Test Equipment, Calibration, and Verification
Equipment basics
Accurate aggregate testing depends on properly maintained equipment: balances (with readability appropriate to the sample mass), ovens (temperature-controlled and monitored), sieves (correct aperture, not damaged or blinded), mechanical splitters, measures of known volume, and thermometers. Candidates should know that each has a calibration or verification interval and that records must be retained.
Calibration versus verification
Calibration establishes the relationship between an instrument's reading and a reference standard; verification confirms that the instrument still performs within tolerance. Traps include: treating a verification check as a full calibration, using an out-of-tolerance balance, and failing to record the reference standard used. Sieves are verified by checking aperture size and condition; a damaged sieve must be removed from service, not merely cleaned.
Traps in questions
Watch for: (1) using a balance whose readability is too coarse for the sample; (2) an oven whose temperature is not monitored; (3) a measure whose volume was never verified; (4) assuming that a certificate of calibration is the same as an in-house verification. Also watch for questions about traceability - calibration must trace to a recognized standard.
Worked example
A balance reads 500.0 g for a 500.0 g reference mass, then 500.3 g on repeat. The repeatability error is 0.3 g. If the tolerance for that balance class is ±0.1 g, the balance fails verification and must be adjusted or removed from service before use.
Rule of thumb
Match the balance readability to the smallest mass change you must detect - a balance that cannot resolve the required increment cannot produce a valid result, regardless of calibration status.