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4-20mA Water Flow Meter Electronic Transducer Telemetry Wiring Diagrams
Quick Answer: Use a two wire loop powered 4-20mA water flow meter for most industrial telemetry jobs. Connect a 24 V DC supply in series with the PLC or RTU analog input and the flow transmitter. Shielded twisted pair cable with one end grounded will carry the signal over 500 m without major noise issues.
Why 4-20mA Works for Water Flow Meter Telemetry
Most water and wastewater sites use 4-20mA because one pair of wires carries both power and signal. A flow rate of zero equals 4 mA. Full scale equals 20 mA. Open wire faults are easy to spot. If the PLC reads 0 mA, the loop is broken.
The signal is immune to most electrical noise on a water site. VFD cables and pump starters create noise, but a shielded twisted pair handles it well. A 4-20mA loop can drive 250 ohm PLC inputs and long cables without a repeater. In practice, we have seen runs of 600 m work fine with 1.5 mm2 cable.
Here is the thing. A 4-20mA loop is not a voltage signal. You are sending current, so cable resistance matters less than with voltage. That is why it works over long distances.
Electronic Transducer Wiring Basics for a 4-20mA Water Flow Meter
Most Silver Instruments water flow meters use a two wire loop design. The transmitter has a positive terminal and a negative terminal. The loop supply is normally 24 V DC from a PLC, DCS, or standalone power supply. Connect the positive of the power supply to the positive terminal of the flow meter. Connect the negative terminal of the flow meter to the positive input of the PLC analog card. Connect the negative input of the PLC card back to the negative of the power supply.
Use shielded twisted pair cable. 0.75 mm2 works up to 500 m. 1.5 mm2 works up to 1000 m. Ground the shield at the control panel end only. Do not ground both ends. A ground loop will cause a drifting signal.
Typical 4-20mA Wiring Sequence for a Water Flow Meter
For a standard electromagnetic water flow meter, terminal 1 on the transmitter goes to the positive of the 24 V DC supply. Terminal 2 goes to the positive analog input on the PLC. The negative analog input on the PLC goes to the negative of the 24 V DC supply. The shield connects to earth ground at the PLC cabinet only.
If the flow meter is four wire, the signal output uses a separate pair. Connect the signal positive to PLC AI positive. Connect the signal negative to PLC AI negative. Power the transmitter from a separate 24 V DC supply.
4-20mA HART and Remote Telemetry Wiring
Many Silver Automation Instruments electromagnetic water flow meters include 4-20mA HART. HART is a digital signal superimposed on the same two wires. You can read flow rate, signal strength, and empty pipe alarms from the transmitter without opening the terminal box.
For remote telemetry, feed the 4-20mA signal into an RTU or cellular SCADA unit. Some water authorities in Latin America use solar powered RTUs with one analog input for flow rate and one pulse input for totalized volume. The pulse output from the flow meter connects to a digital input on the RTU. The dual output gives both real time flow and total flow in the same system.
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Field Wiring Rules Most Engineers Learn the Hard Way
Do not run 4-20mA signal cable in the same tray as VFD motor cables. A water plant in Vietnam had a 20.5 mA reading on a DN200 electromagnetic flow meter. The cause was a VFD cable running parallel for 80 m. We fixed it by moving the signal cable to a separate steel conduit.
Check the loop resistance. With a 24 V DC supply, the total loop resistance must stay below about 800 ohm for most transmitters. A 250 ohm PLC input leaves room for long cable runs. If the supply is 12 V DC, the maximum resistance drops a lot. Use 24 V DC.
Most engineers skip this part, but polarity matters on loop powered devices. Reversing the wires will not damage a good transmitter, but it will not measure. If the PLC shows negative mA or no value, check the plus and minus first.
Silver Instruments Water Flow Meter Options with 4-20mA
Silver Instruments supplies several water flow meter types with 4-20mA output. The electromagnetic flow meter is the main meter for raw water, potable water, and wastewater. Sizes from DN10 to DN600. Accuracy of 0.5 percent for DN50 to DN300. Liners in PTFE or hard rubber. Electrodes in 316L, Hastelloy, or titanium. It ships with 4-20mA HART, pulse output, and RS485 Modbus RTU.
The clamp on ultrasonic water flow meter suits large pipes from DN15 to DN1000. The clamp on unit is useful when you cannot cut the pipe. This meter outputs 4-20mA and RS485. We have supplied this unit to a desalination plant in Oman and a food factory in Thailand.
For high pressure water injection or feed water, a vortex flow meter with 4-20mA works up to 250 degrees Celsius and 100 bar. Tell us your pipe size, flow range, pressure, and temperature.
FAQ: 4-20mA Water Flow Meter Wiring
Can I connect a 4-20mA water flow meter directly to a PLC?
Yes. Use a PLC analog input configured for 4-20mA. Supply 24 V DC in series with the loop. If the PLC input is voltage only, install a 250 ohm resistor across the input.
What cable type is best for a 4-20mA flow meter?
Use shielded twisted pair, 0.75 mm2 up to 500 m and 1.5 mm2 up to 1000 m. Ground the shield at the PLC cabinet only.
Why does my 4-20mA signal read below 4 mA?
This usually means an open loop, bad terminal, or sensor fault. Check the power supply and wiring first. HART diagnostics can show open circuit faults.
Is 4-20mA or pulse output better for water telemetry?
Use 4-20mA for flow rate and pulse for total volume. Most Silver Instruments water flow meters include both, so you can wire both to the RTU at the same time.
Can I use a 4-20mA flow meter on a solar powered remote site?
Yes. The loop uses very little power. A small 24 V DC solar system with a battery works. Add a surge protector if the site is in a lightning prone area.
Send us your pressure in bar, temperature in °C, pipe size in DN, and flow range in m3/h. Include your cable distance and controller brand. Our engineers will recommend a water flow meter with a wiring diagram for your PLC or RTU. Contact Silver Instruments by phone at +86-135-1213-1526, WhatsApp at 86-135-12131526, or WeChat at +86 13512131526.
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