Observed → 60 °F · API MPMS Ch. 9.1 & 11.1
The temperature at which the hydrometer / densitometer reading was taken. The result is reduced to the 60 °F base.
How density and API gravity vary with temperature, and the MPMS 11.1 base-density iteration
API MPMS 11.1 uses a base-density iteration: from the observed density it computes α₆₀ and CTL, divides observed density by CTL to estimate ρ₆₀, and repeats (~5 passes) until converged, then reports API@60.
Liquids are less dense when warm, so a hydrometer reads higher API at high temperature than the true 60 °F value. The correction removes this effect.
Measurement: API MPMS Ch. 9.1 (ASTM D1298). Reduction to 60 °F: API MPMS Ch. 11.1 (ASTM D1250).