Half Rate on the Warehouse Floor
A technician must treat 350 m2 of floor surface along the walls and beneath pallet racks in an empty warehouse. The label states: 'Dilute 20 mL of product per litre of water. Apply 4 L of finished spray per 100 m2.' He carries a 7.6 L backpack sprayer with no pressure gauge, a worn nozzle tip that produces a streaked pattern, and he pumps 'by feel'. On a marked 100 m2 test strip, spraying water at what feels like his normal walking pace, he uses 2.0 L. He has 500 mL of concentrate on the truck and one hour before the warehouse crew returns.
1. Diagnose the calibration problem and explain what he should and should not change.
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Diagnosis. He is delivering 2.0 L per 100 m2 where the label requires 4 L per 100 m2 - half the labelled volume. The label fixes the dilution and the volume per unit area; both are part of the authorized use.
What he must not do: double the concentration to 'make up for it.' That produces a solution stronger than the label allows - an offence - and it does not fix coverage: the floor still receives half the wetting, with higher residue, odour and exposure in the areas that do get hit, plus a real risk of the pest surviving in the dry gaps.
What he must do: bring the application rate up to the label, by technique, then re-measure:
- Slow the walking pace to roughly half his test pace, and/or
- Fit a nozzle that delivers about twice the flow, chosen to give the labelled pattern and droplet size;
- Keep pressure constant and stop pumping 'by feel';
- Re-run the 100 m2 strip with water and confirm close to 4 L before mixing any product.
Only when output matches the label does the mix calculation mean anything.
2. Calculate the mix for the 350 m2 job: total spray volume, total concentrate, and how to load the 7.6 L tank.
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Step 1 - total finished spray. 350 m2 ÷ 100 m2 = 3.5 units of area; 3.5 × 4 L = 14 L.
Step 2 - total concentrate. 14 L × 20 mL/L = 280 mL. He has 500 mL, so supply is sufficient.
Step 3 - tank loads (7.6 L tank).
- Tank 1: full - 7.6 L water + (7.6 × 20) = 152 mL concentrate. At the calibrated rate this covers 7.6 ÷ 4 × 100 = 190 m2.
- Tank 2: remaining area 350 − 190 = 160 m2, needing 160 ÷ 100 × 4 = 6.4 L finished spray, so 6.4 L + (6.4 × 20) = 128 mL concentrate.
- Check: 152 + 128 = 280 mL concentrate and 7.6 + 6.4 = 14 L spray. Consistent.
Practical points: add half the water first, then the concentrate, then top up, per the label order; mark the floor into 190 m2 and 160 m2 blocks so the second load lands where it should; recheck output mid-job as pressure falls. If he finishes the job with spray left over, he under-applied - record it and fix the calibration, do not pour it out or 'touch up' an already-treated area.
3. What should he do about the worn nozzle, the missing gauge and the record-keeping, and what if the concrete is too rough to wet with 4 L per 100 m2?
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Equipment. A streaked pattern from a worn tip means uneven delivery - some bands get double, some get nothing, which is both an efficacy failure and an illegal over-application in the wet bands. Replace the tip with the type specified by the label/manufacturer, carry spares, and check all tips before the job. Without a gauge he cannot hold a constant pressure, so output drifts all afternoon: fit a gauge or use a sprayer with a pressure regulator, calibrate at the working pressure, and re-verify at intervals and after any nozzle change. Check strainers and hoses for leaks before filling.
Records. Log date, site, product name and PCP number, dilution, total volume, area treated, applicator and licence number, equipment and calibration figures, start/finish times, re-entry interval and who was notified. This is what an inspector and the client's health-and-safety file will ask for.
If 4 L per 100 m2 will not wet the surface: do not simply apply more. Read the label for a rate range, an alternate application method or a restriction on porous surfaces; if none applies, stop and check with the manufacturer or select a product labelled for that substrate. Over-application to chase coverage is still an offence, and puddling on a warehouse floor is a slip, runoff and re-entry problem. Also confirm the crew stays out for the full re-entry interval, with ventilation running.