When a dosing pump is connected to a controller, PLC, or water treatment system, it receives a signal that tells it how much to dose. The type of signal used — whether pulse-width modulation (PWM) or a 4-20mA analogue current loop — determines not just how the instruction is communicated, but how the pump physically responds to it. The two approaches have different resolution characteristics, different failure modes, and different implications for dosing accuracy across the operating range. Choosing the wrong signal type, or misunderstanding how the selected type behaves at the extremes of the dose range, is a common source of overdosing and underdosing in industrial chemical treatment systems.
How PWM Works
Pulse-width modulation is a digital control method. The controller sends a series of on/off pulses to the pump, and the pump responds to each pulse with a single stroke or a defined action. The dose rate is controlled by varying either the frequency of the pulses — how many pulses per minute — or the duty cycle — the proportion of each pulse cycle during which the signal is on versus off.
At 100% duty cycle or maximum frequency, the pump runs at its maximum stroke rate. At 50%, it runs at half stroke rate. At 10%, it runs at one-tenth of its maximum rate. The output is controlled by the timing of the pulses rather than by the magnitude of an electrical signal.
PWM is inherently digital and therefore immune to the analogue signal errors — noise, resistance variation, cable length effects — that can affect 4-20mA systems. Each pulse either arrives or it does not. The pump either strokes or it does not. This makes PWM robust in electrically noisy environments.
The limitation of PWM at low dose rates is stroke resolution. At very low pulse frequencies, there are long intervals between strokes. If the required dose is very small relative to the pump’s stroke volume, the pump may be delivering acceptable total volume over time but doing so in discrete slugs rather than in a smooth continuous flow. In applications where mixing downstream of the pump is limited, this pulsing behaviour can create local concentration variations — momentary overdose at the injection point followed by a period of no dosing.
How 4-20mA Works
A 4-20mA signal is an analogue current loop. The controller sends a continuous electrical current ranging from 4mA (representing zero or minimum output) to 20mA (representing maximum output). The pump’s drive electronics read this current and translate it into a proportional output — either stroke rate, stroke length, or both, depending on the pump design.
The 4mA live zero is an important feature. A 0-20mA signal cannot distinguish between a zero command and a broken wire, because both produce zero current. A 4-20mA signal, by using 4mA as the baseline, allows the pump controller to detect a broken wire or failed transmitter as a fault condition — any signal below 4mA is an error rather than a valid command. This makes 4-20mA inherently more fault-tolerant for safety-critical dosing applications.
The analogue nature of 4-20mA means the pump receives a continuously variable command signal and can respond with a continuously variable output. There are no discrete pulse intervals. In principle, this allows very smooth modulation of dose rate across the full operating range.
However, 4-20mA signals are susceptible to electrical noise, loop resistance errors, and cable length effects that can introduce small errors in the received signal. At long cable runs or in electrically noisy environments, a 4-20mA signal may not deliver the same current at the pump that the controller intended to send, introducing a dosing error that may not be obvious without careful calibration.
Turndown Ratio and Dosing Accuracy at Low Rates
Turndown ratio is the ratio between the maximum and minimum controllable dose rate of the pump. A pump with a turndown ratio of 100:1 can be controlled accurately down to 1% of its maximum output. A pump with a turndown ratio of 10:1 can only be controlled accurately down to 10% of its maximum output — below that, the dosing becomes unreliable.
This matters because chemical dosing requirements change with process conditions. A cooling tower that requires maximum biocide dose during a heat wave may need only a fraction of that dose under mild conditions. If the dosing pump’s minimum controllable rate is too high relative to the minimum required dose, the controller cannot achieve the low dose accurately. The result is either a dose that is higher than required — overdosing, which wastes chemical and may cause side effects — or a pump that cycles on and off at low duty, producing the slug-dosing behaviour described above.
Digital motor-driven dosing pumps, such as those in the Injecta range with stepper motor drives, typically offer higher turndown ratios than conventional motor-driven pumps with PWM or basic 4-20mA control, because the stepper motor can be controlled with very fine resolution across the full stroke range. This is particularly relevant in applications where the required dose varies across a wide range, or where very low doses must be delivered accurately without slug behaviour.
Selecting the Right Signal Type
For most industrial chemical dosing applications in stable electrical environments, 4-20mA provides smooth modulation, fault detection via the live zero, and compatibility with the majority of PLCs and water treatment controllers. It is the default choice for proportional dosing in closed-loop control systems.
PWM is appropriate in electrically noisy environments where analogue signal integrity is a concern, in applications where on/off batch dosing rather than proportional control is used, and in systems where the controller outputs only digital signals.
The interaction between signal type, pump design, stroke volume, and turndown ratio determines the practical dosing accuracy across the full operating range. Getting this right at the system design stage prevents the chronic underdosing and overdosing that results from a mismatch between control signal behaviour and application requirements.
To discuss control signal selection and dosing pump specification for your application, contact us at info@autoflotechnology.com.