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Flow Meter with Analog Output Setup: Calibrating 4-20mA Current Loops for Remote PLC Integration
Quick Answer: A 4-20mA loop sends one analog value from the flow meter to a remote PLC input card. Calibrate the loop by checking the 4mA zero point, the 20mA span point, and the PLC scaling. Record the flow range, engineering units, and loop resistance before installation so the analog output matches the process value.
Most process plants in Southeast Asia, the Middle East, and Latin America still choose 4-20mA for flow meters because the signal survives long cable runs. A shielded twisted pair can carry a 4-20mA signal 500 m to 1000 m without losing the measurement. Digital buses work well, but field wiring and spare PLC cards are simpler with analog current loops.
Hardware You Need for a Clean 4-20mA Loop
Use a multimeter with mA range, a 24 V DC loop power supply, a precision 250 ohm resistor for voltage input PLC cards, and the flow meter configuration manual. If the meter has HART, a HART communicator or the local display pushbuttons can set the 4mA and 20mA trim. For hazardous areas, check the meter rating before opening the enclosure. Silver Automation Instruments supplies flow meters with ATEX Zone 1 and Zone 2 approvals for oil and gas sites.
A 4-20mA loop can be powered by the PLC input card or by an external supply. Most electromagnetic flow meters and Coriolis mass flow meters from our factory use separate terminals for 24 V DC power and the 4-20mA output. For example a DN15 electromagnetic flow meter may have terminal 1 and 2 for power and terminal 3 and 4 for the loop. Check the wiring diagram. Do not connect the loop negative to earth at both ends because ground loops create offset errors.
Step by Step Loop Wiring Tips
First measure the total loop resistance. A typical PLC analog input has 250 ohm input impedance. Add cable resistance plus any barrier or isolator. The flow meter analog output has a maximum load, often 500 ohm or 750 ohm. If total resistance exceeds that limit, the loop cannot reach 20mA. Most engineers skip this part and then see a clipped signal at high flow.
Wire the flow meter analog output in series with the PLC input. Use shielded twisted pair cable. Ground the shield at the PLC cabinet only. Keep 4-20mA cables away from VFD motor cables and power lines. A paint manufacturer in Southeast Asia had a Coriolis meter reading drifting by 3 percent because the signal cable ran close to a mixer VFD cable. The fix was 200 mm separation and grounded shield at one end.
Zero and Span Calibration
For a new flow meter, the factory sets the 4mA value to zero flow and the 20mA value to the full scale flow. Write down these values before installing. If the PLC range is set to 0 to 100 m3/h, the flow meter must match the same 0 to 100 m3/h range. A mismatch in engineering units is the most common scaling error we see on customer sites.
To calibrate zero, stop the flow and wait for the meter to settle. For an electromagnetic flow meter the pipe must be full. For a Coriolis meter the process temperature must be stable. Use the local display or HART communicator to force the analog output to 4.00mA. Measure the current in series with a multimeter. If the reading is 3.95mA or 4.05mA, adjust the 4mA trim until the meter output equals 4.00mA. Repeat for span. Force the output to 20.00mA and adjust the 20mA trim.
Do not confuse zero flow
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Scaling the PLC Input Correctly
In the PLC program, set the raw counts for 4mA and 20mA. A typical PLC input card has 0 to 27648 raw counts for 4-20mA on a 16-bit channel or 0 to 4095 on a 12-bit channel. Map 4mA to zero flow and 20mA to the full scale. If the flow meter full scale is 250 kg/h, set 20mA to 250 kg/h. If the PLC expects litres per minute, convert the range before commissioning.
One water utility in Vietnam replaced a mechanical meter with a Silver Instruments electromagnetic flow meter on a DN100 line. The PLC showed 60 m3/h when the actual flow was 42 m3/h because the PLC range was set to 150 m3/h but the flow meter was set to 100 m3/h. Scaling the PLC range fixed the reading without any wiring change.
Analog Output Options on Silver Instruments Flow Meters
Our Coriolis mass flow meters, electromagnetic flow meters, vortex flow meters, and thermal mass flow meters can ship with 4-20mA output. Many models also include HART, pulse, or Modbus. The analog output is usually passive or active. An active output supplies loop current. A passive output requires an external 24 V DC supply. Confirm this during ordering because it changes the PLC wiring.
For fuel oil with viscosity up to 20 cP or seawater with conductivity of 45000 µS/cm, the analog output remains stable. Thermal mass flow meters from Silver Automation Instruments use a PT100 sensor for gas temperature and still transmit mass flow through a 4-20mA analog output. Pressure transmitters from Silver Automation Instruments also use the same 4-20mA wiring, so your maintenance team can use one calibration procedure across flow and pressure loops.
FAQ: 4-20mA Flow Meter Calibration
Q1: Why does the PLC show negative flow when the pump is off?
A: The zero trim is set too low or the pipe is not completely full. Check the fluid condition first, then force 4mA and measure the loop current.
Q2: Can I use a 250 ohm resistor to measure 1 to 5 V from a 4-20mA loop?
A: Yes. Connect the resistor in series with the loop and measure voltage across it. 4mA gives 1.00 V and 20mA gives 5.00 V. Use a precision resistor with 0.1 percent tolerance or better.
Q3: How far can a 4-20mA loop run?
A: Typical distance is 500 m to 1000 m with shielded twisted pair cable. The limit depends on loop resistance, wire gauge, and the flow meter maximum load rating.
Q4: What does a 3.5mA signal mean on an electromagnetic flow meter?
A: Many units use 3.5mA as an empty pipe or fault alarm. Check the pipe fill status and the error code on the local display.
Q5: Do I need HART to calibrate the 4-20mA output?
A: No. You can calibrate from the local display on most Silver Instruments flow meters. HART makes remote trim and diagnostics easier, but it is not required for a basic loop setup.
Get a Quotation Based on Your Process Data
Send us your pressure (bar), temperature (°C), pipe size (DN), and flow range. We will confirm the analog output type, loop power requirement, and calibration range for your remote PLC. Tel: +86-25-68650347, WhatsApp: +86-25-52155837, WeChat: +86 15365082610. Mention your PLC input card type for faster selection.
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