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Flow Meter 4-20 mA Output Configuration: Wiring Analog Loop Signals to PLC Controllers
Quick Answer
For a standard 4-20 mA flow meter output, you normally run a two wire loop from the flow meter to the PLC analog input and connect a 24 V DC supply in series. The PLC input reads 4 mA as zero flow and 20 mA as full scale. If you have a four wire active output, you land the signal and common on the PLC input card without adding loop power.
You Need to Know the Output Type Before Wiring
Here is the thing. We see this on customer sites many times. A technician assumes every flow meter has the same output, then the PLC displays a fixed value or no value at all. The first thing to check is the transmitter wiring diagram on the flow meter label or in the manual. Silver Instruments ship every flow meter with a wiring tag inside the terminal cover.
There are two main analog output types in the field: loop powered two wire and four wire active. A two wire flow meter has only two terminals for the 4-20 mA signal and receives power from the same loop. A four wire flow meter has separate power terminals for 24 V DC or 110/220 V AC and separate signal terminals for the analog output.
Two Wire Loop Powered Flow Meter Wiring
Most engineers skip this part until the startup day. The correct loop has three elements in series: a DC power supply, the flow meter transmitter, and the PLC analog input resistor. A common supply is 24 V DC with at least 50 mA per loop. The positive terminal of the supply connects to the flow meter plus terminal. The flow meter minus terminal continues to the PLC input plus terminal. The PLC input minus returns to the supply negative terminal.
This is the simplest loop. If the PLC input is configured for 4-20 mA, it typically has an internal shunt resistor, often 250 ohm. At 20 mA the voltage drop across the input is about 5 V. The transmitter needs the remaining voltage from the 24 V supply to operate. For most Silver Instruments electromagnetic and oval gear flow meters, the minimum loop voltage is 12 V DC, so the 24 V supply works well for cable runs up to 300 metres. For ATEX Zone 1 installations, put an intrinsically safe barrier between the flow meter and the PLC input.
Four Wire Active Flow Meter Wiring
With a four wire flow meter, the 4-20 mA output is powered by the flow meter itself. You run two separate signal wires from the flow meter output terminals to the PLC analog input terminals. Do not add an external 24 V supply in the signal loop. The signal output usually shares the same negative reference as the flow meter power supply.
One issue we saw last year in a Vietnam paint plant was ground loop noise. The PLC input was reading 6.3 mA at zero flow. The cause was a difference in earth potential between the flow meter and the PLC panel. The fix was to bond the flow meter body to the same ground bar as the PLC and use shielded twisted pair cable with the shield grounded only at the PLC end.
PLC Input Card Settings
Different PLC cards use different terminal names. On a Siemens S7-1200 analog input module, you often land the signal on 0+ and 0- or 1+ and 1- terminals and configure the channel for current. On Allen Bradley MicroLogix or CompactLogix cards, the input may require a jumper between a current and voltage terminal. Check the card documentation before wiring because a voltage measurement can produce a 0 mA or overrange reading.
In practice, most current inputs accept 0 to 20 mA or 4 to 20 mA. Set the channel for 4 to 20 mA so the PLC can flag a broken wire at 0 mA. This small configuration step saves callouts later.
HART Signal and the 250 Ohm Resistor
Some flow meter loops carry a HART signal on top of the 4-20 mA current. A handheld communicator or a HART modem needs a minimum loop resistance, typically 250 ohm. Many PLC input cards already have this resistance, but not all. If the input card has a low resistance, you can add a 250 ohm resistor in series with the loop. Put the communicator across the resistor.
Silver Instruments electromagnetic flow meters often ship with HART as an optional output. We recommend HART only when the customer needs remote zero adjustment, diagnostic data, or multivariable
Specific Silver Instruments Models and Output Wiring
For a conductive liquid line from DN15 to DN200, the Silver Instruments electromagnetic flow meter works with a two wire 4-20 mA output. The water or chemical must have conductivity above 20 µS/cm for the meter to read correctly. The signal is scaled by the flow range, for example 0 to 30 m3/h. The same meter can also output a pulse for total flow to a PLC digital input.
For diesel, fuel oil, or viscous chemicals, our oval gear flow meter can supply a 4-20 mA output and a pulse output. Most oval gear meters handle 5 cP to 100,000 cP oils, but check the flow range first. A customer in Indonesia used a DN25 oval gear meter to measure diesel transfer from a day tank. The PLC read the 4-20 mA signal for flow rate and counted pulses for total litres. The important wiring detail was a separate 24 V DC supply for the 4-20 mA loop because the local panel only had 220 V AC.
For compressed air or natural gas, the thermal mass flow meter can be ordered with a 4-20 mA output scaled to kg/h or Nm3/h. Add a Pt100 input if the process temperature changes more than 10 °C during the batch. This meter often uses a four wire connection because the sensor needs more power for the heater circuit.
Cable, Shield, and Field Checks
Use shielded twisted pair cable with a cross section of at least 0.5 mm2 for runs up to 100 metres. For longer runs, use 0.75 mm2 or 1.0 mm2. Route the signal cable away from VFD motor cables. If the cable crosses power cables, cross at a right angle. On the PLC panel side, ground the shield at one point only.
Before powering the loop, measure the loop resistance with a multimeter. A dead short or open loop is the most common wiring fault. We have seen this on customer sites many times. A quick check at the flow meter terminal and the PLC terminal saves hours of startup time.
Quick Commissioning Checklist
First, confirm the flow meter output type. Second, set the PLC channel for 4 mA to 20 mA. Third, apply 24 V DC and check the loop current with a mA meter in series. Fourth, compare the zero flow reading. At zero flow the PLC should read 4.00 mA or very close to it. Fifth, apply a known flow or use the flow meter simulation function if available. Many Silver Instruments meters have a current simulation output in the menu. Set it to 4 mA, 12 mA, and 20 mA and confirm the PLC reads the same values.
FAQ
Question: Can I connect a 4-20 mA flow meter directly to a PLC without a power supply?
Answer: No. A two wire loop powered flow meter needs an external DC supply, usually 24 V DC. A four wire active flow meter includes its own signal power but still needs separate power for the meter electronics.
Question: How do I know if my PLC input is active or passive?
Answer: Check the analog input wiring diagram in the PLC manual. If the PLC input provides voltage to the loop, it is active. If the PLC input only measures current and needs an external supply, it is passive or loop powered.
Question: What happens if I reverse the 4-20 mA wires?
Answer: The PLC may read 0 mA, a negative value, or a fixed error. Reverse the two signal wires at the PLC or at the flow meter and check the reading again.
Question: Why does my PLC show 20 mA at zero flow?
Answer: The flow meter output may be configured for 20 mA at zero flow, or the PLC channel may be set to 0 to 20 mA instead of 4 to 20 mA. Check the scaling parameters in both devices.
Question: Can I share one 4-20 mA signal with two PLC input cards?
Answer: You can wire two current inputs in series if the total loop resistance does not exceed the transmitter load limit. Each input adds its shunt resistance, so check the total loop resistance value first.
Request a Quote from Silver Automation Instruments
Tell us your process fluid, pipe size (DN), flow range, pressure (bar), temperature (°C), and power supply. We will select a flow meter with the right 4-20 mA output configuration for your PLC. Call Silver Automation Instruments at +86-135-1213-1526, WhatsApp 86-135-12131526, or WeChat +86 13512131526. You can also visit flow-meter.com.au for more product details.
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