Metering Pump 4-20mA vs Pulse Control: Choosing the Right Signal

The first decision in automatic dosing is the signal. Metering pump 4-20mA control is an analogue input that suits continuous adjustment from an analyzer, such as residual chlorine or pH. Pulse control is a contact input that suits flow-paced dosing from a water meter, where the pump strokes in proportion to the water passing. Wiring either one is not difficult. Scaling the signal and setting priorities correctly is where most commissioning time goes.

This guide is for electricians and operators commissioning solenoid diaphragm pumps. The same logic applies to mechanical pumps with a variable frequency drive.

Metering pump 4-20mA or pulse: how to choose

A metering pump 4-20mA signal is a current loop. 4 mA is the bottom of the range and 20 mA the top. The pump converts the current into a stroke frequency between its set minimum and maximum. Current loops resist interference and work over long cable runs.

A pulse signal is a series of on/off contacts, usually from a water meter or flow meter with a pulse output. Each pulse triggers one or more strokes, or one stroke after several pulses.

A simple way to decide:

1. Dose follows water flow, as with coagulant on a filter plant: use pulse input, or the flow meter's 4-20mA output.
2. Dose follows a water quality reading, as with chlorine or pH: use metering pump 4-20mA input from the analyzer or controller.
3. Both flow and quality change a lot: let a controller combine them into one 4-20mA signal for the pump.

Wiring metering pump 4-20mA input

The external control socket on a solenoid pump is usually a multi-pin connector on the side or base. Three points matter.

Polarity. The current loop has a positive and a negative. Reversed, the pump sees no signal. Some pumps alarm, others simply sit idle.

Active or passive. Most analyzers provide a powered (active) loop, and most pumps accept a passive input. Two active devices or two passive devices on one loop will not work. Check both manuals before wiring.

Scaling. Many pumps let you set the frequency at 4 mA and at 20 mA, and some allow reverse action. During commissioning, inject 4, 12 and 20 mA with a loop calibrator and confirm the pump responds as expected.

Stroke length stays manual. Check in advance that at full metering pump 4-20mA signal and the chosen stroke, the pump can still cover the highest dose.

Pulse control: multiply and divide

Pulse wiring is usually two cores from a volt-free (dry) contact in the meter to the pump's pulse input. Some meters give a powered pulse instead; confirm the voltage is within the pump's input rating, or the input can be damaged.

Pump menus normally offer two modes:

• Multiply: one pulse gives N strokes. Useful when flow is low and pulses are sparse.
• Divide: N pulses give one stroke. Useful when flow is high and pulses are frequent.

Work out the ratio from the volume each pulse represents, the chemical needed for that volume and the volume per stroke. If incoming pulses exceed the pump's maximum stroke rate, the extras are lost and the system underdoses.

Reading the wiring diagram and setting priorities

Identify every terminal first: power, metering pump 4-20mA or pulse input, pause or remote stop, level switch and alarm output. Diagrams usually reference pin numbers; match them to the numbers moulded on the plug.

Get the priority logic right

When remote control, pause and level alarm are all connected, the pump follows its own priority rules. Two common mistakes: the low-level switch is set to warn only, so the pump keeps running after the tank is empty; or the pause input is set normally open when the panel is normally closed, so the pump stops the moment it is connected. Our guide to low level alarm setup covers level wiring.

Cable and screening

Run metering pump 4-20mA cables in screened cable with the screen earthed at one end only, and keep them out of power cable trays. Long pulse runs need the same care, because false triggers cause overdosing.

Safety: isolate and lock off power to the pump and panel before wiring. Never connect mains voltage to a signal terminal.

Commissioning checks for metering pump 4-20mA and pulse modes

1. In manual mode, confirm the pump starts, stops and delivers.
2. Switch to external mode. Test the metering pump 4-20mA input at 4, 12 and 20 mA and check frequency changes as configured.
3. In pulse mode, apply a few pulses by hand and count the strokes.
4. Disconnect the level switch and confirm the pump stops or alarms as set.
5. Calibrate actual output.

Pump series differ in what control they offer; the NEWDOSE DFD and DP comparison is a good example of manual versus pulse models. For manual frequency adjustment, see solenoid metering pump flow adjustment. If you need a controller or an analyzer with an output, look at our dosing controllers.

Conclusion

Pick the signal before you pick up a screwdriver. Pulse input fits flow-paced dosing, metering pump 4-20mA input fits analyzer-based dosing, and a controller handles cases where both matter. Check polarity and active or passive loops, set scaling or multiply and divide, sort out level and pause priorities, then prove it with a loop calibrator and a drawdown test. Send us your dosing point, signal source and pump model, and contact our engineers to review the control scheme.

FAQ

Can a manual-only pump accept a metering pump 4-20mA signal?

Usually not. Choose a model with external control, or add a controller in front of the pump.

What happens if the metering pump 4-20mA signal is lost?

That depends on the setting. Some pumps stop and alarm below 4 mA, others run at minimum. Confirm this during commissioning.

How long can a pulse cable be?

Dry-contact pulse lines can run a fair distance, but use screened cable and keep it away from power cables to avoid false pulses.

What is the alarm output for?

It sends pump fault or low-level status back to the control room or PLC, so a stopped pump is noticed straight away.

Can one flow meter drive two pumps?

Yes, if its output can drive two loads, or through a signal splitter. Each pump gets its own multiply or divide setting for its chemical.