Every dosing duty starts as a concentration target in mg/L and has to end as a pump setting in litres per hour or strokes per minute. This page gives the conversions that get you there: dose mass from flow and concentration, solution flow from product strength, dilution, flow-pacing and pump-stroke setting — each with units stated and a worked example.
From flow and concentration to kilograms of active chemical per day.
The foundation of every dosing calculation. A concentration target (mg/L = g/m³) applied to a flow gives the mass of active chemical you must deliver per unit time.
Ṁ = mass of active chemical to deliver (kg/h)Q = stream flow rate (m³/h)C = target dose as active chemical (mg/L, equivalent to g/m³)Flow Q = 120 m³/h, ferric coagulant target C = 45 mg/L as product.
Converting the chemical mass into a volume the pump must deliver.
A dosing pump delivers volume, not mass. Convert the required mass rate into a solution flow using the delivered solution's active-chemical concentration. Neat liquid products are quoted as % w/w plus density; dilute solutions as g/L.
q = solution (pump) flow rate (L/h)Ṁ = active chemical mass rate (g/h) — note grams hereS = active chemical concentration of the dosed solution (g/L)% w/w = product strength by weight (e.g. 40 for 40% ferric)ρ = product density (kg/L, e.g. 1.43 for 40% ferric chloride)Need Ṁ = 5.4 kg/h = 5400 g/h. Product is 40% ferric, density 1.43 kg/L.
Making a weaker working solution — and what it does to pump flow.
Diluting a concentrate improves pump turndown and mixing but proportionally increases the volume the pump must move. The mass-conservation rule is unchanged.
C₁, V₁ = concentration and volume of concentrate takenC₂, V₂ = concentration and volume of final working solutionWhen preparing dilute acid or caustic solutions, add the concentrate to water, never the reverse — the dilution exotherm can boil and spit concentrated reagent. Confirm material compatibility of the make-up tank.
Dilute 572 g/L ferric to a 100 g/L working solution for better turndown.
Holding concentration constant as plant flow varies.
If the dose target (mg/L) is fixed, pump output must track flow. This is the basis of feed-forward control — see dose-rate & control.
S is the dosed-solution strength (g/L).k = C ÷ S (L of solution per m³ of flow).Turning a required flow into a control-panel number.
A metering pump's output is its maximum capacity scaled by stroke length × stroke frequency. Set the controllable fraction to the duty.
qmax = pump's rated capacity at the working back-pressure (L/h)| Convert | Formula | Note |
|---|---|---|
| mg/L → kg/h | Q(m³/h) × C ÷ 1000 | active mass |
| kg/h → L/h solution | 1000 × kg/h ÷ S(g/L) | via solution strength |
| % w/w → g/L | 10 × %w/w × ρ | needs density |
| L/h → mL/min | L/h × 16.67 | small-pump scale |
| ppm | = mg/L | dilute aqueous |
Send us your flow range, dose target and product data sheet and we will return sized pump flows, stroke settings and a recommended dilution — with the working shown.
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