Inside SOP 2: air buoyancy, the invisible force in every weighing

Every object weighed in air is buoyed up by the air it displaces — roughly 150 parts per million of its mass for stainless steel at sea level. For most work that vanishes into tolerance; for precise mass calibration it is often the largest correction applied, and NIST's SOP 2 is the procedure for applying it.

The correction depends on the density of the air (computed from temperature, pressure, and humidity), and on the difference in volume between the standard and the unknown. Two weights of identical nominal value but different alloys displace different volumes, and the buoyancy difference lands directly in the result. This is also why conventional mass exists: OIML R 111 specifies weights against an agreed 8000 kg/m³ reference density at standard air, so that everyday commerce can ignore what precision labs must compute.

SOP 2 in the library carries the full air-density equation, the correction formulas, and worked examples — with the environmental instrumentation requirements that make the correction honest.