Solenoid diaphragm pumps
The workhorse of small-system dosing. An electromagnet pulses a diaphragm; each stroke displaces a fixed micro-volume through ball check valves, and dose rate is set by stroke frequency (and on better pumps, stroke length). Typical range 0.4 – 20 L/h at pressures to 16 bar, with control options from manual through pulse-proportional (paced by a water meter) to 4 – 20 mA. Virtues: no motor, few moving parts, compact, economical, and precise at low flows. Limits: the delivery is inherently pulsed (a sharp squirt per stroke), which matters for mixing and for downstream instruments; duty above ~20 L/h is outside the technology's comfort zone; and at very low stroke settings repeatability suffers (the sizing article's 20 – 80% rule exists for this reason).
Motor-driven diaphragm pumps
The same diaphragm-and-check-valve principle, driven by a motor and cam instead of a solenoid. That mechanical change buys: continuous industrial duty ratings, flows into the hundreds of L/h, smoother (though still pulsating) delivery, better turndown behaviour, and robustness under 24/7 operation. This is the standard technology for plant-scale dosing: antiscalant on an RO train, coagulant at a works, pH control on a process line. The cost is what you would guess: bigger, dearer, and worth it precisely when the duty is continuous or the flow outgrows solenoid territory.
Peristaltic pumps
A different principle entirely: rollers squeeze chemical through a flexible tube, and nothing touches the fluid but the tube bore. The consequences are distinctive. There are no check valves to jam or vapour-lock, which solves the classic hypochlorite problem: NaOCl releases gas bubbles that sit in a diaphragm pump's head and stop it pumping (losing prime silently), while a peristaltic just pushes the bubble through. They self-prime reliably, meter accurately at extremely low flows, handle viscous and shear-sensitive fluids, and run dry without damage. The trade-offs: the tube is a scheduled consumable (its life shortens with pressure and chemical aggression), and pressure capability is modest, typically a few bar, well below diaphragm territory.
Choosing
| Duty | Sensible pump |
|---|---|
| General dosing to ~20 L/h, ≤ 16 bar | Solenoid diaphragm |
| Continuous plant duty, higher flows | Motor-driven diaphragm |
| Hypochlorite and gassing chemicals | Peristaltic (or degassing-head diaphragm) |
| Very low or very viscous dosing | Peristaltic |
| High discharge pressure | Diaphragm (solenoid or motor per flow) |
Two cross-cutting notes. First, the pump head's materials matter as much as its mechanism: PVDF heads with the right seal elastomer per chemical (covered in its own article). Second, every technology here is a positive displacement pump: dead-head any of them against a closed line and pressure climbs until something yields. Relief valves and proper injection fittings are there to prevent that.
Rule of thumb: solenoid for small and standard, motor-driven for big and continuous, peristaltic for gassing, low-flow and awkward chemicals. If the chemical is hypochlorite and the complaint is "the pump keeps losing prime", the fix is a peristaltic or a degassing head, not a bigger diaphragm pump. And whatever the type, acid and hypochlorite get separate pumps, lines and injection points, never one pump switched between them.