Flow Meter Pulse Output vs 4-20mA: Which to Use and When

Direct answer: use the pulse output when you need an accurate total (litres, m³ consumed per shift or batch), and use 4-20 mA when you need the live flow rate for control, trends or alarms.

Pulses are counted, so a total built from them has no integration error beyond the meter’s own K-factor. A total built by integrating a 4-20 mA signal accumulates the small offset, scaling and sampling errors of the analog chain. Many plants wire both: pulses to a counter or totaliser, 4-20 mA to the PLC loop, and a separate flow switch contact for a no flow alarm.

Who this is for: instrumentation and utility engineers wiring water, cooling water, fuel oil, chemical dosing or effluent meters into a PLC, panel totaliser or energy monitoring system.

The two signals in one line each

• Pulse (frequency) output: one pulse per fixed volume. Pulse frequency is proportional to flow rate; pulse count is proportional to total volume.
• 4-20 mA output: a current that maps the flow range linearly, 4 mA at zero (or range low) and 20 mA at full scale. The live-zero 4 to 20 mA range is standardised in IEC 60381-1:1982 (IEC webstore).

K-factor: turning pulses into litres

The K-factor is pulses per unit volume, given on the meter’s calibration certificate. A KEP application note puts it simply: the certificate gives the average K-factor, for example 200 pulses per US gallon or 150 pulses per litre, and that number is entered into the totaliser (KEP, Calculating and Using K-Factors).

Worked example (illustrative): a cooling water meter has K = 50 pulses per litre.

• Total: 1,250,000 pulses counted over a shift ÷ 50 = 25,000 L = 25 m³.
• Rate: pulse frequency 125 Hz ÷ 50 = 2.5 L/s = 9 m³/h.

Field check of K, as Seametrics describes: reset the total, set K = 1, run a known volume into a container, read the pulse count, divide pulses by volume (Seametrics field calibration bulletin). For example 500 pulses into a 10 L can gives K = 50 pulses/L.

Some meters, such as turbine meters, have a K-factor that varies across the flow range. Meter manuals state linearity; one turbine sensor manual lists ±3% of reading linearity and ±0.5% repeatability (GF Signet 2100 manual). If that matters, use a totaliser with multi-point K-factors.

4-20 mA scaling

Rate = range low + (mA − 4) ÷ 16 × (range high − range low).

Example: 0 to 50 m³/h range, signal 12 mA. Rate = (12 − 4) ÷ 16 × 50 = 25 m³/h. You can check your own numbers with our 4-20 mA loop scaling calculator.

Why integrating 4-20 mA drifts

To get a total from 4-20 mA, the PLC samples the rate and adds rate × time each scan. Errors add up:

1. Zero offset: a meter at true zero sending 4.05 mA reads 0.05 ÷ 16 × 50 = 0.16 m³/h. Over 24 h that is about 3.75 m³ of “flow” that never happened. Use a low flow cut-off.
2. Span error: analog input and transmitter tolerances scale every reading.
3. Resolution: an input with limited counts over 4 to 20 mA quantises the rate.
4. Sampling: if flow pulses or changes fast between samples, the sum misses it.
5. Restart loss: a PLC restart without retained memory can reset the total.

Pulses avoid points 1 to 4 because each pulse is a counted, fixed volume. They still need retained counters and protection against missed pulses.

PNP, NPN and open collector outputs

Pulse outputs are typically open collector (NPN, sinking), PNP (sourcing), or a passive contact (reed). The input must match:

• NPN / open collector: the output pulls the line to 0 V. The input must be a sourcing type (it supplies current that the output sinks), or you add a pull-up resistor to supply. Check the meter’s maximum switching current and voltage.
• PNP: the output drives the line to supply voltage. Use a sinking type PLC input (current flows from the output into the input).
• Reed or mechanical contact: low frequency only, and contact bounce needs input filtering.

IEC 61131-2:2017 defines Type 1, Type 2 and Type 3 24 V DC digital inputs, each with its own on and off voltage and current limits; read the exact thresholds from your PLC input datasheet (IEC webstore). A pulse output has to cross both thresholds cleanly.

Maximum frequency

At full flow, pulse frequency = K × max flow. Example: K = 50 pulses/L, max flow 5 L/s gives 250 Hz. Check three limits:

• The meter’s maximum output frequency and minimum pulse width
• The input’s maximum counting frequency (ordinary PLC digital inputs are filtered; a high speed counter input may be needed)
• The input filter time, which must be shorter than the pulse width

If frequency is too high, many meters let you set a pulse scaler (one pulse per 10 L). Each pulse then represents more volume, which lowers total resolution but stays accurate.

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Cable length and noise

• 4-20 mA is a current loop, so voltage drop along the cable does not change the reading as long as the loop supply covers transmitter minimum voltage plus load and cable resistance. That makes it tolerant of long runs. Check the loop voltage budget on the transmitter datasheet.
• Pulse signals are voltage edges. Long cables add capacitance that rounds edges and can pick up spikes from VFD and motor cables, causing false counts. Use shielded cable, earth the shield at one end, keep it away from power cables, and use input filtering suited to the max frequency.
• In a steel plant or rolling mill bay with large drives, route signal cables in separate trays from power. In process plants with long field runs, 4-20 mA for rate is often the easier signal to keep clean.

Use both, plus a flow switch

A common pattern in Indian utility and process plants:

The flow switch is a separate discrete device (or a configurable meter relay output) so that a loss of cooling water trips a furnace or compressor even if the PLC analog chain fails. Wire it fail safe: contact closed when flow is healthy.

If the meter supports a fieldbus or Modbus output, it may give rate and total over one cable. That is outside this guide’s scope.

If you want a panel display for this, the Em Bose EB series has digital inputs for pulse signals and analog inputs for 4-20 mA, and a Flow Totalizer application. Check the live product page for input ratings against the maximum pulse frequency you worked out above.