A pump that delivers 0.18 millilitres instead of 0.25 makes a 50 millilitre serum last shorter and a 200 millilitre lotion feel thin near the end. Dose per stroke is a specification, not a detail. This note sets out the tolerance to expect, the variables that move it, and a ten-stroke test any brand can run on incoming goods.
1. What the dose number means
Output per stroke is the mass or volume dispensed from a full prime to the end of travel. For lotion pumps it is usually quoted at a defined viscosity and temperature, with a tolerance of plus or minus 5 to 8 percent. A pump quoted without a viscosity reference is only half a specification, and the missing half causes most disputes.
2. Priming and the first strokes
The first three to five strokes usually run short while the dip tube and chamber fill. Measurement should start at stroke five and average strokes five to ten. If a brand measures stroke one, every pump looks defective. Write the priming protocol into the incoming inspection sheet so a new QC operator repeats the same method.
3. Viscosity window
Every pump is designed around a viscosity band, often 1,000 to 8,000 centipoise for lotion pumps. A formula thickened above the band returns a short dose and a slow recovery; a thin formula squirts and drips. When a formula changes thickener level, re-run the dose test before approving the pack, even if the pump part number is unchanged.
4. Dip tube length and cut
The tube should sit 2 to 3 millimetres above the base, cut square. A tube touching the bottom restricts flow; a tube cut too short leaves several millilitres unusable and produces the classic “half a bottle left” complaint. Tube length is bottle height minus 3 millimetres, measured on the actual moulding, not the drawing.
5. Airless versus lotion pump
Airless systems dose more consistently across the life of the pack because the piston follows the product and no air returns to the chamber. A well-matched airless pack typically holds dose within a few percent to the last 5 percent of fill. Lotion pumps with a dip tube lose dose as the head space grows, which is why the end of a bottle feels different.
6. Temperature drift
Viscosity roughly halves for many emulsions over a 20 degree rise, so a pump tested at 25 degrees will over-dose at 35 degrees in a summer warehouse. Test at 5, 25 and 40 degrees where the market demands it. Record the temperature on the report; a dose figure without it cannot be compared between labs.
7. The ten-stroke weighing test
Weigh a clean beaker, prime five strokes, then deliver ten strokes and weigh again. Divide the mass by ten, then divide by the measured density to reach millilitres. Report mean, range and standard deviation. A range wider than 10 percent between strokes points to a valve fault or trapped air rather than a formula problem.
8. Fill height and head space
Overfilling leaves no air for priming and the first strokes pull product only; underfilling means the dip tube uncovers early. A head space of 8 to 12 millimetres at the shoulder is a practical target for a 200 millilitre bottle. Check it against the fill line on the drawing, not against the label position.
9. Complaint patterns worth reading
“Pump does not work” is usually a priming issue or a gelled product at the nozzle. “Too much comes out” is usually a viscosity drop from heat exposure in transit. “Nothing comes out” late in the pack points to tube length, evacuation or a clogged orifice from a formula with suspended powder.
10. Where it fits
Dose testing belongs with the pack, not with the lab alone. See body lotion and facial essence for pump formats, and recent operations notes for line checks.
