the PLC terminals just as you would with your multimeter if you were troubleshooting an operating 4-20 mA loop and couldn't risk opening the circuit to put a milliamp meter in series.
The signal isolator you use could be powered via the output terminals from the remote PLC or locally from 24/120, just make sure the input is high impedance and isolated.
Thanks all for your kind suggestions.
The signal I'm trying to duplicate is the Natural Gas Pressure at the inlet of F9 GT (FPG1).
How about using a clamp leakage current sensor, like the one shown in this link (if there is a good brand with suitable range)?
I see no reason why this transducer shouldn't work but I think it's overkill. A conventional 4-20 mA signal isolator will work just as well. Use one that will not effect the integrity of the primary loop if the secondary loop powers down or becomes open circuit.
I was suggesting something similar by measuring the Voltage across the Primary 4-20 mA input resistor, adding resistors to the leads ensures that whatever happens to the secondary e.g. a short circuit there is no effect on the primary. It could be done safely on the fly; whereas to add your transducer you have to break the circuit to thread the wire through the transducer.