Flow Meter Output Signals: How to Send Flow Data to a PLC, SCADA or the Cloud

A flow meter output signal is the way a meter passes its reading to another system: a PLC, a SCADA screen, a paperless recorder or a cloud platform. Picking the wrong output is one of the most common reasons a new meter "works on the display" but shows nothing, or the wrong number, in the control room. Most industrial flow meters offer several outputs, so the question is which one fits the job. The short answer: use 4-20mA for flow rate, pulse for totals, Modbus RS485 when one cable must carry several values, and GPRS when there is no cable at all.

Quick Answer

What Is a Flow Meter Output Signal?

A flow meter output signal is an electrical signal, analog or digital, that represents a measured value such as flow rate or totalized flow. The transmitter converts the sensor reading into that signal so another device can read it without anyone looking at the display.

Five output types cover almost every installation:

• 4-20mA: an analog current loop proportional to flow rate.
• Pulse / frequency: a stream of pulses, each worth a fixed volume or mass.
• RS485 (Modbus RTU): a digital serial link that carries many values on two wires.
• HART / Profibus: digital protocols used in larger process plants.
• Relay: an on/off contact for alarms and simple control.

Which Output Signal Should You Choose?

The right output depends on what the receiving system needs and how far the signal has to travel. The table below compares the five types.

Distances above are typical industry values; actual limits depend on cable, power supply and receiver.

4-20mA: The Default Signal for Flow Rate

A 4-20mA output sends a current between 4 mA and 20 mA that rises in proportion to flow. In a standard setup, 4 mA means zero flow and 20 mA means the upper range value set in the transmitter.

Example: a meter is ranged 0–50 m³/h. A reading of 12 mA means (12 − 4) ÷ 16 × 50 = 25 m³/h.

Points to check before wiring:

• Active or passive (2-wire or 4-wire). A 2-wire (loop-powered) transmitter draws power from the loop, so the PLC input or an external 24 V DC supply must power it. A 4-wire transmitter has its own supply and drives the current itself. Connecting an active output to an active PLC input, or a passive output to a passive input, is a frequent reason for a dead loop.
• Range in two places. The 20 mA value in the transmitter and the scaling in the PLC must match. A meter ranged 0–50 m³/h scaled as 0–100 m³/h in the PLC will read exactly half.
• Fault signal. Many transmitters drive the loop below 4 mA or above 20 mA (for example 3.6 mA or 21 mA) to show a fault. Set the PLC to raise an alarm instead of reading those values as flow.

A 4-20mA signal shows how fast the fluid is moving right now. Adding up that signal over time inside a PLC gives a total, but small offsets add up day after day, so the PLC total drifts away from the meter's own totalizer. For totals, use pulse or Modbus.

Pulse Output: The Right Signal for Totalizing

A pulse output sends one pulse for every fixed quantity that passes through the meter, such as 1 pulse per litre or 0.25 mL per pulse. The receiver counts pulses and multiplies by the pulse value (often called the K-factor) to get the total.

Pulse output flow meters are the usual choice for:

• fuel and oil consumption,
• batching and filling,
• chemical dosing at low flow.

In one dosing project for a water treatment plant, the required flow was only 8–50 mL/min, so the micro flow meter's pulse value had to be small enough that the PLC still received several pulses per minute. In another, an additive dosing line specified that the pulse output must be PNP to match the existing PLC inputs.

Things to set or ask for:

• Pulse value. Pick a value so the pulse frequency at maximum flow stays below what the PLC counter input can read.
• PNP, NPN or open collector. Match the PLC input type. Open-collector outputs need a pull-up resistor and a supply, wired the same way as in this mag meter PLC wiring example.
• Pulse width. Very short pulses can be missed by slow inputs or by PLCs with a long scan time.

Modbus RS485: Several Values on One Cable

Modbus RTU over RS485 is a digital link: the PLC or SCADA system (the master) asks the meter (the slave) for values held in numbered registers, and the meter replies. One twisted pair can carry flow rate, total, temperature, density, alarms and diagnostics, and one bus can connect several meters.

Settings that must match on both sides:

Wiring tips:

• Connect A to A and B to B; swapped A/B lines are the most common reason a new meter does not answer.
• Use shielded twisted-pair cable, connect meters in a daisy chain (not a star) and fit a 120 Ω termination resistor at both ends of long buses.
• Check the protocol, not only the port. Some meters offer RS485 with a manufacturer protocol as well as RS485 with Modbus. For a PLC or SCADA system, order the Modbus version.

HART and Profibus: When the Plant Already Uses Them

HART adds a digital signal on top of the 4-20mA loop, so the same two wires carry the analog flow rate and digital data such as totals, diagnostics and configuration. HART is useful when the plant already has HART-capable DCS inputs or handheld communicators.

Profibus-PA and Profibus-DP are fieldbus networks used in larger plants, often in Europe. Choose them only when the control system is already built on Profibus; otherwise, 4-20mA plus Modbus covers most needs at lower cost.

How Do You Get Flow Data to SCADA, a PC or the Cloud?

Flow data reaches SCADA, a PC or the cloud by one of four routes, depending on where the data must end up and what already exists on site.

Route 3 suits sites that need records rather than control. Paperless recorders accept 4-20mA and pulse inputs, store data and send it to a PC over RS485 or Ethernet.

Route 4 suits remote sites. A GPRS electromagnetic flow meter runs on battery and sends readings over the mobile network, so no power or signal cable is needed. When the meter has only pulse or Modbus outputs, a third-party 4G or NB-IoT gateway can read it and forward the data to a cloud platform. Check that the gateway supports the meter's protocol and that the reporting interval suits the job: hourly data is fine for billing, too slow for control.

Output Options by SILVER Flow Meter Series

The table lists the outputs available on each series. Exact combinations depend on the model code, so state the required output in the inquiry.

In past orders, steam users often chose vortex meters with both 4-20mA and RS485, so the control room gets a fast analog rate and the energy system reads totals over Modbus.

Common Wiring Mistakes and How to Fix Them

Not Sure Which Output You Need?

Send us four details and our engineers will suggest a meter and output combination within 24 hours:

• the meter type, or the fluid and line size,
• what will receive the signal (PLC model, SCADA, recorder or cloud platform),
• the cable distance and available power supply,
• whether you need flow rate, total, or both.

Contact our engineers with these details, or attach a photo of your PLC input card or wiring diagram.