Moisture Content Disagreement Between Two Ovens
Two technicians determine total moisture on the same 2,000 g sample of 1 in. coarse aggregate. Technician A uses a forced-air oven set to 230°F and dries to constant mass over 18 hours, reporting 3.1%. Technician B uses a ventilated oven set to 230°F but only dries for 4 hours, reporting 2.4%. The sample contains visible clay lumps and some porous chert. The scale used by Technician A was verified that morning with a 2,000 g check weight reading 1,998.5 g. Technician B's scale was last verified three weeks ago. The specification requires moisture content within ±0.5% of the supplier's certified value of 3.0%.
1. Which result is more defensible and why?
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Technician A's 3.1% is more defensible. Drying to constant mass is required; 4 hours is insufficient for 1 in. material with clay lumps and porous chert, which release absorbed water slowly. Technician B's lower value indicates incomplete drying. Technician A's scale error is small (1,998.5 vs 2,000 g, about 0.075%), within typical tolerance for a 2,000 g sample, and was verified same-day. Technician B's scale verification is stale, adding uncertainty. A's result (3.1%) is within ±0.5% of 3.0%; B's 2.4% is outside the acceptance band.
2. What would you require before accepting either value as final?
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Require a constant-mass confirmation (successive weighings differing by no more than the method's specified amount, typically 0.1% of sample mass, after additional drying). Re-verify Technician B's scale with a check weight traceable to a standard. Confirm oven temperature with a calibrated thermometer (not just the setpoint display) and that the oven is ventilated. Document sample mass, oven ID, temperature, drying time, and balance ID. If the discrepancy persists, re-test on a freshly reduced sample.